f ‘ E T — DEPARTMENT O HE mTEEIOE U . s. GEOLOGICAL SURVEY

C H A R L ES l)

GEOLOGY AND ORE DEPOSITS

O F T $ E

MININGD ICT J RS NC NTY MONTANA ISTR . EFFE O QU ,

’ NV A L T E IR I—I A R V E X \V E H U

‘ V IT H A N A P P E N D IX O N

THE MICROSCOPICAL PETROGIH PHY OF THE DISTRH‘ T

’ J O S F J I H 1 3 1 \ l { I i I CL L

EX R A C F R O M T H E WEN Y -S EC O N D A N N UA L R E PO R O F T H E S U R V E Y 1900- 1901 T T T T T . PA R T II—O R E pE PO S IT s

W A S H I N G T O N

G O V E R N M E N T P R I N T I N G O F F I C E 1 9 0 2

GEOLOGY AND ORE DEPOSITS OF THE ELKHORN MINING D C J F F N UN Y MON ANA ISTRI T , E ERSO CO T , T

W A L T E R H A R V E Y W E E D

WIT H

AN APPENDIX ON THE MICROSCOPICAL PETROGRAPHY OF THE DISTRICT

J O S E P H B A R R E L L

C N N O T E T S .

h a r Introd u tion C pte I . c F ield work and acknowl edgm ents G eographic pos ition

h t r II G n r eo i eatur f h C ap e . e e al g log c f es o t e district and its relation to th e sur rounding region a t r III I n rock f th i C h p e . g eous s o e d strict G ranitic rocks G ranite (quartz-m onzonite ) D efinition Exte nt and character of surface D escription C hem ical com position

A plite-granite

D efi nition C h em ical com position O ccurrence

D iorite D escription E xtent and area covered Q uartz-diorite-porphyry D efinition E xtent and character of surface A ndesitic rocks Extent and character of surface D escription IVeathering D iorite-porphyry r D escri tion of th e se dim entar rocks C hapte IV . p y A lgonkian rock s Turnley hornsto nes ( Spokane shale?) C am brian rocks A lpreston quartzites ( F lathead quartzites) Starm ount lim es tones H obo G ulch lim e-shale

22 G E O L PT 2— 01 26 , 402 C O N T E N TS .

— r IV D tion of th e sedim entar rock ontinued . C hapte . escrip y s C — C ambrian rocks C ontinued . C em etery lim es tone Elkh orn h ornstone Siluro-D evonian rocks

U nion shale

M adison lim estone Q uadrant form ation

C row R idge series r ation f ro k m C hapte V . R el s o c asses Sedim entary foundation of th e district G abbro stock of Black Butte D iorite stock of C em ete ry R idge D iorite -porphyry laccolith of Q ueen G ulch Turn ley R idge quartz-diorite-porphyry stock A ndesite m asses of E kh orn Peak ueen C re ek and Elkh om G u c l , Q , l h A ndesite M arble cliffs G ranite batholith G laciation V nera c of rm m m i f k f th C hapter I. G e l effe ts the al eta orph sm o the roc s o e district

A rg illaceous rocks C alcareous rocks F um arolic alte ration M in ra s of th e o de o its of th distri t C hapter V II . e l re p s e c O re m inerals G alena ( sulphide of lead ) Bournonite

T tr ed ri e r co r or antim onia u i f r e a t a e e s de o co e . h (g y pp , l lph pp ) Pyrite ( iron sulphide ) Tetradym ite ( sulphide and telluride of bis m uth ) Bism uthite ( sulphide of bism uth ) Blends s a erite zinc su ide ( ph l , lph ) C alam ine ( hydrated silicate of zinc) C erussite (lead carbonate ) L inarite ( hydrated sulphate of lead and copper) C halcopyrite G old Sil ver P ro us ite b ack ox ide of man anese y l ( l g ) 0 A zurite ( blue carbonate of copper) M alachite (green carbonite of coppe r) H em atite and m agnetite Lim onite (bro wn hematite ) D escloizite ( vanadinite of l ead and zinc) A urichalcite ( bas ic carbonate of z inc and Oopper) P rr otite m a netic rite iron su hide y h ( g py , lp ) G angue m inerals Q uartz (crystalline silica) wa n ] C O N T E N T S . 403

f — th re de sits o th d i n n . C hapter VII. M inerals of e o po e istr ct C o ti ued n u m — i Ga g e inerals C ont nued . C alcite (lim e carbonate ) D olom ite ( carbonate of lim e and magnesia ) G arnet ( variety grou ularite ) Diopside (lim e-m agnesian pyroxene) Serpentine ( hydrated silicate of magnesia) A x inite ( boro-silicate of alum inum with varying am ounts of iron nd m an a s a g ne e ) . Tourm a ine bas ic or subsiicate of boron a um ina otas im e l ( l , l , p h , l , m agnesia ) E idote ort osiicate of im e a um ina and iron p ( h l l , l , ) Wollastonite ( silicate of lim e ) M icrocline

Th E k m in C hapte r V III . e l h orn e H isto ry of the pro perty G eneral features of the ore deposit Topographic relations G eological relations O utcrop H anging-wall h om stone F oot-wall dolomite

Th e ores G eneral characte r Value Mine workings O re shoots and cham bers G eneral characters M ain ore s hoots F orm s of th e ore shoot Peculiarities of th e hanging wall N orth ore shoot South ore shoot Interm ediate ore bodies G eneral characte r of th e ore of the m ain ore sh oots Isolate d ore bodies in th e dolom ite or foot-wall lead cham bers O ccurre nce F illing D escription of a foot-wall ore body N ature of the ore deposit Structural relations of th e ore shoots O re shoo ts constitute saddle depos its A lteration of country rock M ode of form ation of ores O rigin by replacem ent E vidence of re placem ent sh own by thin sections of the ore Source of th e m etallic contents of th e once Secondary sulphide en richm ent Age of th e ore depos it r I N ot s on o r m in of th e di trict C hapte X . e the es s in C . and D . m e

Keene m ine 404 com .

— Notes on the oth er m ines of th e district C ontinued. C hapter IX . U nion mine

Dolcoath m ine R elief m ine

H omestake and D unsto ne m ines

n r L axenbur and M ountain iew M o te C ist o, g, V

J acquem in m ines E lkhorn Q ueen F uture of th e district

i tr ra b J ose Barre ] A endix . M icrosco ca e o pp p l p g phy , y ph l Introduction U naltered igneous rocks

R ocks of th e Ce m ete ry R idge sto ck

D ifferentiation in place A ndesites E lkhorn M ountain intrus iv e andes ites E k orn M ountain lavas breccias and tufts l h , ,

Turnley G ulch andesites Diorite-porphyry laccolith T urnley R idge stock Batholith at Elkhorn

V ariations in com position

Elkhorn M ountain aplite A plite stock A plite dikes north of Black B utte A plite dikes south west of Black Butte C ontact dikes of aplite in T um ley G ulch Segregations of to urmaline Alte red sedim entary ( contact-m etam orphic) rocks U se of nam es

Pure quartzite s Q uartzitic

Lim e-silicate group O rder of crystallization of th e lim e-al um ina-silieate group O ccurrence of m inerals M arbles L L U A N I S T R T I O S .

T r hic m a of th e E k orn district P L A TE XL II . opog ap p l h Th E kh orn m ine and m i XLIII. e l ll I Taiin s dam s of th e E k orn m ine XL V . l g l h V G i m a of th e E k orn district s owin area distribu L . o c X eol g p l h , h g l

V I T ica ranite area at ead of T urn e C reek sh owm bow ders XL . yp l g h l y , g l due to disintegration along fault planes V II A P int of R ocks s owin or e cut in andesites be ow to wn L . o X , , h g g g l f Elk orn B Ic Lake and m arb e c if s of E k orn o h ; , y l l f l h

II i n of th e E k orn district s owin ot etica V I . C ross sect o s XL l h , h g hyp h l underg round structure C ross sections of th e E k orn district s owin ot etica l h , h g hyp h l underground structure A i w o in nature of bow der m oraine on w ic Elk L . e s w , V h g l h h orn is buit wit te rmina m oraine dam m in val e seen h l , h l g l y in m idd e distance B Beddin ane between s ate and l ; , g pl l do om ite - oot eve E k orn m ine l , f l l , l h h E k orn s a t s owin the ski and m et od of ore LI . T e l h h f , h g p h hoisting Cerussite cluster from th e Elk horn m ine Native sil ver from th e Elkhorn m ine A V ertica trans verse section across th e nort an in -wa , l h h g g ll bod E k orn m ine B ertica transverse section across th e y, l h ; , V l - sout an in wal bod E k orn m ine . h h g g l y, l h A C am ber in th e - oot south sto e E k orn m ine B , h f p , l h ; , Sout end of th e 950- oot sout ste e E k orn m ine ookin h f h p , l h , l g

V I Th 850- oot an in -wa to E k orn m m e B Th e L . A e s e , f h g g ll p , l h ; , oot sto s in ead c am ber E k orn m ine f p l h , l h M ap of th e workings of Elkhorn m ine Photogra ph of specim en of hanging-wall breccia with pyritic

m atrix E kh orn m ine . , l I G a na ore E k orn m ine s owin im estone breccia cem ented L X . e l , l h , h g l by one Th e ore breccia E k orn m ine s owin structure of th e oot , l h , h g f r wall o e bodies . A uartzose ore s owin im estone ra m ents cem ented b , Q , h g l f g y uartz B C e ular uartz rom E k orn m ine orm ed b q ; , ll q f l h , f y dissol ving out of lim estone fragm ents of deposit like that s own in A C C e u ar and com act uartz due to in h ; , ll l p q fill g of O pen spaces of B by late r deposition of silica A A i - nite s owin se re ations of microsco ic au ites LXII. u te s e , g y , h g g g p g sus pended in m icrocline and zone of iron-poor augites on th e borders of th e se re ations B S ecim en s owin a ite g g ; , p h g pl in an i in r f h a i strin er enetrat des te rov ate a e o t e te . g p g , p g l g pl 405 406 IL L U STR A T IO N S

2 x M n n in t n f kh rn ri F IG . 7 . Inde m a of o ta a sh ow oca io o E o dist ct p , g l l Section m m th e sout ore s oot of th e E kh orn m ine t rou th e h h l , h gh

i h uth r d f th E k m in n - 74 . Sect on of t e so o e bo y o e l horn e o the foot

- 75 . Sections across th e 2 200 oot ore bod of th e E kh orn m ine s owin , f y l , h g typical form s of replacem ent deposit f Wil n r bod - t f E k or min 76. F ace o so o e oo sto es o n e y, f p l h D i r m h o occurrence of or sh in itchin ar or 77 . ag a to s w e oots p g ches folds

78 . T in section of ore showin re acemen of d omit a ena h , g pl t ol e by g l GEOLOGY AND ORE DEPOSITS OF THE ELKHORN MINING F F N COUN Y M N ANA DISTRICT J E ERS T , T . , O O

R V E D By W AL T E R H A E Y WE .

C H A P T E R I .

IN T R O D U C T I O N .

C W L M E F IE L D W O R K A N D A KN O E D G N T S .

The Elkhorn district was first vis ited by th e write r in the summ er 1 896 w e stu n the enera re at ns the ran te area in of , hil dyi g g l l io of g i

r. W r . su e w c the Butte ore de s ts ccu M r. a te e e rin hi h po i o l S K ll y, p ten ent the rn mine cal e attent n to the a ue an d of Elkho , l d io v l of n est ati n t s r ert w se e tens e e e ment and nter i v ig o of hi p op y , ho x iv d v lop i esting geological occu rrences make it in some respects unique am ong It a th or e the ore deposits of M ontana. was found th t e e d posit rs on the r ers a reat area ran te and t at in the occu bo d of g of g i , h immediate vicinity of the depos it there has been success ive intra n us rock rea n u t r u h e se iment r nd s ions of ig eo , b ki g p h o g fold d d a y a h tr c t ff re e ul r earlier igneous rocks . T e dis i t hus o e d p c ia ly favorable opportunities for astudy of the relations of the igneous rocks to one another and of the effect produced by such intrus ions upo n adjacent n s m d se m entar r c s . I 1 897 a sec i t was a e to the str ct di y o k ond v i di i , and a fortnight was spent in a reconnaissance of the entire area and rm th e m r sa ent e t res it e a dete ination of o e li f a u of s g ology . The ser at ns m a e at t s t m e c n rm e n ns rev us rme ob v io d hi i o fi d opi io p io ly fo d, d e n an tu the str ct were et rm ne u an d tailed mappi g d s dy of di i d e i d pon. r H a man 9 . . was eta ed to m In the summ er of 189 M R . Ch p d il ake a topographic map covering the more i nteresting geological features of e a u l rn T he write r an to h the region imm diately bo t E kho . pl ned do t e eta le e ca ma n m se but the necess t urt er stu d i d g ologi l ppi g hi lf , i y of f h dy of the Butte copper depos its prevente d his spending the summ er upon i D r r t s w r . ere re he t o to the str ct . se a rel hi o k Th fo o k di Jo ph B l , who was act n as e assistant for the seas n and went er the i g fi ld o , ov r un wit him n cat n the is t nct ns to be m a e and the r g o d h , i di i g d i io d p ob h r th m lems to be worked out . T e a eal distribution of e rocks was apped r a n h nt s A u t nd m r nd D . rre ur t e m u s a e te e a reat by B ll d i g o h of g S p b , g 407 408 E L H O RN M N N G ST R IC M O N T A N A K I I DI T , . credit is due to him for the care with which the obscure boundaries of the fferent r c s a n ra A t th h di o k h ve bee t ced out . e same time e made care u stu es the re at ns r c masses the c an es in e f l di of l io of o k , of h g th i ne us r c s res u t n r m c ntact metam r sm and ss e g o o k l i g f o o o phi , of po ibl m n m a h assi ilatio of ateri l by t e intruding m agm a. The detailed facts u n w c m an the n uct ns ma e in t s re rt rest w re po hi h y of i d io d hi po e , D r rr r t ere re at ere . a e . The un e r un w r was n h fo , g h d by B ll d g o d o k do e

h r. The r ra h r w by t e autho pet og phic study of t e ocks as mad e by D r. h e l r e r t u r n . L . Pir n In Barre un e t ec s s r . V sso the l , d di p vi io of P of , petrographic laboratory of the Sheffield Scientific School of Y ale h U n ers t . T e eo cal n tes ta ne D r. Barre ] t et er iv i y g logi o ob i d by l , og h t t s re rt u on the etr ra the r cks rm e the as of wi h hi po p p og phy of o , fo d b is a thesis prepared by him in partial fulfillm ent of the requirem ents for r T he r t r r the degree of docto of philosophy . pape hus p epa ed has been largely drawn upon by the writer in preference to the original fie n tes w e the etr ra h c re rt is the sam e as t at th ld o , hil p og p i po h of e es s sa e n that m uc the ure s eculat e rt n has een th i , v o ly h of p ly p i v po io b om itted . h r c h r II n T e topog aphi map of t e dis t ict (Pl . XL ) includes a area

21 m es on a sca e or a ut 2 n e of 1& by } il , l of bo i ch s to a h - r W r h m e t e re e e n s wn 50 t contou s . e e t e h rn il , li f b i g ho by foo h Elk o dis trict is m entioned in these pages this area is m eant . A ll the m ore important prospect pits are located and the m ore im portant geological n u h features are i cl ded wit i n the area selected . The chemi cal analyses in this report have been m ade in the labora h r H ur . to . tory of t e G eological S v ey by D . N . S kes d r an s are es ec a ue to M . Wa te r . el e rm er m ana er Th k p i lly l S K l y , fo g of the rn m ne for his s ta t and for n nesses too num er Elkho i , ho pi li y ki d h n r ous to m ention . T e author is i debted to him fo the use of the mine m a s and for v a ua e s ec m ens m nera s and res a s f r the use p l bl p i of i l o , l o o ' of a room kindl ace at his s sa as wel as for man ers na y pl d di po l , l y p o l n r d e M m rm r r t es . a a a u W D a e e e s ta s e ls to r. a ho pi li i Th k o illi v y , fo fo

m an of the m ne and to M r. m ac istant rem an for m an i , Tho Ty k, as s fo , y courtesies . This report will be regarded by som e as an account of an exhauste d and w r e -out str ct as the k rn m ne the c e r ucer o k d di i , El ho i , hi f p od , a ter a r uct e er a ut twent ears was c se wn f p od iv p iod of bo y y , lo d do n d r and a a ne . I n 1 901 we er it was re ene an mises b do d , ho v , op d , p o to be a a n a r ucer h t r r r ar t in the r s g i p od . T e o he p ope ties e as ye p o ect sta e and t u rom s n not ffer suffic ent ex sures p g , , ho gh p i i g, did o i po and ata for a com l r r I h a sa e we er to e t. t as seeme d p ete po d dvi bl , ho v , resent s me at u l h a ts n rn n the rn m ne as t e p o wh f l y t e f c co ce i g Elkho i , h y will be of inte res t in connection with the future development of other r ert es in the ntr he e t to w i the m ne was w r e p op i cou y . T d p h h ch i o k d and the lateral exte nt of the developm ent ofiered peculiarly favorable ' m‘ aa

T O P O GR A P H I C M L KH O R N D I S T R I C T

Topography by R H Ch a p m an n un : 18 99

$ m l G E O G R AP H P O S T O N IC I I . 409

con t ns for stu and no a is n e fo r r di io dy , pology eed d p esenting an account of a mine which has yielded s uch large am ounts of the pre cions m eta ls and which m ust ever be regarded as one of the big m ines n of M o tana. G E O G R A P H P O S T IC I IO N .

The k rn distr ct is s tuate in the cente r J effers n o u nt El ho i i d of o C y , in the central portio n of the m ountain region form ing the western ar h t t e tate M ntana. T he t wn l rn w c is in th e p of S of o o of E kho , hi h center the str ct l es a ut 1 8 m es eas t wnsen a to wn on of di i , i bo il of To d , the M issouri R iver and on the m ain line of the Northern Pacific Rail t way . I is 20 m iles southeas t of Helena and about the sam e distance t It i n h northeas of Butte . s access ible by a bra ch line of t e Northern

— lo i of lkh rn distri F IG . 72 . Index map of Montana, sh owing cat on E o ct.

Pac c R a way and is ab ut 1 2 m es east the t wn ul er ifi il , o il of o of Bo d which is on the M ontana C e ntral di v is ion of the G reat Northern Rail h n the dis T he acc m an n m a . 72 s ws t e s tuat way . o p yi g p (fig ) ho i io of triot with respect to the m ain drainage li nes and m ountai n ranges of r and l rn ea s r se to an e e at n eet the State . C ow E kho p k i l v io of f e to the n rt and n rt eas t the t wn e n the only a s hort dista nc o h o h of o , b i g ese two ea s are the cul highest peaks for m any m iles around . Th p k minating sum m its of a group of m ountains lying west of the M issouri R iver V alley and isolated from the neighboring ranges of the R ocky

l e s on the n rth eas t s ut and west . M unta ns y r a en a , , , o i b b o d , op v l y o o h

r u to w c the nam e C r w ree M unta ns or r w M un This g o p , hi h o C k o i C o o a has a crest ne in s a e s m ew at resem n a tains is ofte n pplied , li h p o h bli g w t the en n ac n eastwar and trenc e b C r w horseshoe , i h op i g f i g d h d y o S C T M O N T A N A . 41 0 EL KH O RN M IN IN G D I TR I ,

ree a tr utar the M issouri. s m unta n str ct w c C k , ib y of Thi o i di i , hi h its e at n r m ar to the n se m entar r c s but owes el v io p i ily foldi g of di y o k , chiefly to the durability of thé finer-grained igneous rocks which cap it summ its is ra ne stream s w c are enera ra a to its s , d i d by hi h g lly di l w r a n n rt -s ut au t m ar e an s . n the west e e fl k O , ho v , lo g o h o h f l , k d by the ra na e wa r ck ear ree and u er a e se arates d i g y of P i ly P C k Bo ld V ll y , p it from the complex of m ountain peaks and ridges known as the t i n t at the r w M unta ns not n Boulder M ountai ns . I s thus see h C o o i o ly lie w n th m nta n re n the tate but are a un t t ell withi e ou i gio of S , i bo h to o r a and e ica and not m ere a art s m e ar er p g aphic lly g olog lly, ly p of o l g

district .

T O P O G R A P H Y .

T he strict is one reat re e as is we s wn on the t ra c di of g li f , ll ho opog phi I I e e t u ste e are not reci m . . In enera th s s a P XL , , p, l g l lop ho gh p p p W l th r c s re usua we e se t ere is c ns er t na. e e a i o hi o k lly ll xpo d , h o id n h e at n th r n i a e l and e etat n. w to t e e e e t bl soi v g io O i g l v io of gio , receives abundant precipitatio n in the form of rain and snow and is often the scene of m idsummer snowstorm s ev en when the adj ace nt

ow an s ar at e in suns ne. A s m t t ere ore be m a ne l l d e b h d hi igh h f i gi d, n h n F or t the climate is somewhat rigorous a d t e wi ters are long . his reas n t ere is an a un ant ra na e w c is all concentrate in the o h b d d i g , hi h d str m n wn as rn ree w e mm e ate uts e the area ea k o Elkho C k, hil i di ly o id of r r e al t r eads mapped C row Creek and Prickly Pea C e k so take hei h . The str ct t u sma s ws the usua ecul ar t es ra na e di i , ho gh ll , ho l p i i i of d i g ne r n t r noticed where igneous rocks and limesto s a e fou d close toge he . In e rm er roc s the stream s are erenn a w le in the atte r the th fo k p i l , hi l drainage ways carry running wate r only in tim es of heavy rain or r i n r ant as s ciati n ant f e the t n n . e e s a c co mel i g s ows Th o d o o of pl li , limestones being in general bare and arid and the igneous areas us u and ere w t an a un ant er a e but t s is ally well wooded cov d i h b d h b g , hi n ent on ra n a w c is a unct n a t tu e so t at the en depe d i f ll , hi h f io of l i d , h g eral statem ents given above are true only where the elevations are l e r r rn ree w ere it enters the r. In h w a t simila t e o p of Elk ho C k , h r a en a e u er R er the areas oth ne us and b o d , op v ll y of Bo ld iv , of b ig o r t n lim estone rocks are arid and devoid of all arbo eal vegeta io . Elkhorn is one of the m os t picturesquely situate d m ining towns of It e near the ead a m unta n a e at a nt w ere the State . li s h of o i v ll y poi h n r n n ccur the s ut the u c is narr w and a s udde b oade i g o s . To o h g l h o can n e w e a e it c an es r m a m ra ne- amm e r e to yo lik , hil bov h g f o o i d d go g N est n c se u un er rn and a high m ountain amphitheate r. li g lo p d Elkho r w ea s at an e e at n eet a e the sea it rece es C o p k , l v io of f bov , iv tr ute r m m an c u anks w c r t n acr ss the wer ib f o y lo d b hi h , d if i g o lo

m unta n c untr to the west are cau t rn ea . T he o i o y , gh by Elkho P k

s ener is not t ca a ne t u ru e and ctures ue . T he c y ypi lly lpi , ho gh gg d pi q w r m the town is we er a r a one nc u n a wi e stretc v ie f o , ho v , b o d , i l di g d h h of valley and a mountain range to t e south .

4 E L H O R N M N ST T N 1 2 K IN I G DI R ICT , M O N A A .

t r th r rt w s n r e ut In 8 the M ntana cor whe he e p ope y a ot wo k d o . 1 8 8 o oration s out to a new c m an r an z e in L n n w c to p old o p y o g i d o do , hi h ok the nam e and r e rt the c r rat n rem e e the m and p op y of old o po io , od l d ill , ns t tute a o c r us e e m ent un er w c the m ne has i i d p li y of vigo o d v lop , d hi h i e n s r nd n s sin yield d ou ces of ilve a ou ce of gold ce 1 888 . h r t n h h n m u M r Wa r n te te . T e total p oduc io of t e m i e as bee co p d by . l S ’ e e n e h r r r m th m n Th the ma a r t e e t e c a s s . e y , K ll , g of p op y f o o p y book ures are en in r un num ers n and t se for the ear er fig giv o d b o ly , ho li period are only approxim ate . r m 1 884 to anuar 1 900 th e m ne was in c nt nu us O erat n F o J y , , i o i o p io , sa e for a s r t m e in 1 886 w en an acc ent to the um s resu te v ho t i , h id p p l d h h th - t e e for se er n in t e floodi ng of t e m ine up to e 500 foo l v l v al mo ths . In 1 896 the ore in s t was near e auste and re arat ns were igh ly xh d , p p io r le h t h m n b t r . im m a e to a an n t e e u M . Wa te e w o at at t e d b do i , l S K l y , h ecam e enera m ana er the r ert care u e l rat n w r b g l g of p op y , by f l xp o io o k isc se new ore es and c nt nue to w r the r ert unt d lo d bodi , o i d o k p op y il h e utum n 8 9 it ecam a rent h D ecem er 1899. In t a 1 9 e a t at t b , of b pp h e e e nse um n w c necess tate ea ue s c m ne xp of p pi g , hi h i d h vy f l bill , o bi d w t the sm a e tent and low ra e the ore in s t w u not i h ll x g d of igh , o ld rr n h t h Th c s th r wa a t t e further opera ion of t e m ine . e o t of e o e extracte d had steadily risen d uring the last few years until in 1 898 it reac e a to ta er ton w e the e ense m l n ncrease h d l of p , hil xp of il i g i d a re re e e t s wn n to per ton. It w s the fo d cid d o clo e do a d abandon the r er p op ty . The developm ent of the Elkhorn has playe d an im portant part in m e Th m nt the settlem ent and better nt of the State . e o hly pay roll aggregated over as bes ides the m en em ployed at the m ine and a l r r r wer r r r h mill a ge numbe of wood choppe s e egula ly at wo k . T e town rn a sett em ent w t a u at n a ut 600 was of Elkho , l i h pop l io of bo , u t and m a nta ne ract ca t s one m ne and the su es b il i i d p i lly by hi i , ppli shippe d to the dist rict for m ine and town kept the railroad line busy Th s n nt s t r t reat r for m any years . e pas i g i o hi o y of his g p operty is e t not nl at the to wn rn but in the ranc es for w se f l o y of Elkho , h , ho ro uce it urn s e a m ar et and in the arts the State r m p d f i h d k , p of f o e which it drew s uppli s . T he rm e r nacces s t th e str ct s tuate as it was in the fo i ibili y of di i , i d heart of th e m ountains and unti l recent years rem ote from rail road c m m un cati n ete rre act e r s ect n for a n t m e m re er o i o , d d iv p o p i g lo g i ; o ov , in the ear ears the rn m ne s u es art cu ar Sa t were ly y of Elkho i , ppli , p i l ly l , r m n w v er e ens e so t at the c st t eat e t as ea y . y xp iv , h o of h v In 1 886 the Northern Pacific Ra ilway built a branch line to WVickes - r s n ss the A ta nd m t m n to get the ore car ying bu i e of l a Co e i es . A bout two years later the m arvelous developm ent of the Butte copper deposits induced this com pany to begi n an extens ion of this branch

wean. ) H IST O R Y A N D PR O D U CT IO N . 41 3

he t m he n n s ut war to t at c t . A t t sam e e t M nta a e tral now o h d h i y i o C , a art the G reat N rt ern s stem was un er c nstruct n and a p of o h y , d o io , wa - n s t th t e ra u c nte t e an w t u te as e ec nt . il y b ildi g o b g , i h B obj iv poi Th e M ntana e ntra secure the m re a ra e r t wa and the o C l d o f vo bl igh of y , rthern a fi ne a t r n uit Bern ce was not c m et c c e e to e . No P i li , f b i g b l i , o pl d B t r a s as s t r u o u er the c unt seat 1 2 m es west o h o d p h o gh B ld , o y , il of rn so t at a ear ater the rt ern ac c m an wa Elkho , h y l No h P ifi Co p y s induced to build a branch line th rough the Boulder Valley and up the r The n t s r a was a ffi gorge of Elkhorn C eek . buildi g of hi o d di cult ece en neer n as the rad e to be ercom e was e ces s e but pi of gi i g , g ov x iv , by the use of short loops a road was constructed on which short trains r w cou be au e . tr wee t a n ser ce as esta s e ma n ld h l d A i kly i vi bli h d , ki g it possible to s hip the silver- lead ores which were discovered in great abun ance in the ee w r n s the h rn m ne and w c d d p o ki g of Elk o i , hi h were a s un to a m te e tent in t er r ert es in the tr t l o fo d li i d x o h p op i dis ic .

R P O DU C T IO N .

T he success ful developm ent of the Elkhorn m ine resulte d in very energetic prospecting of the adj ace nt claim s during the first few y ears n e T h r rt h r of its existe c . e epo of t e D irecto of the M int for 1 884 m en t ns 1 2 m nes all c se to the rn w ic were sa to s w we io i , lo Elkho , h h id ho ll defined ore es carr n a ues in s er and met m es in bodi , yi g good v l ilv so i and c er and w c were at that t m e e e e r s ect gold opp , hi h i d v lop d by p o p

s a ts er 20 eet ee . t se n te in t at re rt the ee ne h f ov f d p Of ho o d h po , K ,

n n d D . a e e n e e nto m n r r U n a d . an e e es m w c o io , C h v b d v lop d i i f o hi h e has een s e at ar us tim es but the r ert es ha e t us far b hipp d v io , p op i v h r e sa o nt n to the wners and t e r e has ee n in i p ov d di pp i i g o , h i yi ld b s g n nt m re w t th rn T he . and D ifica c a t at e m n . i o p d i h h of Elkho i e C . s h n one r m w n r t rn r n t e o ly f o hich a y e u s of p oductio are available . A sm all sm elter was built in 1 886 and run for a short tim e upon the ores T h r u t f r n in from this m ine . e p od c o 1886 is give the repo rt quoted f F or th m e r es th as w rt o u n. 1 887 e sa aut t e o h . b llio ho i y giv l in r t h nd D . n nd s er uc t e . a m e as in a p od of C i go d ilv , and for 1 888 as in gold and in s ilver.

he . nd h n r In ater ears t C a D . as bee ease at a us t m es and l y l d v io i ,

the ore e tracte has een s e to custom sm e ters . The ore x d b hipp d l , we er has een too low ra e to w r r ta and it is n ho v , b g d o k p ofi bly, o ly when pockets of galena have been found that any profit has bee n m ade r r T r r in th e workings of the p ope ty . he oth e prope ties of the district a e een w r e at ar us tim es in the ast twenty ears but n h v b o k d v io l y , o ly

for s rt er s and a e not een e e e nto r uc n m nes . ho p iod , h v b d v lop d i p od i g i A n e ce t n s u be ma e the h rn ueen w c es er x p io ho ld d of Elk o Q , hi h li ov the r e e n ueen G u c ust uts e the m ts the area idg b yo d Q l h , j o id li i of m a pped . 4 EL O R I N N ST R C T M O N T A N A . 41 KH N M I G DI I ,

T he following tables give the yearly production of the Elkhorn mine

Y r r i m - 9 ea ly p oduct on of E lkhorn ine from 1 882 1 88 .

Y ea rs . Silv er. Gold .

D olla rs.

a O unces .

Y rl roduction l - ea y p of E khorn m ine f rom 1 8 90 1 900 .

ilv r. S e G old .

Lm lb

Y ea rs . iném ill ull n m il i M io . S eltin ore . M l ull on b g b .

T ota 4 42 2 084 349. 97 3 494 . 874 2 428 . 920 l , , , , ,

m n a O re sh ip e ts .

In addition to the above there was an average of about pounds th r of copper contained in e m ill bullion each yea . warm R O T O N ] P DU C I N A N D MILL I G . 41 5

n o rn b ri P roductio f E lkho mine y pe ods .

Y ears (inclusive ) .

G S , 902 , 000

n un f l a A d po ds o ead .

r m w at h ee n state it w be seen t at the ec n m c e F o h as b d , ill h o o i g ology the str ct is alm st ent re an acc unt the rn m ne and of di i o i ly o of Elkho i , , in ee not n are the w rk n s of the t er r ert es smal but d d , o ly o i g o h p op i l , r num m n they were at the tim e of vis it inaccessible . A la ge ber of i ing claims have been located and prospecti ng pits sunk over nearly every rt th tr ct us far we er the n ore e s ts w rt a e s . p of di i Th , ho v , o ly d po i o hy of serious attention appear to be in the areas of alte red sedim entary ns e een un near the summ t rn rocks . Small vei hav b fo d i of Elkho ea and in the am t eater e w it but u to the resent t m e no P k phi h b lo , p p i ore bodies large enough to warrant exte nsive developm ent work hav e h m i in t l w een sc ere . T e J ac ue n m es s uate a ut 2 m es est b di ov d q , i d bo i rn on a ranc urn e ree a e s e c ns era e of Elkho , b h of T l y C k , h v hipp d o id bl erru n us ore w c was too low ra e to w r on acc unt its f gi o , hi h g d o k o of rec us -m eta c ntent a ne but w c was nee e at the sm e ter as p io l o lo , hi h d d l h r n m nes on the n rt s e rn ea are a so a flux . T e i o i o h id of Elkho P k l w rt attenti n as t e ex se ar e d es rat er ure iron o hy of o , h y po l g bo i of h p h r n n t r t as low ore . A s a str ct t e utu e ca o be sa to be the di i , f id b igh , n grade characte r of the ores precludes their treatm e t at pres ent prices . l h er e exce t n The ore the D o coat is we a ss . gold of , ho v , po ibl p io

M IL L IN G .

— Th The Elkhorn ores are of two clas ses sm elting and m illing . e sm elting ores consis t chiefly of argentiferous galena and are shipped to h m r The m n r a m ure s er su es and t e s elte . illi g ores a e ixt of ilv lphid

ra c er and are treate the r nar c r di n r cess . A g y opp , d by o di y hlo i zi g p o care u se arat n the ore is c urse m a e in the m ne and f l p io of , of o , d i , t s rtunate is m a e the m re readi ecause the m l n ore hi , fo ly , d o ly b il i g

- is almost wholly quartzose and com es from the hanging wall bodies . The m illing ore is treated at the m ine in an ordinary pan amalga m n m l atio i l . In m illing the ores the treatm ent has varied somewhat in recent In nera ears w n to t e r ncreas n re ract r c aracter. e y , o i g h i i i gly f o y h g l , the cess c ns st in a dr stam n the s ce us ore sa t e n pro o i s y pi g of ili o , l b i g E L R N M N N G ST C T M O N T A N A . 4 1 6 K H O I I DI R I , a e to th e ore as it is fed to the stam s w ence the dr ul is dd d p , h y p p r T he ar e am unt r conveyed directly to the Briickner oasters . l g o of g ay copper and galena in the ore treated in the last few years has made r In h r r r it necessary to use as m uch as 1 5 pe cent of salt . t e f ee o es f om r ffi n Th r the upper levels of the m ine 5 pe cent was s u cie t. e oasted ore is ta en in cars to the coo n r w ere it is e t in ea s for ten k li g floo , h l f h p to teen m nutes t en e b nsert n a bar ert ca and t en fif i , h hol d y i i g v i lly h

ri nta m a n a ree air assa e th r u the center the ea . ho zo lly , ki g f p g o gh of h p The heap is subseq uently leveled down to a thickness of 1 to 2 feet Th ore i t n c n e and then trenched by ditches . e cooled s he o v yed in w ee arr ws to the an r m w ere it is m e wit sa t and sul h lb o p oo , h ix d h l

’ huric ac and ut in ama amat n ans m e e n a e to c ear p id p lg i g p , li b i g dd d l r ter r n in in the ans the ‘ ul is rawn nto the quicksilve . Af g i d g p p p d i sett n ans w c w en a u ul are fi e u w t water li g p , hi h , h h lf f ll of p p , ll d p i h h i t n u and sti rred slowly for half an hour . T e top peg s he p lled out r n and a stream of wate r u in . T he cost of treatm ent has s teadily increased as the ores becam e r r s r e due to the ncreas n am unt salt more ref acto y . This i la g ly i i g o of us in 1 9 h c st sa t er ton ore m e was used . Th 8 6 t e o of l p of ill d in r s n to in 1 897 and to in 1 898 . ue c st but i i g , F l o m 1 896 r se to in 1 897 and to in 1 898 . ese two te s , o , Th i in 9 t in account for the increase f rom 1 8 6 o 1 898 . h m n m an er M r Wa ter e T he care u returns m a e t e e a . S . e f l d by i g , l K ll y , to the c m an s w in reat eta the c st u f treatm ent er to n o p y , ho g d il o p h l w n ta e w c is c e r m ore m e . in c ns er n t e of ill d o id i g fo lo i g bl , hi h opi d f o ’ th e m ana er s re rt for th e ca en ar ear 1 896 it s u be rem em g po l d y , ho ld r m a T he ta e s ws the am unt bered that the ore was ext e ely b se . bl ho o

ore treate u n r duce etc. of d , of b llio p o d ,

Work performed by E lkhom mill in

- Batte ries 20 stam in service 315 da s 9 ours . ( p) y , h

Pans in service 357 da s 15 ours . y , h

d 13 762 dr tons . re crus e . O h , y

Salt used tons .

A v erage am ount crushed per stam p per day tons . f ounces si ver r n A verage as say value o pulp l pe to . r n P ulp panned d y to s . n A verage assay val ue of tailings ou ces . r c nt Portion sav ed according to as say pe e . r n Portion saved according to bullion returns pe ce t. 48 721 fin i ver roduced 6 e ounces . S l p , fin un G old produce d e o ces .

1 8 E L xH O R M N N S 4 N I I G DI T R ICT , M O N T A N A .

T he ue c st is ncreas e a s w n to ncreas e e t th f l o i d l o , o i g i d d p h of e m ne the ar er am unt water due to new water c urses cut in i , l g o of o t n u r e e n h r s ec e v s a d t e ncrease use c m resse air. p o p i g pp l l , i d of o p d

A na l sis o cost o m inin er ton o ore hoisted rom 1 8 96—18 98 inclus ire y f f g p f f , .

[Based upon total ex penditure in m ine ]

’ Su erinte ndence inc udin office ex e nse p , l g p

M ine labor

Ex plosives

Illum inants G eneral repairs and supplies Surface ex pe nditures

T axes insurance and e a ex e nses , , l g l p

5 T ota 12 . 1 4 2 1 15 . 5941 l $ 384 . 0

The cos t m n n was in 1 896 and su se uent ears er reat , y of i i g b q , v y g ly increased over that of form er years by reason of the exhaustion of the ar e o re o es rmer w r e the c st was ar e ncreas e a so y ; , l g b di fo l o k d o l g ly i d , l r the ore m m m an sm a n by eason of co ing fro y ll stopes a d faces .

ld A m ount contents and ralue o are so . , , f

S ilv e r old d T o t l alue Y ea r. . G . L ea . a v .

T ons . 0 77new. P ound s . 2 . 2 309 214 8 89 30. 62 5 4 . I 00. 90 6 3 29 104 945 83 , 7 , , $ ,

$67 . 78

50 355 . 59 , ,

A peculiar feature of the reduction plant at Elkhorn is shown in I he ta n ar m un e er m u lt u XL V . T s e a s es a s PI. ili g i po d d by i of d b i p of n T h s re in c the sa t sacks filled with ta ilings sa d . e bag a those whi h l is t r r tr n and w en s the m is rece ed . e r c st e e e u ed by ill iv Th i o h fo ifli g , h w s am s are filled with sand they form excellent retai ning alls . The e d made neces sary by the fact that the water is used by the ranchm en f rr n downs trea m or i igatio . - . 8 . G EO LO GIC A L S U RVEY M NT Y EC O N D A N N A RE R T PAR T II XU V U S U L PO PL.

F H R N IN I R F K H R W T H T W T A IL IN G S D A M S O E L K O M E N G O G E O E L O N C R E E K B E LO E O N .

rr ll e re r l av a lte r d rm e n and s ro h - r rr - S h o e s n arrow rg of th e v a e y wh c e e k e e s e I sto e s cut t h ugh t e q uart z d o te po rphyr) T arlrngs da m s arr O ppos rte s lrde of po rp h yry d e b rrs Marn w ago n road rs se e n In bott o m of gu lch H rgh e r ood e d are a on m o u n tarn cove rs C a rbo rrrfe rous Irm e stone Sm oot rounded sum m it of C ro Pe a se e n In w h . w k d istance

H A P T E C R I I .

G E N E R A L G E O L O G I C A L F E A T U R E S O F T H E D I S T R IC T A N D

IT S R E L A T IO N T O T H E S U R R O U N D IN G R E G IO N .

T he Elkhorn district comprises a sm all part of the southwest end a r u m n ns er m m n r of g o p of ou tai of v y co plex co positio . F o the be nefit of the reader who m ay des ire to know the relations of the str ct to the Surr und n re i n a s rt r sum the c n t ns di i o i g g o ho é é of o di io , so ’ far as t e are n wn r m the writer s stu es wi be en in th s h y k o f o di , ll giv i c a ter t et er w t a r e acc unt the ead n eatures the h p , og h i h b i f o of l i g f of l Th str t on h r er dis trict itse f . e di ic lies t e bo d of the great Bo ulder ran te area and nc u es a art t at area and a so the r er of g i , i l d p of h l bo d s n r e m es t nes h e w c e th a a ea s a s etc. c m os e n of fold d li o , l , , hi h o p lo g rn ridges lying eas t of Elkho and exte ndi ng to the M issouri Ri ver. This area of folded rocks i ncludes rocks of nearly all geological periods and ar e lit l c c m s t n all e e in n arc es and of v i d ho ogi o po i io , fl x d lo g h nter en n tr u s rm n the S -s a e s suc as are un in i v i g o gh , fo i g h p d fold , h fo d n r h s n r w w r the easter anges of t e Sta te . Th ese fold ca be t aced es t a d and the n ua rm at ns rea l we to the l rn str ct i divid l fo io dily fo lo d E kho di i , where the norm al lithologic character of the strata is lost as a result T h n he stra o er of contact m etam orphism . e foldi g of t ta is clearly ld than the e trus n the ne us r c s w c rea u t r u x io of ig o o k hi h b k p h o gh them . T he Elkhorn m ining dis trict includes a wide variety of rock form a t ns and smal as it is w t n its or ers is un e ence s w n io , l , i hi b d fo d vid ho i g the nature of a long series of profound disturbances which have m ate riall m e the e t s art the te n on the y odifi d g ology of hi p of Sta . Lyi g western or er a reat area e se m entar r c s the b d of g of fold d di y o k , form ations seen within its lim its include a wide variety of lithologic t es now r un a tere ater m etam r s m but in art at yp , p ofo dly l d by l o phi , p least traceable uni nte rruptedly i nto the no rm al unalte red formations ’ r T h sed m entar re r th e r r a mile o two away . e i y co d of e arth s ea lie h s to r is we er etter stu e in the re n mme ate east i y , ho v , b di d gio i di ly , s nce the h rn re n is ecu ar a ra e for a stu not i Elk o gio p li ly f vo bl dy , of se m entat n or the e r r c n w c s uccee e but di io p iod of o k foldi g hi h d d , of the m any and varied e vents which occurred in a period of dynamic act t n t ate can c ut rea s w c r m e i ence re ivi y , i i i d by vol i o b k hi h , f o v d p sente e sew ere in the tate are n wn to a e c m m ence in ear d l h S , k o h v o d ly ert ar t me and a ter the u l t the re n a o e the sea and the T i y i , f p if of gio b v a n form tion of mou tai n ranges by foldi ng and synchro nous erosion. 419 N G ST R CT M O N T A N A . 420 E L KH O R N M IN I DI I ,

T he evidence of the m agnitude and power of the forces at work during what may fittingly be called the dynamic period of the his torv of th re n is e er w ere a arent and the ar e en m e na wh c e gio v y h pp , v i d ph o i h res ulted are exhibited here better than at any other place k nown T he eta en e t ere in the m a n the to the writer. d iled evid c ga h d ppi g of areal geology confirm s by abundant proof the deductions m ade during the reco nnaissa nce of the district as to the relative age of th e rocks and the m etam orphic effects produced by the i nj ection of great m as s es of molten rock . The sedim entary rocks are de v oid of all but the m os t fragm entary ss rema ns and are so enera m etam r se t at t e r r na fo il i , g lly o pho d h h i o igi l

ar ter is ar e m as e . e ert eless t e r se m enta r c arac ch ac l g ly k d N v h , h i di y h ter is e r ect a arent t e r e n st nct and the att tu e p f ly pp , h i b ddi g di i , i d of

th e e s eas nrade out. M re er w e e act correlat n eds b d ily o ov , hil x io of b is not alwa s oss e t ere is a enera c rres n ence etwee n the y p ibl , h g l o po d b se uence ser e ere and t at at ca t es near n ta near q ob v d h h lo li i by , o bly e ena and near W te a and the ar n er us rm at ns ave H l hi h ll , C bo if o fo io h been identified not only by foss ils but by traci ng the beds into the region of unalte red foss il-bearing s trata a few m iles southeas t of the

town . T he sedim entary rocks embrace a part of the Algonkian series known ‘ as the e t terrane .and the am r an and t er Pa e ic rm at ns B l , C b i o h l ozo fo io h t re u n u ted r r en r T h recognized in t e Bel R ange a do b ly ep es ted he e . e uras s c has not een s t e y ent fie and the reat se r es sev J i b po i iv l id i d g i , era t usan eet t c a tere ar i ace us s ce us r c s w c l ho d f hi k , of l d g ll o ili o o k , hi h directly overlies the C arboniferous m us t certainly be in part C reta ceous for a t u no a e nt ca e e nce was ta ne the strat , , l ho gh p l o ologi l vid ob i d , i i T he rti r - graphic co rrelation is convi nc ng . Te a y lake bed deposits found in the large r i nte rm ontane valleys of wes te rn M ontana do not ccur in the str ct escr e t u un but a few m les to the o di i d ib d , ho gh fo d i h r n r a rm e a an at the t m h so uth . T e egio p ob bly fo d highl d i e t e T er tiar a es e ste r m w c the was te s u aer al er s n and y l k xi d , f o hi h of b i o io probably also the lighter mate rial poured out from volcanic ve nts were was e wn nto the ert ar a e t at fille the r a en h d do i T i y l k h d b o d , op a r r th u er R e r be h r v lley t ave sed by e Bo ld iv low Elk orn C eek . T he ne us r c s are se era n s enera coarse ra ne but ig o o k of v l ki d , g lly g i d , n T he r of very widely differi g appearance . ocks are m os tly of com

on t es and res ent no fficult in c ass e r m yp p di y l ification . Th y a e of dif ferent age and represent s uccess ive events occurring in the Tertiary - r h r T h r o r pos t Tertia y history of t e egion. e oldes t ocks are gabbros nd r te the un s are t n h a s e t a c ra tes . T e rm er r dio i , yo g pli i g i fo b eak i rregularly up th rough s edim entary rocks and i nclude fragm e nts of em in t e r m ass or res t n u o n t e r s ur ct th h i i g p h i face . Volcanic a i vity

1 om n la ture and d esi nation as terrane se e Wa lco tt : Bu ll G eo l So m V o l X 1 899 F or n e c . c. A . g . . . pp . - 199 244 . '

GEOLO G ICAL MAP O F T ING D ISTR IC T. MONTA NA

W lt r H r‘ v W l r r o r u n-r: a e a ey eed Geo ogs J ose ph Ba rroll F reld Ass rs rant

C H A P T E R I I I .

I G N E O U S R O C K S O F T H E D IS T R I C T .

The igneous rocks cover about half of the area shown on the geological

m a Pl. XL V the rn str ct and as n cate in the stru p ( ) of Elkho di i , , i di d c ture sect ns t e un er e the se m entar roc s in s m reas w r io , h y d li di y k o e a he e

the atter are seen on the sur ace . e r ers e c aracte rs t r l f Th i div h , hei re ati ns and t e r effect u n the se m entar r c s and u n on l o , h i po di y o k po e n t er m a e them the ead n eature the e the r a o h k l i g f of g ology of dist ict . T he ore de sits are r a er e r m the i ne us ntru s ns but po p ob bly d iv d f o g o i io , r n n n n tr Th n u definite p oofs of this con ectio ca ot be aced . e ig eo s rocks are all un er t an the se m entar nes and r nar r ce ure yo g h di y o , if o di y p o d were followed their description should follow that of the sedim ents ; but nas m uc as the atter are all altere and metam r h se th , i h l d o p o d by e eat the ne us ntrus ns and ence owe t e r resent c aracters h of ig o i io , h h i p h to t em it has seem e est to i e a enera es cr t n the ne us h , d b g v g l d ip io of ig o r c s rst reser n the acc unt t e r m cr sc ca etr ra o k fi , vi g o of h i i o opi l p og phy

n e r D r. B re for a append d pape by Joseph ar ll . The ne us r c s em race a num er r c t es ar n reat ig o o k b b of o k yp , v yi g g ly in a earance and in c em ca and m nera ica c m s t n as a res u t pp h i l i log l o po i io , l of original differences in the com position of the magma and of widely varying physical conditions under which the molten m aterial has con t The r rn and r w s and solidated i n o rock . ocks of Elkho C o peak those seen at the mouth of Queen G ulch are m ai nly volcanic and represent w nd th s e e men n er t n T h t r lava flo s a e loo e j cta ta of volca ic up io s . e o he igneous rocks are of i ntrus ive origin and have been consolidated ’ beneath the earth s crust . The m ost abundant rock type is one of granitic habit and form s a art the ranite area e ten n for m an m es n rt west and p of g x di g y il o h , , n T he t er r c s ar r i t n u e r m s south of Elkhor . o h o k e eadily d s i g ish d f o thi and r m the an es tes the e e a ne but ess rea r m one f o d i by y lo , l dily f o r W t th t n th r - rite r s w c w an t e . e ce e a c s o h i h ex p io of g bb o dio o k . hi h ho w e ariat ns in the sam e r c m ass the r c s are a r un rm in id v io o k , o k f i ly ifo r u ntrus n characte througho t each i io . 422

u m ] IG N E O U S R O C KS . 423

G R A N IT IC R O C K S .

- N G R ANITE (Q U AR T Z M O Z O N IT E ) .

- n er h nam n te are nc u e t se coarse D efi nition. U d t e e gra i i l d d ho ly ranu ar r cks intru e and c ns ate at c ns era e e t s e w g l o , d d o olid d o id bl d p h b lo ’ the eart s sur a e w c a e a n rma ran t c structure and c ns st h f c , hi h h v o l g i i o i

- uart rt c ase so a im e el s ars rn en e and b t te . of q z , o ho l , d l f d p , ho bl d , io i

u te t tan te and ma netite are resent as access r c nst tuents . A gi , i i , g p o y o i The Elkhorn granite is exactly like the granite at Butte in which the nd l r n ur The atter r c has een re eate copper a si ve v ei s occ . l o k b p dly — described as a basic granite one nearly like a diorite in com position. The adoption of the name monzonite for such intermediate rocks m a es the new nam e uartz-m n n te a ca e but the er name k q o zo i ppli bl , old $ ” ran te e n m re enera un erstoo w be us e e ce t in g i , b i g o g lly d d, ill d x p h Th r c is rea rec the petrographic descriptio n of t e rock. e o k dily og nized as it is the coarsest - ra ne rock the str ct and c nta ns , g i d of di i o i w te e s ars as s uart ac or ar - reen rn en e and hi f ld p , gl y q z , bl k d k g ho bl d , r -br wn da k o mica . — E s tent and cha racter of surf ace The granite area shown on the map is m erely the eas te rn border of a very e xtensive granite area lying to the west exten n r m H e ena on the n rt to a o nt 26 m es , di g f o l o h p i il

sout Butte and r m h rn wes t to the D eer o e V a e . h of , f o Elk o L dg ll y T s reat mas s ran te to w c the nam e ou er at t h hi g of g i , hi h B ld b holi h as 1 een a l e the wr ter is s m ew at s ar e ned and w t its b pp i d by i , o h h ply d fi , i h a t c rm s is the un est m em er the ne us ser s at rn pli i fo yo g b of ig o ie Elkho , though in m any parts of the m ain area west of Elkhorn it is cut by t rhyolite dikes and covered by extrus ive masses of that rock . Wi hin the Elkhorn district the granite form s the western flanks of Elkhorn Peak and the rough country west and northwes t of the town of Elk rn Th r ere it are rea l rec n e the reat ho . e a eas cov d by di y og iz d by g h m in I X . T e t r w ers w c strew the s es as s wn P . V e bo ld hi h lop , ho L I i b , too is ecul ar n cati e in its manner r wth cons ist n ma n , p i ly i di v of g o , i g i ly the o e e ne P inus m urra cma w c rare atta ns a ame of l dg pol pi ( y ) , hi h ly i di t r ed n e exce i g a foot.

— - D es cri tion . The res r c is coarse ranu ar t ra p f h o k ly g l , of ligh g y c r and c m ose ar - reen rn en e ar - r wn t te olo , o p d of d k g ho bl d , d k b o bio i , - - co r ess uartz and wa w te and a e flesh co re e s ar. lo l q , xy hi p l lo d f ld p i h rn l n e i m n am eter. T e e s The grains average about 4 m . di ho b d r r rare and is more abundant than the biotite . A ugite occu s ve y ly r m a l ra n e w surrounded by pa amorphic hornblende . S l g i s of y llo , n r m t n T he roc is not a s ute un rm in tita ite a e so e im es see . k b ol ly ifo a earance but s h ws sm ear strea s and s ts ar er c r pp , o y k po of d k olo , r h which are particularly abundant near the m a gin of t e mas s .

i i ini 787-76 l nit c rocks of Butte, M ont. , and v c ty , by w. H . Weed : J our. 0001 V ol. VII. 1899, pp. 0. E L K H O R N M I N I N G D IST R IC T , M O N T A N A .

T he r c is res and has s u ffere tt e c em ca a terat n e en o k f h d li l h i l l io , v

W e re s nte rat n is r m n n h di i g io p o i e t . ( Vic/M ea l — A fres h s pecim e n from the western part of the area s w n o n the b r N e m n e . to in h a a a y y D . . S s t e ho p, l z d H k S urv e v labo rato ry a e the win result : , g v follo g

‘ A nal s is o ra i a u - il o m nea r lkhorn rl z n te r. a rtz on o c r m f v m z n E . t. y g ( q ) f ,

s itu nt Pe r ce nt . C on t e .

0 Wh eth e r S o r S Q , u nd e te rm ined

T he analy s is is typical o f q uartz -m onzo nite and co rrespo nds alm ost h o f h utte n te T he m in r l o iti n e xactly w ith t at t e B gra i . e a com p s o

l i i e r ne r he s e s tha of h o n ro a cula ted fo r t s v y a l y t am a t t e Elkh r ck . ' ‘ — lh e granite dis integ rates readily u nder o rdinary

athe rin r ce s s e s and as it is trav e rs ed b y inte rs e ctin o ints g p o . g j ro u nde d bo wlde rs thickly co v e r th e g rou nd whe re v e r the s lo pe is s u ffi c ie ntly s teep to adm it of the rem o v al o f the s and i nto which

dis n e T he es ul is e culi rl ru e s ne r the roc i ie rat s . r t a a y ce k g p gg d v .

m r s of he o l typical of g ranitic rocks in any pa t t w r d .

R elat e to s urro undin r cks the ran tes are the o un es t of the iv g o , g i y g

s s s s e ns dis tric t and h ave broke n ac ro s all ea rlier m as e . Th e e r latio r are fully discussed elsewhe e . m o . 0 4 ) IG N E O US 3 0 11 8 . 25

- A PL IT E G R A N IT E .

The term aplite is used to designate gra nular or finely granular i T h s il ceous granites cons isti ng chiefly of alkali feldspar and quartz . e nam e is here applied to those lighte r-colored gra nites nearly f ree from au te and t te usua s m e w at ner ra ne t an the n rm a gi bio i , lly o h fi g i d h o l r c and ccurr n as es and intrus i ns in it or in the r c s near o k , o i g dik o o k i o e r e r c i w e r te sl t fine r ts r . T h s en r s c eam w b d o k , h f h , y hi , igh ly

ra ne than the ran te s u ar te tu re and c nta ns er fe w and y , y g i d g i , of g x o i v T h r c ar s eat how sm a ra ns t te and rn en e . e e r ll g i of bio i ho bl d o k v i g ly , e er in te ture and a earance and as s wn in an t er c a te r in v , x pp , , ho o h h p

scus s n its re at ns s is ss b ca enr c e m ate r a di i g l io hip , po i ly lo lly i h d by i l

d n c absorbed an ass im ilated from the rocks with which it is in co ta t. s t It It vari es from very finely gra nular tructure to a pe gm a ite . weat ers in r un e sur aces but es not o rm the w ers c ara c h o d d f , do f bo ld h

ri ic the uart -m n n e I i t to s nte rat n and te st t s . t s s u ec of q z o zo i bj di i g io , r i e l when so alte ed s as i y q uarried and dress ed .

n an s s m a b r to es s we the o lo w n c m s t n A aly i de v D . S k ho d f l i g o po i io

A nal s is o a lite ro m E lkhun a J f oul. y f p f ,

o ns itu nt P e r e nt . C t e . c

C a( )

N azO

° at 1 10 C

° H 0 above 1 10 C T iO

co,

no,

1 00 13 T otal .

a T otal 8 . 426 E L KH O R N MIN IN G D IST R IC T , M O N T A N A .

- D IO R IT E G A B B R O G R O U P .

These rocks form a se ries in which gabbro and diorite form the end

m em ers wit roc s nterm e ate c aracter etween . T he areas b , h k of i di h b occu e eac r c a e een m a e se arate but the sm a er pi d by h o k h v b pp d p ly , ll ntrus ns near the town are nte rm e iate c arac ter w e the eo i io of i d h , hil g logical evidence indicate s that they are all parts of one great mas s r r o unde lyi ng the altered sedim enta y rock seen n the surface .

G A B BR O .

' initzom — A a r is a ranu ar cr stall ne r c c ns stin c g bb o g l y i o k, o i g ssent a im e-s a e s ar nd r x ne mm n au e i lly of l od f ld p a py o e (co o ly gite) . The h rn a r is a ar u s - ra r c ar n r m fine to Elk o g bb o d k bl i h g y o k , v yi g f o

c arse ra n . The m st as c ar et un at ac Butte s ws a o g i o b i v i y , fo d Bl k , ho e te m ass sat n a ra r te cr sta s w t irre u ar r u n e f l d of i y l b do i y l , i h g l ly o d d ra n n ns er r m n g i s of augite a d co id able m agnetite . G abb os co mo ly s how decided variation in m ineralogical com position in parts of one

m as s and the ca m ficat ns m t be escr e as s ec a t es . , lo l odi io igh d ib d p i l yp T he bas ic type jus t noted varies to one showing hornblende and biotite re lac n au and n s m rtz T h ur the r c p i g gite showi g o e qua . e text e of o k is art cu ar we s wn on weat ere sur aces on w c the ar p i l ly ll ho h d f , hi h d k u t n in r h - n ars st a gi e sta ds elief above t e dull gray feldspar. I the co e grained form s the augite grains are so large and abundant as to give h r r n r r r n th finer- ra ne t e ock a st o gly po phy itic appea ance . I e g i d form s the augite grains are m inute and the rock has a pepper-and-salt appearance and texture . Chem ical com osition — A n ana s s a t ca rm the r c p ly i of ypi l fo of o k ,

r m ac utte has een ma e in th e ur e a rat r D r. f o Bl k B , b d S v y l bo o y by

H . . to es w t the w n result : N S k , i h follo i g

A nal sis o abbro rom Black B utte E lkhorn M ont. y f g f , ,

Constituent. Per cent . C onstituent.

47 44 N one

M go N one

0110

N a,0

K, O

° H ,O at 1 10 C

° H ,O above 1 10 C

T io,

a Undeterm ined wh eth er sulph lde or sulph ate.

E L K H O R N M IN IN G S T R IC O N T A D I T , M N A .

e to ether a m atr w te e ar t u th s is ess a h ld g by ix of hi f ldsp , ho gh i l ppar nt in the m r m m fin - ra n e e c n er e rm he r . o o o , g i d fo s of t ock — Intent and a rea co vered The m ai n diorite area occurs eas t of the t wn rm n the r e and the s a w ra s a , s y m t eate r n rt o fo i g idg h llo , g phi h o h u e l h T he r r r of Q e n G u c . ock weathe s eadilv and decomposes to a s t and o r us m ass so t at t u e s ures are abun ant res of p o , h , ho gh xpo d , f h una tere m ater a is r t n l d i l ha d to ob ai . T he dike rocks als o weather rea i and are c m m n c ncea e the m est ne r s and d ly o o ly o l d by li o déb i soil . The area co vered bv it is about feet across and s om ewhat longer in the rect n the str e the s trat e r c s the s a h di io of ik of ifi d o k , h pe of t e i ntrus ion evidently bei ng i nfluenced b v th e attitude of the sedim e ntary beds . T he expos ures in the railroad cut for one-fou rth mile south of the t n s w an a terat n t s r t Nuc ea m sta io ho l io of hi dio i e . l l asses are s een sur r un e s t and m uc crac e w te a e- el w or e , y y wis o d d by of h kl d hi p l lo , llo h

s ree a terat n r uct . The sam e en m ena we re bse r e in the g n, l io p od ph o o v d he s u e m ne w r n s eet e w t r ace . T h v s w t at the i o ki g , f b lo f ho h cause a terat n is a ee -seate one and t at it is not a m ere s ur of l io d p d , h face n m n In the ra lr a cut ust m enti ne th lter r phe o e o n. i o d j o d e a ed dio ite is ac n s ut war but m est nes a ear cu t r te es s ix , y . l ki g o h d li o pp b dio i dik , es ar n r m 5 to 60 eet in w t e n n te ntrus e in y , dik , v i g f o f id h b i g o d i iv r i r the uptu ned strata n the seco nd qua ter m ile .

U A R T Z -D IO R IT E -P O R P H Y R Y Q .

— uart - r te- r r — us n the n m r D efi n ition . Q z dio i po phy y i g a e po phy ry in the b road se nse adopted by the G eological Survey to des ignate a porphyritic texture— is a fine-grained rock havi ng the composition of a uart - rite t at is c m s e ess entiallv a c as e e s ar q z dio ( h , o po d of pl gio l f ld p and ree uart w t access r rt c ase and err ma nes an m in f q z , i h o y o ho l f o g i erals w c c nta ns en cr s ts or ar e st nct cr sta s us ua ) , hi h o i ph o y l g di i y l , lly of plagioclas e and l ess comm only of augite o r oth e r i ron m agnes ian

s cates in a nel cry s ta ne m atr . ili . fi y lli ix — The l rn uart - r E ctent a nd d iam eter of s ulfi t ce. E kho q z dio ite porphy ry covers a co ns idera ble area in the southweste rn portion of

he s tr ct w ere it rms the r e etween urn e and l rn t di i , h fo idg b T l y E kho creeks and the sharp ridge to the eas t where the rail road crosses from I r u - r in een to l rn u c es . t m s ta s c ve e s es art Qu E kho g l h fo l o d lop , p

w e but character st c a earance as s wn in PI. V so ood d , of i i pp , ho XLI , that the area cove red by it is in a general way easily discrim i nated T he m as s is cut t r u t f rom that of the other rocks . h o gh by bo h r le v and rn creeks e s n a s ect n 800 eet ee acr ss T u n Elkho , xpo i g io f d p o T he ent s n s um m t west the m ut A l reston the m as s . g ly lopi g i of o h of p i ere s and e eta t n so t at e sures are G ulch s cov d by oil v g io , h good xpo mm n not co o . U. 8 . G E G IC S VEY - X VII OLO AL UR TWENT Y S ECO N D ANNUAL REPO RT PART II PL. L

P O IN T O F R K N O R U T N D I ES W H A . O C S , S H O W I G G G E C IN A ES T B E LO T O W N O F E L K O R N

Y K N IF F K H R N P A B IC LA E A D M A R B L E C L S , EL O E K.

T he w h ite are a as an inc lude d m ass of m arble m ande site ; th e c liffs on the ugh t are h igh ly alte re d Me so zo ic rock s ws 429 s IG N E O U S R O C KS .

A N D E S IT IC R O C K S .

D efin ition — Andes ites are volcanic rocks of porphy ritic or felsitic te ture w se cr sta z e m nera s are a i c as e w t e t e r t te x , ho y lli d i l pl g o l i h i h bio i ,

rn en e or au te or an two or all th ree the m nera s . T he ho bl d , gi , y , of i l andesitic rocks em brace a wide variety of types of both intrusive and r s r ffer reat in a earance but are all cl se e t u ive n . e x o igi Th y di g ly pp , o ly related in com pos itio n and ge nesis and fo rm part of the great form a tion coveri ng large areas in the Crow Creek M o untains north and eas t of Elkhorn ; they also form the hills to the southwes t of the town and h n and r e n wn as u M unta n w c is a c nt nu t e lo g high idg k o B ll o i , hi h o i l e ti e t e re resent the r ucts a ation of th ese bils . Co ll c v ly h y p p od of l w h s n le e r r us can c act t . ere as e se ere in t e i g p iod of vigo o vol i ivi y H , h tate t ese roc s c nst tute reat m unta n c s w c a e een S , h k o i g o i blo k , hi h h v b e u n the ear er roc s in reat m as ses and w c were r pil d po li k g , hi h p ob a arts c nes now ss ecte so t at the e s t n m unta ns are bly p of o di d , h xi i g o i m e rely res idual masses of the old volcanoes . — r s t h E cten t and character of s urf ace. These ocks con ti ute t e highest r tr e rm r t m hi and m ost ugged peaks of the dis ict . Th y fo g ea a p thea ' ters w ere stee clifls and r ect n uttresses are in str n c ntras t , h p p oj i g b o g o In e to extensive talus slopes form ed by the dé bris of the rocks . g n

era] the r c s rm s ar accentuate t ra c eatures . In the , o k fo h ply d opog phi f go rge below Elkhorn they are cut by the creek in the narrow canyon

w t a ru t wa s see n in P I. V A . n rn and r w i h b p ll , XL II , Alo g Elkho C o r e rm t e mm ts he w e r n c eeks th rocks fo he high s t s u i of t hol a ge .

scri i — The r c s s w a w e ran e in m nera c m s t n D e p t on . o k ho id g i l o po i io and te ture ar n r m uart - r te to a as e and r m an es te x , v yi g f o q z dio i di b f o d i r r es to tufls but are m st t ca an es t c r c s ar us po phy i , o ly ypi l d i i o k of v io n w r m r m d h ki ds . O ing to the eheating and m eta o phis ue to t e prox imit the reat ran t c m ass to the west the ri na c aracter y of g g i i , o gi l h of the r c s has een scure and t e r resent te ture and mineralo o k b ob d , h i p x g m ti n r ar e th r su t suc m m r T he ical co pos i o a e l g ly e e l of h eta o phism . l r s are en e r r t u nd as t m st t ca c s a c e a e c. e o ypi o k d , d k olo d , o gh , l i Th y vary from light gray to dark in colo r and from rocks devoid of en cr sts to st nct r r t c rms the atter e n the m st ph o y di i ly po phy i i fo , l b i g o D s e th m e m r m w com m on . e pit e ta o phis to hich they have been sub ected t e st l e t e ence t e r r na c racte r a a j , h y il xhibi vid of h i o igi l ha s l va '

ws recc as tufls and ntrus e s eets . e are ense fin flo , b i , , i iv h Th y d , e ra ne r c s w t m ierocr stalline te ture and c m m n s w g i d o k , i h y x , o o ly ho en cr sts w te abra r te - fe s ar and m re rare au ph o y of hi l do i ld p , o ly of gite Th n n h r and hornblende . e determ i atio of t e p edomi nant type as andesitic in habit is confirm ed by the careful petrographic study m ade r ] i D r. ar e and a u escr t n t em s en in the a en x by B l , f ll d ip io of h giv pp di . mi i — T h m ca m s Che cal comp os tion . e che i l co po ition of these rocks 43 0 E L K H O R N MIN I N G D IST R IC T , MO N T A N A . probably presents the sam e variations s hown in thei r mineralogical h c aracter. But two ana ses a e een ma e : The rst t at t e h ly h v b d fi , h of n h t n rma an es te t e sec n at the a te re a es te . o l d i ; o d , h of l d d i

n t A alyses of andes iic rocks .

C onstituent .

° at 1 10 c

° abov e 1 10 C

99. 68 100. 80

ntains oth rite and rrh otite am ount und eterm ined but etween F 0 8 and 0 F es a C o b py py : . b 1 . , ,

d F to 0 F e s . an eS , g s in h n i l i is v l n b F eO can not be directly d eterm ed . T e per ce t F e,0 , , nc ud ng F e,0 ,, eq ui a e t of

eO lies etwee n and and F e 0 etween and 0. F eO and m lphides . F b , , b

cUnd etermined wh eth er sulph ide or sulpha te .

— Weathering The andes ite was traversed by a close system of fine n r n n joints that break on weatheri g into sha p and a gular débris . O the highest and m ost rugged m ountain slopes the individual blocks are cr ss and the s e ex osures rm ru e s tairwa s w e several feet a o lop p fo d y , hil the solid rock form s rocky buttresses that extend beyond the general r m ta u In the e s ure at cliff wall or j ut out f om s ooth l s slopes . xpo s r e t ns the s r c rms br en cl ff m as ses as s wn in lowe el va io olid o k fo ok i , ho I the netw r c se s ace nts caus n the roc 1. A P XLV I , , o k of lo ly p d joi i g k n to break on weathering i nto rath er sm all fragm ents . I genera l the rock breaks into fine débris compos ed of pieces but a few inches across r t ta us s es w c w ere ste e are com m n a and form ing g ea l lop hi h , h p, o ly b re i n but w ere atter are c ere rass and a scatterin of v egetat o , h fl ov d by g g m er growth of ti b . we an ] IG N E O U S nocxs . 43 1

The roc is rea a tere r nar weat er n the au te- t te k dily l d by o di y h i g , gi bio i and other dark minerals changing to chloritic aggregates and the feld rs a a ra n i o spa clo uding from incipient kaoliniz tion. This lte tio s s wide spread that it is impossible to procure specim ens of the perfectly fresh h t er n th na a te rat n the r resu in r c . O n t e a e c ts o k o h h d , fi l l io of o k l h th r m n The r is er merely a thin skin upon t e s urface of e f ag e ts . ock v y res istant and with the poss ible exception of quartzite offers m ore resist ance to ordinary disintegrating agencies and to the wear and tear of r n er r n h stream t ansportation than a y oth ock k own in t e State .

- D IO R IT E PO R P H Y R Y .

In the extrem e southwest corner of the district there is a m ass of fine-grained gray rock which is distinguis hed from the andesites with w c it is ass c ate its m re cr sta ne te ture s tte a ear hi h o i d by o y lli x , po d pp

ance and un rm t c aracte r. It is we e se in the ra r a , ifo i y of h ll xpo d il o d ut at the m ut urn e G u c and a s in the wer ne in ue c o h of T l y l h , l o lo li Q en T h r n in h ra r at urnl u c s G ulch . e expos u e see t e il oad loop T ey G l h hows b tr in an s r c T he r i a r te- r it to e in us ive de itic o ks . ock s dio i po phyry c ns cu us a earance w n to its m ar e s tte or m tt e of o pi o pp , o i g k dly po d o l d c aracter w c resu ts r m the resence ar c usters rn h , hi h l f o p of d k l of ho en e and t te w c w t cr stals a ra r te lie in the fine bl d bio i , hi h , i h y of l b do i , h r n rm r grained gray groundm ass . T e ock is u ifo th o ugho ut the entire m ass the u er rt n s win a s t ar er or lu s t nt and , pp po io ho g ligh ly d k b i h i t n r n tte r o t at th r c rm s s T h res s weat e e s e e e . i i g h i g b , h o k fo bold l dg e upper 1 00 feet show distinct lam ination planes parallel to the andesitic

contact . C H A P T E R I V .

D E S C R I P T I O N O F T H E S E D IM E N T A R Y R O C K S .

The strat e r c s c er a re at e sm a art the str ct and ifi d o k ov l iv ly ll p of di i , thei r normal characters have been in large part obliterated by the m etam orphism they have undergone as a result of the h eat and m i neral n t n h n w r ro u t m izi g ac io of t e ig eous m asses hich have b oken th gh he . A s a rea n te th e a tere strata are wev er c nt nu us trace y , , y l d o d l d ho , o i o l able i nto the unalte red form ations which m ake up the great folds form ing the m ou ntain ridges east of Elkhorn and extend to the valley of th M s r e W n h m h r t i i ff u t e s u R r. t t e ts t e st c t s c i o i iv i hi li i of di i di i l , if not m ss e to ent all the r ns rec n a e in the una tere i po ibl , id ify ho izo og iz bl l d area and its m rtance id not ust the t m e necessar to t race , i po d j ify i y n h r r i t r out s uc c nect n . In ene ra t e ade s t nc ns ec n e h o io g l , b o d i io og iz d in the una tere se r es can be s cr m nate w e the C ar n er us l d i di i i d , hil bo if o lim es tone is identified by foss ils in the imm ediate vicinity of the granite c ntact and the er n uart tes a e ee n we s ut to o . ov lyi g q zi h v b follo d o h unaltered strata . The r c s are all tite and the areas s wn on th e m a nc u e o k l d , ho p i l d l cks r e n off r m the m a n t u in enera the m ass es b o b ok f o i body , ho gh g l are all part of the eas tern flank of a broad fold com posed of all the s edim entary strata from those of proba ble Algonkian age to the C re T h e s w t n the s tr ct are t ere re not r z nta b taceous . e ut b d i hi di i h fo ho i o l , ° have a general eas tward dip at angles of 30 to The gene ral lack of foss ils and the m etam o rphis m of the rocks make the dete rm ination of their age alm os t imposs ible save by correlations and by tracing n r them out to the u alte ed areas . The sedim entary s eries at Elkhorn m ay be see n in an unaltered con h s a a m e or m re s ut eas th T h dition in t e hill h lf il o o h t of e town . e ' sequence there seen does not difler m aterially from that which prevails in the neighbo ring mountain ranges . A few notes upon the co rrelation T h of th e form ations are therefore given . e Algonkian beds are part the e t terrane r m whic ra m entar ss rem a ns a e een of B l , f o h f g y fo il i h v b 1 r T he am r an r c s the ne r n ran es nta n repo ted . C b i o k of ighbo i g g co i a un ant ss s w c r e t eir M e am r an a e ne t er the b d fo il hi h p ov h iddl C b i g , i h r m r n a na n re r n T h Lo we r nor U ppe Ca b ia f u s be i g p es e ted . e Silu rian

1 - W t ull l m l - P ro C am rian foss ilife rous terranes b C . D . alco t : B . G eo . S oc. A V o . X 1899 199 244 b , y , , pp . . 432

E L K O R M IN IN IS T R I T N H N G D CT , MO N A A .

nera liz tion o th e im a r k o th E o m di rict Ge ed sec e s d ent roc s e kh st M ntana . f y f l , o

N am e .

retace ous R id h n inol C e beds . ornsto es ad es g , , and quartzites ; to p capped by andes ites f o C row Peak .

N eoca rb Q uad rant formation sandstones and quartzite s with inte rbedded lim estones al Carboni erous r d f te e to a lim e silicate hornfels . M ad ison lim es tone : in places alte red to

m arble .

Devoniarr? U ni n : and r n . o shal e black s hales ho ns to e K ee ne lim estone : in part argillaceous E lkhorn hornstone C e m etery lim estone includes th e white nular foot-wall lim estone of th e Elk orn lode and th e underlying bl ue lim e

C am brian stones . H obo G ulch shal y lim esto ne includes a ri i m ch racte st c li estone flags . Starm ount lim estone : m ostly arg illaceo us

r b e uiva n A lpreston quartzite . p oba l eq le t f o F lathead quartzite . A lgonkian? ( b) Turnley quartzitic hornstone ( a ) Turnley shale : red sandy ; indurated

n by d Basal beds co cea led an esitic lav as .

A L G O N K IA N R O C K S .

— T h urn r Tur nley horns tones (Spoka n e s halef ) . e T ley beds a e the Th u r n on th s oldest s trata of the region . e v occ o ly e s lope above the

urn e ace r in the s ut wes tern art the s tr ct w ere t e T l y pl o h p of di i , h h y b form . a block separated from the m ai n mass of s t ra tified rocks v a

t n ue ran te . In c r c m s t n and re at n to the e r o g of g i olo , o po i io , l io ov lying quartzite the rocks correspond to the red Spokane s hale of the e t te rrane seen at W te al 20 m es s ut at T ownse nd to the eas t B l hi h l , il o h , , \ T h w r s t ta c ns is t r and at e ena on the n rt . e e e r s tra c s H l o h lo . of o of o k

i s sl T h r t at weat er to a s fornr n en ras es . e e h h good oil . g op , g y op ov lying q uartzitic hornsto nes are m ore res is tant and thei r debris is cons picuous on the su rface . T he wer s n is 200 eet t c and c ns sts s a e meta lo divi io f hi k . o i of h l m orphosed to a very dense horns tone com posed of light - b ro w n biotite ar b d m ure r n ore 20 to 30 eet t c ccurs in and qu tz . A e of i p i o f hi k o rm the m iddle lower part of the fo ation. The q uartzitic hornstones ove rlie the bas al beds j ust note d and are we l be e are ery e nse and h k The r c s t u . e t ic . 200 f et o k , ho gh l dd d v d

n ar a ra - ac c r so t at thev clos e l res em b e the hard a d e of g y bl k olo , h v l was p . ) C A M BR I A N R O C KS . 43 5

n s . T he r i fine s cc ar a the ractur n a es te te tu e s a e c c da . d i x ly h oid l , f o hoi l T he rocks are well exposed in a road cutti ng near the s outhern end of r M cr e w the r c to n thei outcrop . i oscopica l xam ination sho s o k co sist of u rt an a m m - q a z d s all a ount of olive gree n m ica .

C A M B R IA N R O C K S .

i i — T a A lp res ton qua rtz tes (F lat/Lead qua rtz tes ) . he qu rtzites are 1 25 eet thick near w te in c r and w n to t e r s u eri r ar ness f , ly hi olo , o i g h i p o h d

are we e se rm n the crest the l wes t the t wn. ll xpo d , fo i g of hil of o T he rock is an altered sandstone and is correlated with the Flathead 1 san st ne the ase the am r an ser es the State . d o , b of C b i i of

t o n e s e r s ra e r h S arm u t Zim es tmws . Th es lim e ton s a e epa t d f om t e be s us t escr e an ntrus n ran te - r r but corre d j d ib d by i io of g i po phy y , spe nd in character and position to the M eaghe r limestone of the Little ” e t s ect n t u s t e ent ficat n is m s s e and it seem s B l io , ho gh po i iv id i io i po ibl best to give the form ation a local nam e derived from that of a mine in i T h t 0 - r t t . e wes be s c ns st a ut 25 eet t a n lo d o i of bo f of ligh g y , hi ly e e ar l ace us l m est ne w c is rat er s t and weat ers wn b dd d gi l o i o , hi h h of h do in even s lopes be neath the first pe rs istent lim estone d iff of the sedi s In h rn th r ks re m re o m entary e ries . t e Elkho expos u res e oc a o r les s m eta morphos ed and com mo nly altered to a light greenis h -gray horn e s reat r nes T h e r m m r the ser es c m r s n a s . e u e e f l of g h d pp b of i , o p i i g

350 eet r c s ffers r m the wer c e in e n m re m ass f of o k . di f o lo hi fly b i g o iv el e e and a ar er c r the rock e n m tt e w t y b dd d of d k olo , b i g o l d i h spots of a darker color and containing les s s ilica and clay than the w r The r s r s w t an the n in e e s . c es t eat er n et er t u er lo b d o k i h i g b h d ly g, m re s hal strata and rm uffs w c are ten c ns cu us to o o y . fo bl hi h of o pi o p

r i r es v m ent in the rn s tr c is g aph c features . Thei b t de elop Elkho di i t on the s e s ut of t he am s in w c the m tailin s are cau ht hill id , o h d hi h ill g g

A t t a t e r e e se in the stee . A . s ce a e w (Pl XLVII , ) hi pl h y ll xpo d p

r h r s t u r the rm t n slopes above the rail oad li ne . T e la ge expos e of fo a io show s the strata to be separated from the underlying Turnley se ries

a uart - rite - r r nt rus n se eral h un re eet in rea t by q z dio po phy y i io v d d f b d h , so that the exact relation of the two form ations is nowhere absol utely

h rre t n s m be c rrect th rm nt eterm na e . t e c a a su e e ta u d i bl If o l io d o , S o lim es to nes m ust have bee n underlai n by a considerable thickness of s ale c rres n n to the W se s a e but the ntrus e m as s of h , o po di g ol y h l , i iv po rphy ry now found below the Starmount lim esto ne was probably n h r i ntruded also in the s hale and no w conceals it . I t e southwes te n part of the dis trict a block of Sta rm ount lim es to ne has been over

I s W W w M un ain b H . ed ntie th nn h i l elt o t . e : T e A . R . S . l S ee G eolo of t e L tt e B e t U . G eo . gy , y p - f r d isc mion f th e C am ria n fo rm atio ns and th ir relati n S urv e Part III 1900 . 284 287 o u o e o s to th e y . , . pp , b roc ks o o h f t er ages . 2 W d l ee . oc . 285 . . p E L KH O R N M IN IN G D IS T R IC T , M O N T A N A . turned by an intrus ion of andes ite and now lies betwee n the latter

- r r n r s n s ra m ent is «100 eet in rock and a diorite po phy y i t u io . Thi f g f t c ness and has ee n a tered t r u ut to an a re ate arnet hi k , b l h o gho gg g of g , - au te W ast n te and e s ars . r m a s tance the r c s at t s gi , oll o i , f ld p F o di o k hi ace s t a e the a earance and the s tructure t- ra m ass pl ill h v pp of ligh g y , i el e e m est nes t u on near V ew it is seen that the bed v y b dd d li o , ho gh i n anes a e een ent re te rate and t at the r na di g pl h v b i ly obli d , h o igi l stratified nature is recognizable only by slight changes in color or by fferent a weat er n w c e e s a th n am nate s tructure di i l h i g, hi h d v lop i ly l i d r m ar a tere r c s the m e a e are een on exposed su faces . Si il l d o k of sa g s in A l resto n G u c n rt wes t the t wn and at ac utte w ere p l h , o h of o , Bl k B , h the rocks have been i ntensely alte red and fragm ents occur in the r The es cr t n t ese nc u e ra m ents and rem ar s gabb os . d ip io of h i l d d f g k on the i r s ignificance will com e m ore p roperly in the account of contact en m ena ven in the as t c a ter t s a er to w c re erence ph o gi l h p of hi p p , hi h f m s b r n r r r u t e made fo details of the ir m i e alogic cha acte s . — H obo G ale/t h o w-s imia This nam e is applied to a well -m arked group of s trata seen in the ra il road cuts near the tailing dam s and in Queen It is n m rtant r n for m a n it ss s s eas G u c . a as es e l h i po ho izo ppi g, po ily recognizable characters and s eparate s two thick and s om ewhat differ n ent lim es tone form atio s . The lowest m em be r of the se ries cons is ts o f 1 8 fee t of shales which no n r r r A m es to ne near 1 have co s picuously m a ked cha acte is tic. li ly 00 e i t n T h r is near ac t et n c ess succee s the s a e . e c w f hi k d h l o k ly bl k, i h i a un ant s m t oint anes . O n weat er n it rm s an u ar b d oo h pl h i g fo g l , r r h r n m re nt b ra sm t a s w c t e a ue e t ce . oo h , d k déb i by hi h ho izo y f q ly d $ ” The u erm st art the rm at n is the cr n e m es t ne a pp o p of fo io i kl d li o , 35- t bed an e c ert m esto ne t n be e and w t the foo of b d d h y li , hi ly dd d , i h cherty m ate rial in the form of lens es averaging two -thirds of an inch

n n r s i n s is th m t r in thick ess a d 1 o 2 inche n le gth . Thi e os cha ac teristic bed of the Cam brian se ries wherever it has been obs erved in M ntana and c m m n c ns sts ure m esto ne w t enses o , o o ly o i of p li i h l of r h r i t d b c e t m ate r a . T e c n e a earance s no ue as m t e h y i l i kl d pp , igh su se r m the rn e s ures a ne to a en n t ese ppo d f o Elkho xpo lo , b di g of h r c s in w c the m es t ne w u be cur e a u t the ar er s l o k , hi h li o o ld v d bo h d i i ceous e nses as t s c aracter st c has ee n n te w e re the e s are l , hi h i i b o d h b d

er ect ri nta and s w no n w ate e r. In weat er n p f ly ho zo l ho foldi g h v h i g, the lim es tone is m ore read ily attacked and as a consequence the m ure c ert o r ar ace us e nses wealher in re e and the w i p , h y , gill o l li f bo l r r sen a r u s ur ace w t en e-s a e r n h de s p e t o gh f i h loz g h p d dep ess io s . T ese lim estones are so m uch harder than the s trata adj acent to them that the horizon is readily dis tinguishable des pite its sm all thickness and

he at it es no re s ent an m r e r l T t fact th do t p y a k d topographic e ief . he unde rlying Starm ount lim esto nes often show the sam e crinkled appear wn D - O 3 l C A M BR IA N R C KS . 4 7 ance so t at in ent n the ri n it is necessar to for the , h id ifyi g ho zo y look r n unde rlying s hale . This fo rmation seems to cor es po d fairly well in position to that of the Pilgrim lim estone of the eastern M ontana C am r an e os ures in w c case the asa s a e w u be the ar s a e b i xp , hi h b l h l o ld P k h l is w r n n n m r n . s c rre ati n e e ot as e u a t n re ho izo Thi o l o , ho v , b d po y hi g o h t an lithological sequence . — Cem eter limes tone s m esto ne nam e r m its ccurre nce at y Thi li , d f o o the rn cem eter c ns sts 650 eet e s all wh c are Elkho y, o i of f of b d , of i h s s The e n st ue m e n true m e to ne . wer 70 et c s st es li lo f o i of bl li o , fol we bv 50 eet ar - ue ranu ar l m est ne weat er n w t a lo d f of d k bl g l i o , h i g i h lighter-gray s urface and showing sections of brachiopods and crinoid

- stem s . e t s occurs a reat t c ness l t ue t n to th c Abov hi g hi k of igh bl , hi i k e e m es t ne s m et m es m tt e w t ar - ue s o ts in a ter b dd d li o , o i o l d i h d k bl p ligh

r n In e n u c th wer be s are s m ew at a te re g ou d . Qu e G l h e lo d o h l d by

c m r w e cr s n r T h conta t etam o phis m and s ho a decid dly y talli e textu e . e sam e als o is true of the rocks exposed in the hillside west of the Elk rn m w ere the r c s are er ar and cr s ta ne and rea ho ill , h o k v y d k y lli b k into s mall blocks whose dark color prese nts s om e what the appearance

s r It s em r ss of a ba ic e uptive whe n seen from a dis tance . e s ha dly po i ble t at t s ar c r can be due to c ntact m etam r s m inasm uc h hi d k olo o o phi , h as the tende ncy of h eat would be to d rive off the organic material to s s i r i ard a w c t c r i due . s e n t ue t is to e n a hi h hi olo Thi b g , h xpl i decided diffe re nce in the appearance of the rocks near the Elkhorn m and t se in uee n G u c unless the atter is rea th e wer ill ho Q l h . l lly lo n he ul rm T he r s 0 e t lim es to e of t Hobo G ch fo ation. uppe mo t 1 0 f e of the C em ete ry lim estone includes the thin and thick -be dded white or

- t ra m es to nes s w n a cr s ta ne ranu ar te ture . s ligh g y li , ho i g y lli , g l x Thi bed v ar es to a straw- e w m t c mar e w c c nst tutes the i y llo dolo i i bl , hi h o i th It m a b een n h sur ace i foot wall of e Elkho rn m ine . y e s o t e f n

n c n i Quee G ul h a d n the rail road cut east of the town. — Iii/t hor n h orns ton e T he Elkhorn ho rnsto ne is seldom seen in good sur ace e os ures but its cat n has een ete rm ne num er us f xp , lo io b d i d by o prospect pits m ade to trace out the bedding plane between it and the un er n im es t ne the r n on w c the ore es the d lyi g l o , ho izo hi h bodi of Elk in The i e i - horn m e occur. rock s well xposed n a hanging wall crosscut the rn m ne c ns st n a ser es t n- e e strata ar of Elkho i , o i i g of i of hi b dd d , v y in in c m s t n r m near ure e trem e ense and fine - ra ne g o po i io f o ly p , x ly d , g i d uart tes to ca careous and ar a ce u s a e The r c s are n u q zi l gill o s h l s. o k i d rate and m etam r se and in the m ne are r l rn n T h d o pho d i ea ly ho sto es . e th c ness is es t mate at 40 eet t u in the m ne a crosscut 1 20 ee t i k i d f , ho gh i f long is driven thro ugh these and closely s im ilar calcareous rocks which dip cast at 3 8 E L H O R N M N N G m srm o'r N T N 4 K I I , M O A A .

I -D E S L U R O V O N IA N R O C K S .

— Ifeene limes fmm This form atio n cons is ts o f lim estone beds which are s o o r e sed w n to a c e r n m ra na and t e r debris p o ly xpo , o i g ov i g of o i l o h , t at t e r t c nes s is not accu rate n w n bu i e ti 50 y t s s m ate at 0 eet . h h i hi k l k o , d f

The basa l be s co ns s t lu s - ra m es t ne us ual a te re to a d i of b i h g y li o , ly l d

- r n rb T h u e r t ra r fine g ai ed m a le . e pp s ta a e more thickly bedded and r T h rm n i te in c l r. e at s as a w e s tl are nace u and , y s ligh o o fo io hol , ligh o , t u e s s rem a ns is e e e to inc u e the u e rm s t ho gh d void of fo il i , b li v d l d pp o or Y ogo lim es tone of the Cam brian s e ries and the overlying dark

re ranu ar effe rs n m es t ne D n n colo d g l J o li o s of evo ia age .

i/ re o ni s Un m M ale . This readily c g zable horizo n com pris e a basal bed ac s ale 30 ee t t c erlain a m etam r s e s e us of bl k h f hi k , ov by o pho d ilic o

n t i o u e t T he lat r r i r lim esto e hat s ab t 100 f et hick . te ock s alte ed to a l ht - c l re as e r m ate r a t at weat ers in s m t -s ur ace an u ig o o d j p y i l h h oo h f d , g l o s w s e utcr so m e tim es r lo r T he rm ar c m s w es . a bl k , ho o op fo idg fo t n l e t at e w it is ro en i ne us ntrus ns and no co n io , ik h b lo , b k by g o i io ,

e s res o It is s n in n m o c tinuo us xpo u ccur . ee u er us prospe t hole s a nd

om m n be rec ni e the u r e T h e m ay c o ly og z d by d al cha act r. e bes t xposures are t ree - o u rths of a m e no rt o f the t w n where the sha e has bee n h f il h o , l fruitles s ly pros pe cted fo r coal . N ear the igneo us rocks the fo rm atio n is s o alte red as to be unrecog i r e h nizable its t ca c a act r a ne . N ear t e U n n m ne at by l hologi l h lo io i ,

h ead A l res to n u c it is a te re to a t- r r t e G c e c . In h of p l h , l d ligh olo d o k

uee n G u c near the C e m e te r R d e r te st c the s ale is Q l h , y i g dio i o k , h a te re to ar ur l s - red rns to ne w e the e rl n s ce us l d d k p p i h ho , hil ov yi g ili o

m es to ne is an e cee n ar t - ra rnst ne m uc ra cture li x di gly h d , ligh g y ho o , h f d T h ri onc rres n to th e r b m nute nt n . e s e re s s a e v i joi i g ho z o po d Th Fo k h l , n a D evonia horizon .

C A R B O N IF E R O U S R O C K S .

J fi ulis on lim e-ste m —T he g reat lim es tone s eries known as the

M a s o n rm at n is c ns cu us in the rn st r ct as it is di fo io o pi o Elkho di i , r i he e n n A n e r t els ewh e e n t S tat of M o ta a. co s va ive es tim ate of the

n s in he rn r n T h r c n to tal thick e s t Elkho egio is fee t. e ocks o s ist

ere as e sew e re of m es to nes w c in the we r a the h , l h , li hi h lo h lf of

rm at n are dar c lo red and s m et m es ar ace us t u us ua fo io k o o i gill o , ho gh lly

r r o r a the da k colo is due t ca bo n ceous m aterial . The m iddl e po rtion

the s er es c ns s ts er ure m es t nes w c are e r res is of i o i of v y p li o , hi h v y t

nt to weat er n and us ua rm w te cl ffs t at are co ns cu s a h i g lly fo hi i h . pi ou

eatu res the an s ca e . In t s s tr ct o we e r the m es t nes f of l d p hi di i h v , li o

a e bee n recr stall e int m arb es and as a c ns e ue nce t r h v y iz d o l , o q of hei er c arse te ture and r a e nature weat er som ew at rea l and v y o x f i bl h h di y ,

n e no t rm e r str n e r T he n s he c do fo v y iki g xpos u es . best s ectio i found w R e n rt a t h l o n r e s t e t wn rn . T he se r es conta n C o idg , o h of o of E kho i i s WE ED - ' l S IL U R O D EV O N IA N A N D C A R BO N IF E R O U S R O C K S .

ss rem a ns e en in the m ar e e areas and t ese s w the us ua fo il i , v bl iz d , h ho l Lower Carboniferous form s common to the form atio n throughout the

tate . The w n s t n h a b r r ] S follo i g ec io as been m de v D . Ba rel

S ection of the Ma dison limestone on C row R idge

© Som e what s iliceous limes tone ® Light-colored andes itic sheet l \ C oncealed l im estones L i t- i n i - 6 . gh bl ue l m esto e w th dark blue circular spots Pa - i n 5. le blue l m esto e with wh ite seam s and sm all crinoid ste m s - 4 . Pa e b ue m ass iv e im esto ne orm in at t is oint the owest rom inent l l l , f g h p l p

3 B ue im t n i . l l es o e w th thin cherty seam s 2: B ue im es tones som ewhat banded wit lar e crinoid ste m s and w ite c ert l l , , h g h h bands prom inent on weathered surfaces

1 . B ue im estones conta inin near th e m idd e at t is oint beds o din abun l l g l h p , h l g dant needle-like cry stal s of tr em olite

T ota l

A sect n m a e b the wr ter in the s east rn ree a io d y i hill of Elkho C k , few m es e w the t wn w ere the e s are una tere s ws t at il b lo o , h b d l d , ho h th e roc s are s m ar in c aracte r and as the s ect n es u nto k i il h , io go p i the qua rt te the ua rant rm at n and nc u es the red s a es zi of Q d fo io i l d h l , w c s ee m to be ac n in the sect n as e s e on C r w R i e it hi h l ki g io xpo d o dg , is give n in detail :

‘ Sewion o the ( nrbon cro us s trum ex osed in the hills cas t o E lkhorn reek 3 m i b f i/ p f C , les elow h t w t e o n . T h ick nem

in feet .

uartzite rat er coars e rained dark co ored on weat erin and ve rv res ist Q , h g , l h g, n t a .

Lim estone li t ra in co or th e oq orm in a m ass iv e ed e , gh g y l , p f g l g S andstone im ure and s a red is in co or , p h ly, d h l uartzite ra dense outcro m arked b trees Q , g y , , p y Lim estone ra t in and irre u ar bedded c ert , g y , h ly g l ly , h y Q uartzite in heav y beds varying in col or and texture ; th e ro ck often shows sm ooth j ointing and th e weathe red surface is k notte d Q uad rant form ation : In e x posure a soil shows sandy clays o f reddish o r brown color Lim estone ra and im ure and orm in art of a c a -s a e se ries , g y p , f g p l y h l S a e red and ur e h l , p pl I neous intrusion of bas ic au itic rock m uc decom os ed g g , h p uartzite w it 111 o r nd o s i in e to bu c o a cca ona k . Q , h ff l , lly p Lim esto ne s a e radin into im estone at base t in am inaeof lim v m ate ria h l , g g l , h l l alte rnating with arenaceous m aterial Lim estone i t ra in co or wea t ers wit rou sur aces ointed and , l gh g y l ; h h gh f , j , showing m ason ry-like outcrop S hale of buff to pink colo r; lam ina: from one-third to one-eighth of an inch thick ; colo r poss ibly due to igneous intrusion beneath it Igneous int rusion o f augitic rock Lim estone th e u e r 4 eet orm in a ed e outcro , pp f f g l g p E L KH O R N M IN IN IST R IC T A N G D T , M O N A .

9. Im ure im esto ne in art red and dolom itic and carr in sand p l , p , y g

8 . R ed im es tones th e co or due to b otc es and inc uded m a s of reddis l , l l h l sse h m ateria in ra or bu f im ton l g y f l es e . - 7 . L im es tone orm in rou ed e of ink ra co or wit knott and som ew at , f g gh l g p g y l h y h open-te xtured outcrop

6 . Sandstone red and arenaceous weath er1n into red c a s ood ex osures , , g l y , g p

M adison lim estone Lim estone ra c ert brecciated , g y, h y, Lim estone ra t inl bedded l to 3 eet of a s at or b ue co or carry in , g y, h y ( l f ) , l y l l , g abundant chert fragm ents with seal-brown surface

3 . Lim es tones of t ica M adison as ect rat er t ick bedded s owin yp l p , h h ly , h g b ock racture and carr in ar e irre u ar-s a ed ra m ents of c e rt l y f y g l g , g l h p f g h ; th e uppe r portion som ewhat thinly bedde d

2 . L im estone v er m ass iv e wit out traces of beddin th e u er 30 eet of , y , h g, pp f dark -b ue or ra ver dense c ert im es to ne em ittin a etid odor w en l g y, y , h y l , g f h st ruck with a ham m er 1 im n dark ra t rin n i u i r u . L es to e wea e to a bu ra a d w t a ec a t , g y , h g ff g y, h p l ly g te red surface ; ca rries occas ionally black cherty m ass es of ovoid shape and ow Z a hr nti rin i m s s e s c o d ste s and cora s . h p , , l

— T h Q uadran t f orm ation . e Quadrant form ation corresponds clos ely in its s tratigraphic relations and lithologic com pos ition to the rocks wn m w n the te n te rat n kno by this na e elsewh ere . O i g to in se al io to w ch the r c s a e een s u ecte in C r w R e the orm at n hi o k h v b bj d o idg , f io cons ists there of quartzite and ho rnfels ; but if the sam e se ries be he ea n th r ks w b n u a te r stu e at t uee G u c e c e u n e . di d h d of Q l h , o ill fo d l d T he sequence and character of the basal beds are given in the s ections n h r e n a T h r w R e sect n w c is v en o t e ec e . e C p di g p g o idg io , hi h gi w r r arr nd w n 3 e was m eas u e D . e a s s a t c ess 80 b lo , d by B ll , ho hi k of h e n d h feet between t e M adiso n lim s to es an t e C retaceous rocks .

Section of Carboniferow sedimentary strata ( Q uadrant f or mation) $ posed on C ro wR idge t nor heast of E tkhorn . T h ick nes

in feet .

r i n h M z ic 6 . A tern atin u e and ca careo us uartz tes ust be eat t e owest es o o l g p l q , j h l quartzite Q uartzitic hornfels Light-gray quartzite Lim e-silica te hornfels L ight-gray reef of l ustrous quartzite Banded strata of i t-co ored im e-siicate orn e s ran in rom n ear y l gh l l l h f l , g g f l ure im estones to near ure uartzites th e base of th e series p l ly p q ,

T otal

M E S O Z O IC R O C K S .

— Crow R idge series U nde r this heading all the M esozoic strata n r u e as win to the a sence wel -defined t have bee g o p d , , o g b of l li ho or s s rem a ns it has een m ss e to logic units of fo il i , b i po ibl differentiate

E L K H O RN MIN IN G D IS TR ICT , M O N T A N A .

G ranular sandsto nes L i t- ra cross -bedde d ranu ar sandstone th e rains consistin of w at gh g y , , g l , g g h is probably a de vitrified volcanic glass

9. S im iar to abov e but b uis ra in co or l , l h g y l L im esto ne and hornfels B uis - im - - 8 . l h gray l estones with well d ev e loped cleavage and m any rust co lored hornfels bands Light-gray hornfels with m any rust-co lored bands Bluish -black lim estones with well-dev eloped cleavage B uis h orn e s redominant im e wit abundant w ite to rus t -co ored l h f l , p ly l , h h l siliceous segregations Q uartzite series :

4 . Siliceous ornfe s so m ew at banded to o ra ica rom inent h l , h , p g ph lly p C oarse ranu ar uartzite wit som e im urities ly g l q , h p L im e-siica orn e s m ott ed wit m inera se re ations l h f l , l h l g g t ite rat er m assiv e uartzite to o ra ica y rom inent , h q , p g ph ll p

T ota 1 680 l ,

T he p e trographic nature of th ese alte red rocks is f ully describe d in the etr ra c a en to t s re rt and nee not be u rt e r p og phi pp dix hi po , d f h e n m tione d here . R C H A P T E V .

R E L A T O r m as s n I N s o R O C K s .

U nde r thi s head it is proposed to give a brief account of the relations the r c m asses to one an t er nc u n the r a e m anner of o k o h , i l di g p ob bl of

ntrus n the n o ks and t e r r a T h i io of ig eous r c h i elative ge . e rocks them selves have bee n described in precedi ng chapte rs ; in this place a res um e. is gi v e n of all the k now n facts concerning the dy nam ic geology the re n w t s uc c nc us ns as seem to be us t fie b of gio , i h h o l io j i d y the facts . Th e s u rface dis tribution and areal ex te nt of the different rock e r s w n h l X V T e m e n o e l ca m a . h m a as s s a ho t geo ogi p (Pl L ) . p does not c urs e e the n wn v ert ca relat ns s uc m ass es , of o , giv k o i l io of h , exce pt in s o far as they are expres sed by the pos itio ns of the bo undary

n s in re at n to the nt u rs he ur T he acc m n li e l io co o of t s face . o pa y i ng

s cr s s s ec is X and r res e n e r plate of o tions (P . LVIII XLIX) ep t v tical sect ns acr ss the str ct a n nes s w n on the area m a e n io o di i lo g li ho l p, b i g at h m ers o n n T he des ign e d by t e sa e lett n li es a d sectio ns . p rofile of eac sect n is rawn r m the c nt ur nes the t ra c m a h io d f o o o li of opog phi p, and the geologic bo undaries corres pond to thos e of the geological d r at n the r m as s wn o m a . The s e s a e an e c ses n the p iz , h p , l io of o k ho sect ns t u as e u n all th e n wn acts are lar e ea and io , ho gh b d po k o f , g ly id l , it is e i ent t at t s m us t be so nas m uc as es te the m ar e v d h hi , i h d pi k d rugged nes s of the district the natural exposu res embrace ordinarily es s t an eet ert ca stance and etween rn ea l h f of v i l di , b Elkho P k and the lowest poi nt in the dis trict the vertical dis tance is but feet .

S E D IM E N T A R Y F O U N D A T IO N O F T H E D IS T R IC T .

The sedim enta ry beds all lie at s teep angles and have a prev ailingly

T h s nu t s r as s wn on the area n t war . e s e utc or h d dip i o i i of o op , ho l m re l r e du to to ra c re e s nce the s tr ke is er a a a y e , y p, g l pog phi li f i i v n T he r c s are tra un rm e n a r m ate n rt a d s ut . ce ifo , b i g pp oxi ly o h o h o k able into the bed s form ing the big fold eas t of Elkhorn C reek a few e w the t wn w ere the ant c ne is seen to be but one the miles b lo o , h i li of lesser folds of an ex tens ive area in which the rocks form long anti c nes w t nterv en n sync nes and in w c t ra c re e and li i h i i g li , hi h opog phi li f T h c geological structure in genera l coi ncide v erv closely . e blo k of nts at h rn rm s the eastern m f an ant cl na sedim e Elk o fo li b o i i l fold , 443 444 E L KH O R N M IN IN G D IS T R IC T , M O N T A N A .

the western lim b and cres t of the fold havi ng form erly existed in the area now n ne tc occupied by the granite . T he axis of this a ticli pi hed s ut war th wes h n efin te i o h d . Of e te rn limb of t e fold nothi g d i s n wn t u it m a be c n ecture to a e een stee and e to k o , ho gh y o j d h v b p lik ly develop i nto a fault yielding readily to the disrupting force of igneous n r i t us ions . While the original st ructure of the sedim ents res ulting from orogenic m ovem ents which preceded igneous i ntrusion was that of a s m e arc the n the r c s e e o e m an esse r crum n s i pl h , foldi g of o k d v l p d y l pli g

the e s and t ese creas in s and cr n les w t acc m an in s s of b d , h g i k , i h o p y g lip and sm a au ts and crus n the r c s the were ll f l , hi g of o k of big fold ,

natura l y m s m r e n h e w r h r th arc l o t a k d i t e b ds hich fo m ed t e co e of e h . Such a m inor fold is shown on the m ap feet north of the s haft house of the Elkhorn m ine by a m arked be nd in the outcrop : this ‘ tt e n r h In th em e li le fold d velops i to a fault west of A lpresto n G ulc . e C te rv lim es to ne this fold sheared the rock and dev eloped li nes of frac tu re w c were we the e - e ntrus ns ab r w e hi h follo d by dik lik i io of g b o , hil the fault into which the fold developed determ ined the southern lim it

the a r t r m n r crum n s ccur but the two ust s c . O t e of g bb o o k h i o pli g o , j

m ent ne are the r nc a nes ecaus e t e nflue nce if t e did io d p i ip l o , b h y i d , h y not ete rm ne the s t n the ne us ntrus n w t w c the d i , po i io of ig o i io i h hi h ore es are i l e bodi genet cal v connect d .

G A B B R O S T O C K O F B L A C K B U T T E .

T he gabbro stock is believed to be the oldes t igneous m ass of the

h r i u rs in str ct . A s s w n the area str ut n t e c t cc di i ho by l di ib io of o k , o etac e m as ses w t m an s m a sses and r un e ntrus ns rea d h d , i h y ll bo o d d i io b k in u t r u the sedim e ntar r c s t at s urr un the m a n es g p h o gh v o k h o d i bodi , r s n r d to w c a e w in the st ucture sect ns Pls . an hi h ho io , XLVIII XLIX

e rts n T h rr n h m a er ntrus ns b a o e reat . e ccu e ce t e s p of g body o of ll i io , as we as arm s the ar er m asses a n n rt ea s t -s ut west nes ll of l g , lo g o h o h li indicates that lines of fracture in the sedim entary rock were followed the ntrus n t u it is e ent t at the latter roc s were e ten by i io , ho gh vid h k x si l t r h in r s r m th s and t er v e y sha te ed at t e tim e of the t u ion . F o e e o h eta s ccurrence bser e in the fie it is n erre t at the a r d il of o o v d ld , i f d h g bb o underlies a large part of the region drai ned by A lpres to n Creek and t t th ar s e se m asses ar all arts on e -seate m as s ha e v iou xpo d e p of e d ep d , from which the stocks and pipes now seen rise upward like branches m n r fro the tru k of a t ee . The rocks adj ace nt to the exposed stocks show very great altera t n— in act but tt e ess t an t at the nc u e ra m ents seen io f , li l l h h of i l d d f g T h t ta ut h la ut t to war it in the rock . e s ra abo t e B ck B te s ock dip d r m all s es n cat n a sett n in the crust u n the art a f o id , i di i g li g of po p i lly n s uent ntr cte m a m a w er is n t a c e a d c n e c a . s e ool d o q ly o d g Thi , ho v . o satisfacto ry explanation of the broken condition o f the sedim ents south S E C T I O N

S E C T I O N 2

8 0 0 0

7 0 0 0

6 0 0 0

6 0 0 0

5 0 0 0 S E C T l O N 4

5 0 0 0 S E C T O N 5

LE G E N D J UR AT R lAS LEIS TW U IE C R ETAC EOQ B CAR BO N IF ER O US D EV O N IA N 7 C A M B R IA N ALO O N KIAN

- Crowlhdée ua dmn l Ma dis o n Ke e ne Ce metery Hob o Unh h Stat mom Alpres ton l ey ( t ti n lnmfl m c e mu a o onna twn bmc alo n limes tone form a tion limestone quartzite h orn s tone IG N EO U S

- ‘ Pos r cne 7710 5 0 08 . m am a m s N IOCEN S /

‘ lizizfzxzttsil Gmnlte or Q ua rt z Ga bb ro Diorite Andes qua rtz diorite stoek altemd rocks

C R 0 S S S E C 'I‘ IO N S O F T H E E LK H O R N D I S TR IC T Sh owing hypo th e tic a l un d e rgrou nd s tru c tu re

J U U U S B l $ $ C C O K Y u m ] R EL A TIO N S O F R O C K M A S S E S . 445

of ac utte w ere occur s e era sm a ntrus ns t at are not s wn Bl k B , h v l ll i io h ho on the e ca m a w n to t e r m te e te nt nor the c n g ologi l p o i g h i li i d x , of o di

‘ t ns seen u n the west s e lac Butte where the a r s io po lop of B k , g bb o hold so m any fragm ents that in places more than one -fourth of the mas s is m m uc ccurrences seem c ear to n cate the co posed of the . S h o l ly i di taking up of s hattered or b recciated m ate rial res ulting from the frac r n e w o r h t s n tu i g that ope n d a. ay f t e in ru io . B oth the alteratio n of the s edim ents and the coarse grain of the ab r n cate r n e and intense eat n l we s w c g b o i di p olo g d h i g, fo lo d by lo ool It s u b n we r t at he ran t c r c s cause in . e te e t a g ho ld o d , ho v , h g i i o k d ate r re eat n and a terat n t s sam e area and t at the effects l h i g l io of hi , h of therm al m etam orphism no w observable m ay be largely the res ult of

e r It is certa n t at the a ro is ater in a e t an an such lat action. i h g bb l g h y

the s e m entar r c s s nce it cut acr s s th e l e eds w se of di y o k , i o fo d d b ho foldi ng was late r than the tim e of depos ition of the C retaceous beds of the str ct a er certa n not ear er than M e retaceo us nor di i , p iod i ly li iddl C he a m e late r than t L ra i . T he gabbro is k nown to be the oldes t igneo us rock because f rag m ents of it torn off by the force of the late r intrus ions occur in the

T he wes te rn s e the ac utte s toc i other igneous rocks . id of Bl k B k s shar tr unca te the ran t c at t and it is m s s e to te ply d by g i i b holi h , i po ibl ll

o r f th b e e n The e what the f rm e exte nt O e ga bro m ay hav be . ch mical com position of the rock m ight sugges t that it was a vent from which

w but s is no u r the n an es t c a as ere e ecte t t s o t e e e ce . d i i l v j d , hi pp d by vid

In act t e re is reas n to e e e t at the e nt re m ass was rm er f , h o b li v h i fo ly c ere se m ents no w ar e w rn awa and t at no ut et to ov d by di , l g ly o y , h o l h n t r the s urface ever e xiste d . T e i te nse alte ration of adj acen ocks and the coarse grain of the gabbro certainly accord with this hypothes is

e w n o s t and with the vide nce that s lo cooli g f the m a s ook place .

D IO R IT E S T O C K O F C E M E T E R Y R ID G E .

In its relations to the sedim entary rocks the diorite s tock s outheast of the town differs m arkedlv from the gabbro s tock j us t noted . The extrem elv i rregular contact s ee n on the m ap s hows that the rock has broken up th rough the lim es tones and e ntirely replaced large masses e I is t ere ore rem ar a e to find s ate atc es of eene of th m . t h f k bl i ol d p h K

‘ lim es tone lyi ng upon the diorite and to O bserve a block o f Elkho rn s a e e te n n u the r d e far out er the io r te y e t m a nta n n h l x di g p i g ov d i , i i i g

n e O bs er at ns s t t s m as and its origi al pos itio n. Fi ld v io how hat hi s the

e o f ene m t ne are m erelv ca n s e r th rite It patch s Ke li es o ppi g ov e dio . is appare nt that they are parts of a cover of s edim e nts now nearly

e T h re e nc he U n s t h rem ov d by eros ion . e p s e of t ion hale wi hin t e diorite s ugges ts that it was able to res is t the corros ive action of the r r h m m olte n ock be tte than t e li estone . Between the Elkhorn argil lite o n the wes t and the m ass ive lim es to nes fo rm i ng the easte rn wall 446 E L KH O R N M IN IN G D IS T R IC T , M O N T A N A .

the ntrus n at east 600 eet m est ne is m ss n s win t at of i io l f of li o i i g , ho g h the ntrus n not n rce a art the strata but cause the rem a i io o ly fo d p , d ov l

i r r th s er f s reat uant t es m es to ne . It s ce ta n m e e act of g q i i of li i , f o ob v d , t at the ntrus n m us t a e een a er uiet o ne scarce s tu r n h i io h v b v y q , ly di bi g the ove rlying sedim ents ; yet the rem oval of a grea t m as s of the M adi T son lim es tone would i ndicate viole nt rupturi ng of the rocks . he replacem ent of so large an am ou nt can be accou nted fo r in various s 1 It m a a e een ate u war or ee n e ected rom a way . ( ) y h v b flo d p d b j f

h s s u e t n is ne at v e the s it e e n e volcanic ve nt . T i gg s io g i d by po iv evid c that the i ntrus io n was cove red by a sedim entary cap s ubseq ue ntly rem ed by er s n and t ere re t e re was no r m fo r the floatin ov o io , h fo h oo g

n s e n 2 T im es t nes the c s and o s t ect . he of blo k po ibili y of j io ( ) l o , that form erly occupied the place no w h eld by the diorite m ay have een racture the rce of the ntrus n and the c s s un wn b f d by fo i io , blo k k do th rough the liq uid m agm a into the rese rv oir from which the igneous

T he l m es t ne m e en m e and d s m atter cam e . (3) i o ay have b as s i ilat d i

T he t r es is i n o r wh h solved by the m olten rock . la te hypoth s o e f ic

a s ute r is necessar as it is s e to the o s e r at ns m a e b ol p oof y , oppo d b v io d

re e sts the w r e r u the etr ra by sco s of g ologi o ld ov . Tho gh p og phic

stu the r c s s ws no r o t s the acts s e r ed here dy of o k ho p o f of hi , f ob v

s eem to n cate s m e such act n — the a are nt c rr s n or eat i di o io viz , pp o o io

in nt the m est ne bv the r te the ac stu r ance of the g i o li o dio i , l k of di b strata and as t the er une e n strin or s m eary nature the , , l ly , v y v , gy , of

r te ts e w c as is s wn in the e tro ra c a e n v ar es dio i i lf, hi h , ho p g phi pp dix , i

r m s n n te to uart - r te and w ic in the fie d is s ee n to v ar f o ho ki i q z dio i . h h l v

from place to place .

- P D IO R IT E P O R H Y R Y L A C C O L IT H O F Q U E E N G U L C H .

The m as s e r te - r r s ee n at the unct o n urnle ueen iv dio i po phy y j i of T y , Q , and Elkhorn gulches constitutes an i ntru s ion which is laccolithic in

T e tt m of the m s re t u n te r s ed m e nts tar character . h bo o a s s s po al ed i (S

n d s i h n l n W m unt r e s an an e t c r c t e c tact ane S . o ho f l ) d i o k , o p dippi g at The expos ure s een along the railroad loop s hows that the

ntrus n m es u the an es tes the u e r c ntact at this ace i io do p d i , pp o pl dip

lV at T he n es t e r h ee n ar e r m e ping S . a d i ic cov as b l g ly e ov d by

r n but e n u is e t to sh w c earl t at the m ass co nst tutes an e os io . o gh l f o l y h i

n r n n i i ect o ns n i ncli ned laccolith i t us iv e i a des t c rocks . S i 6 a d 8 of

c r ss the acc lith at un a ra e aces s o t at the lacc Pl . XLIX o l o f vo bl pl . h o

s r s e Th lit lithic tructu e i not well s how n in th m . e lacco h rock is

c ear un e r t an the andes ite s bu t as it oes no t com e in c ntact l ly yo g h , d o with any of the other igneous rocks o f the dis t rict no further s tate m ent as to its a e can be m a e t o u it m ht be n erre r m the g d , h gh ig i f d f o com position and m i neralogic character that the rock fo rm ed a part

the ene ra an es t c eru t ns and was t e re re e r t an the of g l d i i p io , h fo old h

- quartz diorite and the granitic rocks .

448 E LK H O R N MIN IN G D IsT R IOT , M O N T A N A .

he ntact w t the an es te w ere an a s s O f n e . A t t evide c co i h d i , h pophy i h r ects nt the atter the r c s ws a s t nct c llte tu re porp yry p oj i o l , o k ho di i hi x , v is ible to the eye and unmistakable when the rock is seen in thin section th h l cre t et west the t wn r h m cr s e. n e s e unde t e i o cop O il , f of o , or h r n u s two m asses a r - r te w c t u too the p y y i cl de of g bb o dio i hi h , ho gh o ha e r u e an c n the r r are t em se es sm a l t v p od c d y hilli g of po phy y , h lv s t es e nc us ns a a sm a sla the slightly m etam orphosed . A h i l io h ve ll b of tarm unt rn e s attac e to t em an as soc at n c mm n in t e r S o ho f l h d h , i io o o h i na s t n it seem s c ear t at t e are ra m ents r en se origi l po i io , l h h y f g b ok loo s T he re at ns the r r to the from the main gabbro mas . l io of po phy y rt - n n r s c e r The c ntact ne is s ar and a care qua z mo zo ite a e al o l a . o li h p ful search of the granite reveals occas ional m asses O f quartz -porphy ry t n l h r It is t er but a few inch es in diam e er i c osed in t e g anite . h efore certain that the porphy ry is an earlier intrus ion and not m erely a dike t s Th n s is reac ed t r e tens n the ran e m a s . e c c u n e e lik ex io of g i o l io h , h re t at the uart - r te- or h r was the atest ntrus n rece n fo , h q z dio i p p y y l i io p di g the n th e great welling forth of gra ite .

A N D E S IT IC M A S S E S O F E L K H O R N P E A K U E E N G U L C H A N D , Q ,

E L K H O R N G U L C H .

' z —A t u reat uant t es an es t c r c s ex in h A n des te. l ho gh g q i i of d i i o k ist t e

' district and rm m unta n m as s es for m an m es a ut it th e r cks fo o i y il bo , o h are alm ost entirely extravasated lavas . A t Elk orn Peak and at urn e G u c ntrus e r c s ccur but the r c s not c ns t tu T l y l h i iv o k o , o k do o i te m asses filling the conduits from which the breccias m ay be s upposed to a e c m e but are ater r c s cutt n the recc as t u the h v o , l o k i g b i , ho gh of m e nera aracter nd er eru t n The s urce m sa e g l ch a p iod of p io . o of ost of thes e andes itic rocks m ust have be en som e point or points wes t of the str ct in the re n now ccu e the B u er at t the di i gio o pi d by o ld b holi h , sm all size of th e fragm ents com posing the breccias and the eas te rly the e s nt n to a s u rce at least se era m es westwar dip of b d poi i g o v l il d . n the t er an the e tens e ies ntrus e an es t c r O o h h d , x iv bod of i iv d i i ocks carryi ng such enorm ous blocks of lim es to ne as that now form ing the m ar e c iff rn ea B nd cate a s urce s u bl l of Elkho P k (XLVII , ) i i o of pply t an clos e a h d . That the andesite s fo rm erly covered a large part if not all of the district s eem s probable from the occurrence of these rocks in Queen

G ulc at a low e e at n and in the r e h rn ree el h , l v io , go g of Elk o C k b ow ut ueen ree Th e s ra h r the m o h of Q C k . phy iog phy of t e egion s hows that it has been extensively faulted since the granite batholith was rm e but es te t s the e ence seem s c e r at h n fo d , d pi hi vid l a th t e a desitic r c s were t r wn out can c eru t ns reat m a n tu e and o k h o , by vol i p io of g g i d , fell upon and covered a region consisting O f highly folded sedim ents ar e int a c untr near as ru e as t at se -d r c v d o o y ly gg d h en to ay . The e seem s no alternative but to accept the co nclusion that thes e rocks cov

wm - l R EL A TIO N S O E BO OK n ew s . 449

ere the s tr ct to a c ns era e e t the sur ace sl n ra l d di i o id bl d p h , f opi g pid y r D he en r n chara ter th e r c towa d the south east . espite t du i g c of o k and the fact that the highest peaks are now form ed of it largely ecaus e t is c aracter it is true t at it waste s and is rem e b of h h , h ov d by stream and r st er ra and en rm us m ass es O f it m ust a e f o v y pidly , o o h v been rem oved and carried down i nto the valley O f the Jeffers on R iver during the long pe riod that has i nte rvened s ince the form ation O f the r s ock . ' — M arble clqf a The andes itic rocks of Elkhorn Peak cons i s t in part — of indurated m etam orphosed fragm e ntal rocks volcani c breccias and ' ufl — rmin its s um m t and the cres t wes twar and u n er n t s fo g i d , of d lyi g m ass e ntrus e r c s w c s m et m es ccur as s eets etween the iv i iv o k , hi h o i o h b h e m rn a The altered breccias and constitute t e gr at as s of Elkho Pe k . latter rocks have apparently been i ntruded during the activity of the an es te v can e n n ecte etwee n th e se m entar at rm d i ol o , b i g i j d b di y pl fo d h r n r m n m te r m a n th rm r n c n an t e ove lyi g f ag e tal a ial ki g e fo e volca ic o e . A m eas ure of the m agnitude of the intrusio n j us t m entioned and O f its force is afforded by the i ncl us ions O f altered lim es to ne found n it ese nc us ns are clear c s torn off r m he n r withi . Th i l io ly blo k f o t u de lyi ng M adison lim es to ne and bo rne i nto the ir present pos ition by the rce h e n n m T he ar es t c is wel s e n fo of t as ce di g m ag a. l g blo k l expo d o th e western flank o f Elkhorn Peak and in the white cliffs which are S o c ns cu us a eature the ea and e a an sn w o pi o f of p k , look lik b k of o ffs r w en seen r m the s ut . ese c a e s wn in . B h f o o h Th li ho Pl XLVII , , which has bee n m ade from a photograph taken on the shores of a lake l n th r t enea the let that fills a hol ow i e amphitheate bo tom . B th white ' mar e elifls the ar an es te is see n wit a sec n or l we r s tratum bl d k d i , h o d o

f t T he ew s w he r -c re n O white m arble benea h it . v i ho s t da k olo d a de sitic rocks proj ecting above the snow bank that o verlies the m arble ff T h h er s e ee on h ri - n s h r cli s . e igh lop s s n t e ght ha d ide of t e vie w a e a s an es te bu t the n e O f c ffs eneat them c ns sts rns t n s l o d i , li li b h o i of ho o e e on n to the M es c or C r w R e s er es w t a t n ue O f b l gi g ozoi o idg i , i h o g t n t em The m s n t e ff aplitic grani e be eath h . rock co po i g h white cli s is a very coarsely granular and glistening m arble whose individual grai ns n n n r T h r m are ofte tim es a i ch ac oss . e m as s is 1 25 feet o ore thick in the c ff and m a be trace acr ss the ent s n s u er O f li , y d o g ly lopi g ho ld k orn ea for a stance O f eet The wer m ar e m ass El h P k di f . lo bl seen in the ew is in som e aces 50 eet t c and is a s tracea vi pl f hi k , l o ble across th e north s ide of the peak f or a distance of not less than eet t u it is in art c ncea e debris and its n rt war f , ho gh p o l d by , o h d exte nsion is inferred from the presence O f occas ional blocks of lim e sto ne and r m e su res aff r e r f o xpo o ded by sev ral p os pect pits . This

wer m ass m ar e is cut tr n nd dikele s e lo of bl by s i gers a t of and s ite . Its eas te rn end is a ru tl truncate the an es te and westwar it b p y d by d i , d ra ua we es out a n n h m Th r t i s s w o t e a . e c u n the g d lly dg , ho p o k , ho gh

22 G E O L PT 2— 01 — 29 , S 450 E L KH O R N MIN IN G D I T R ICT , M O N T A N A . m a n a ure m ar e c nta ns two an s w te rnst ne one 3 i p bl , o i b d of hi ho o , eet and the er 3 nc es in n ese rnst nes are c ear f oth i h thick ess . Th ho o l ly an a terat n r uct inter e e la ers m ure m est ne and l io p od of b dd d y of i p li o , t ere re the r ina strat fi ati n ane th r e h fo S how o ig l i c o pl s of e ock . Th y dip at h h r N E . To t e southwest t e lowe block of m arble rests upon a t c ran te w c has een ntru e etween it and the an es te pli i g i , hi h b i d d b d i , h r r h n r t e latter rock elsewhe e unde lying it . T e upper a d large block O f m arble als o shows by a band of im pu re lim estone that the dip O f h r t e s trata is slightly g eater than the dip of the u nderlying mass . It appears probable that these two blocks were form erly united and ha e een we e a art the ntru s n an es te the re at e v b dg d p by i io of d i , l iv position O f the blocks indicati ng that the wedge ente red from the T he is at n O f these c s at an a t t eral wes t . ol io blo k l i ude of sev thousand feet above the M adis on lim estone and so near the M es ozoic beds s hown in the ate n cates e ther that er r u n au t n has O ccurre pl i di i v y p ofo d f l i g d , or that these blocks have been torn from som e deep-seate d m ass and ate nt t e r resent s t n in the u an es t n us n flo d i o h i p po i io liq id d i e i cl io . D irect upward m ovem ent is im probable in v iew of the wedging apart h r h m r n — of t e blocks f om the wes t . T e o e reas o able hypothesis and O ne suppo rte d by the occurrence of m any sm aller included blocks of m arble seen in the cliffs to the east— is that a great flood of andesitic m a m a we e u r m s m e e nt to the westwar l te the er n g ll d p f o o v d , if d ov lyi g

‘ s d s l we m d h recc a an ntru e t e et en t e an t e M es c s a es . b i , i d d i f b h ozoi h l

In sum m ar the acts a rea resente s w t at the an es tes re t y , f l dy p d ho h d i s upon sedim entary rocks of greatly varying age and that th e contact r ff r n e at n It i n erre r m t t at h occu s at widely di e e t el v io s . s i f d f o his h t e breccias and lava flows com posed of th es e rocks covered a very rugged r The r c n area of sedim entary st ata . b ec ias S how a rude beddi g dipping eastward on Crow and Elkhorn peaks and southward in the m ountain m s ut w st rn ese re at n n cate that th s r as s o h e of Elkho . Th l io s i di e ou ce the m ateria la s m ew ere to the west th e l rn istr ct and of l y o h . of E kho d i , from the character of t he beds there seem s reason to believe that the masses now seen are th e residual parts O f a great v olcanic pile whose er l h r ct n e t cent ay som e m iles to t e west of Elkhorn . D i e evide c O f his has ee n rem e as a resu t the ran te intrus n w t ater u t b ov d l of g i io , i h l plif ault n and su se uent e r s n but arts the weste rn S e by f i g b q o io , p of lop of the form er a ndesitic volcanic cone may be well seen west of Boulder

V alley .

G R A N IT IC B A T H O L IT H .

As n te in the ntr uct r c a te r the E rn istrict nc u es o d i od o y h p , lkho d i l d the border of a great g ranite area extending for m any m iles to the west rn and reac n r m the v c n t H e ena to the of Elkho , hi g f o i i i y of l nt in ut O f Butte s reat m ass O f ran te is H ighland M ou a s so h . Thi g g i ” m d n nam e th B er at l t all one connected ass an has bee d e ould b ho i h . wa s ] R E L A T IO N S O F R O C K M A SS ES . 451

O n the reconnaissance m ap that shows the relations of the Butte dis trict to the surrounding region this batholi th has been outli ned and i s r l t n en A t a num t e a io s have be studied . ber of po ints th roughout the granite regio n this rock is covere d by andes itic rocks which were at first t u t to be m re recent t an the ran te n as t e ho gh o h g i , lyi g h y do n it nd u it . s ate m asses ccur at scattere ca es a in eac c po I ol d o d lo l i , h ase the re at n se m s to be h m N tw thstan n th a r l io e t e sa e . o i di g e lte ed charac ter the an es tes w c u te c ear s we the ntrus n nd of d i , hi h q i l ly ho d i io a m etam r s m O f the ra m e nta rm s no s t e e ence was o phi f g l fo , po i iv vid developed in the central part of the area of a m ore recent age of th e

ran te . L ate r stu es we er at rn at e sto ne H ot g i di , ho v , Elkho , Pip r n s and at a nt m wa etwee n t ese two l ca t es s we Sp i g , poi id y b h o li i , ho d c nc us e t at the an es te is O er t an the ran te as ra m ents o l iv ly h d i ld h g i , f g u i th it of it are incl ded n e gran e m as s near its borders . A bund ant evide nce that the aplite is later than the andes itic rocks was l rn h n un at . T e s ec m e fi ure in PI. L I B s ws fo d E kho p i g d XI , , ho this re at n as a m nute str n er a l te is seen enetrat l io , i i g of p i p

in the racture an es te . es e acts t et er w t the m eta g f d d i Th f , og h i h m or hos ed c n t n the recc as s een u n the ran te s w t at p o di io of b i po g i , ho h the an es tes are in art at eas t the rem nants a rm er c e r to d i p , l , of fo ov th e ranite at t the atter a n r en u t r u an e tens e g b holi h , l h vi g b ok p h o gh x iv area an es t c r cks the eat the ntrus n r duc n a m eta of d i i o , h of i io p o i g

m r s m of ese c s as i h O f all h er n t t T h t r t as t e t s a ace t o i . e o phi h o k , o h dj nature and manner of i ntrusion of such eno rm ous m asses of granite ' are as et m er ect un ersto but the rese nt str ct O flers y i p f ly d od , p di i

e cee n a ra e c n t ns for a stu suc en m ena . A s x di gly f vo bl o di io dy of h ph o , we er the r lem is a ar e one and m us t be stu e in its ent ret ho v , p ob l g di d i y, it is not w t n the r nce t s re o rt to ea w t it e ce t in i hi p ovi of hi p d l i h , x p

uc tures a m m e w t n the rn str ct s h fea s ay co i hi Elkho di i . Withi n the region herein described the graniti c rocks are clearly of

r e are ntrus e in he later origi n than anv othe rocks . Th y i iv t other rr m he l t r igneous rocks and ca y frag ents of t at e . They abut agai nst the truncate en s the sed m entar es and a e un u te l dis d d of i i , h v do b d y placed large am ounts O f both sedim entary and igneous rocks lying to h e he r n m t e rm t n t e w st of t p ese t li i s O f thos fo a io s . h r ni r th ns r d T e g a tic ocks of e batholith are exte ively fis su e . These fiss ures are r u e and are m uc m re rom nent in s m e areas t an g o p d , h o p i o h in t ers closelv s eete r c s ccurr n near areas S w n no s eet o h , h d o k o i g ho i g h he er r n w r m n t a ing. T v y st iki g bo lde s and groups O f o oliths h t distin h r r T h guish t e g anite a ea are due to the presence of these fissures . e fissures belong to two well -marked system s : (1) ‘A system O f vertical nts is m st c mm n and r uces a true s eet the r c the anes joi o o o , p od h of o k ; pl are ert ca or a r m ate ert ca and are ser a e e er w ere v i l pp oxi ly v i l , ob v bl v y h thr u ut the at t t u the c seness the S eet n anes o gho b holi h , ho gh lo of h i g pl ar es reat r m l e to l in urs in ca areas v ac ace . l se s ac cc i g ly f o p p C o p g o lo l , 452 E L K H O R N M IN IN G D IS T R IC T , M O N T A N A . and may res ult in a platy s tructure of the granite ; m ore comm only the fissures are farther apart and do not S how except on weathered s ur

T he re t n the ert fiss ures ar es reat in ifferent faces . di c io of v ical v i g ly d w i In the rn area . their s tr e ar es e arts the at t . p of b hol h Elkho ik v i id ly ,

as is s w n the s tr e and m ar s on the e ca m a . ho by ik dip k g ologi l p, Pl In enera we er t ere is a endenc f or the s eet to . t XLV g l, ho v , h y h approxim ate parallelism with the contact li ne between the granite and w r n the t er r c s . 2 D a na fis s ures are a a s ese t t u o h o k ( ) i go l l y p , ho gh e r re O ften obscu red by the m ore prom inent vertical system . Th i cog es nitio n is depe ndent upon thei r developm ent bv weathering . Th e a na fiss ures in s te rect ns the an e etween t em di go l dip oppo i di io , gl b h r T he res u t is the inte rsec being th ough thei strike is the sam e . l l r ec er ha s w n tion of the granite i nto rudely rhom bic fo m s . B k s ho that suc fissures are au t anes t at t e ne er a e but are m e re h f l pl ; h h y v g p , cracks ; that th ey are no t parti ngs or true joints ; that they s how m ove m ent by occasional S lickens ide faces and by m eas urable displacem ents r T he a re te su usually of less than an i nch for each fiss u e . gg ga of ch m inute displacem ents h e has estim ate d to be se ve ral thousand feet for rr the Sie as .

T he at t r c is rem ar a ree r m nc us i ns of t e r r c s b holi h o k k bly f f o i l o o h o k , rre r t r O n two s m l even where it has broken i gula ly h ough them . ly a l l m esto ne nc us ns a e een seen in the ran te at rn and i i l io h v b g i Elkho , T h r t is r these are shown on the geological m ap. e fi s a block of m a ble about 65 feet wide and 1 00 feet long ; the second a m as s of garne tife r

- ous lim e s ilicate rock s om e 200 feet across . T he peculiarities of rock m etam orphism O bserved o n C row R idge give g round for the belief that the granite batholith exte nds under the se m entar area the str ct the c nta ct n to the east but di y of di i , o dippi g , no rect e ence has ee n O ta ne to S w t at it d e s so ere as it di vid b b i d ho h o h , e t r c s u he r r O f h at do s a othe pla e abo t t bo de s t e b holith . T he andes ites rest upon an eroded su rface of sedim entary rocks as rre u ar as t at seen in the str ct to - da and it is t ere re e v i e nt i g l h di i y , h fo d t at the ran te w ere it is e se near s uc c nta cts was not h g i , h xpo d h o , r e m en r bu t m u a e m a n an c e e s ta r c s s t v c e a st es tes . ov d by di y o k , h o g i d i M re er in the central arts the at t rem nants an an e o ov , p of b holi h of d s t e c er st e ist and the ran te m us t a e eit er enetrate i i ov ill x , g i h v h p d betwee n the andes ites and the bas e upon which they rested or rem oved the asa r c s w ate r e w e r A ns erat n t e r e t e . c s e e a b l o k , h v h y o id io of h l tions and O f the profiles s hown in the cross sections s hows that the batholith cover was neve r very gre at and probably cons is ted of r d s feet o less of an es itic rock . T he form ation of the granite batholith oblite rate d all evidences of rm er h aracte r the western rt O f h th e fo c of pa t e Elkhorn dist rict . S i nce the batholith was fo rm ed the re has bee n no fu rthe r igneous

1 i n f th e S i rra N e v ad a o f a lifo i S t ruct ure of a ort o o e C rn a : B ull. G eo l m . Soc . A V ol. II . 50. p , p

O N N A 454 E L KH O R N M IN I N G D ISTR IC T , M T A . t at n ent the s e h rn ea but th e str ct w as ne er cov h i d id of Elk o P k , di i v ere a enera ice s eet the ac at n e n ure y loca and d by g l h , gl i io b i g p l l of the a n t T he s n ws eac w nter accum u ate to c ns er lpi e ype . o of h i l o id able depths in the am phitheater of Elkhorn Peak and exis t as s now

' an s ate nt the sum m er and t r u the G ac a e c t ese b k l i o , h o gh l i l po h h a mphith eaters were undoubtedly fill ed to a depth O f feet or m r n t rm e sm al a ers ur n the ear o e . This s ow m us have fo d l gl ci d i g ly

e st ce ne e r w c us e wn the r nc a u c es the lar est Pl i o p iod , hi h p h d do p i ip l g l h , g glacier com ing down Elkhorn Creek a couple of m iles and depositing h w e n s u t P re-G ac a er s n t e bowlder m aterial on hich th tow i b il . l i l o io u n u te l urn s ed an a un ance s e m ater a w ic t et er do b d y f i h b d of loo i l , h h , og h w t the l c s l sene r st r m the am h t eater clifi s we re i h b o k oo d by f o f o p i h , borne on the s urface of the ice or carried in its gras p to form the we -m ar e m ra nes now se n The am t eater walls S w no ev i ll k d o i e . phi h ho dence of ice e ros ion other than the rem oval O f the débris and the general c ar n out the m r n e and i e-w rn e es le i g of a phitheate . Rou d d c o l dg of n e t r r h rn e The am a d s i e a e seen nea t e divide east of Elkho P ak . phi t eater floo rs we er s w no r un e sses but are ttere h , ho v , ho o d d bo , li d by r Th te r-fill d lac a bowlders which conceal the unde lyi ng rock . e wa e g i l

l w see n in B and two s m ar e ress ns l n a a m e hol o , XLVII . i il d p io yi g h lf il to the n rt eas t are s s roc as ns t u the accum u at n o h po ibly k b i , ho gh l io of

e s débris pr ve nt pos itive proof . T h e glacier which flowed dow n Elkhorn Peak was form ed by th e ' c n uence two s eets one c m n r m the m ar e clifl am h o fl of h , o i g f o bl p i th eater s uth of rn ea the t er r m the am t ea te r o Elkho P k , o h f o phi h et n rn r T h two un te e w I b wee Elkho and C ow peaks . e glaciers i d b lo cy I te e h wn o n h e . T h rn a et t e s t m a and in . e wes L k l , pool ho p Pl XLV I , B ac er was the ar er and cr w e the esser one t war the east and gl i l g o d d l o d , in its retreat left great bo wlders of g ranite scattered over the andes ite Sl es and u n th m etam r se M es c s es T h e c n uent op po e o pho d ozoi hal . o fl ice sheet flowed down th e valley to a poi nt oppos ite the m ou th of A l reston ulc r a n n el at n ee A t a ate r er G e c a e t. p h , hi g v io of f l p iod its r nt was a ut one - a m e a o e the t wn rn at w c f o bo h lf il b v o of Elkho , hi h point the ice m ust have m ai ntained a nearly constant pos ition for a c ns era le en t t m e as the term na m ra ne at t s nt o id b l g h of i , i l o i hi poi is unus ual ar e for so sm a a ac er e n se vera un re ee ly l g ll gl i , b i g l h d d f t

and e te n n c ea r acr ss the a e . A s seen r m the t wn high x di g l o v ll y f o o , it c m ete am s th s a e as is we s wn in the t ra o pl ly d i v ll y , ll ho pho og ph n r m r h r l s m ta e nea t e ra a stat n P . L A . ra ne s w k f o il o d io , , Thi o i ho s a ste - e r file and c ns s ts ran te wl ers u to 1 0 eet in p lik p o , o i of g i bo d p f ’ am eter w t sm a ler m asses andes te and rnst ne the di , i h l of i ho o , whole rm n m am m ar umm c s w t nter en n ett e h fo i g ill y h o k i h i v i g k l oles . This term inal m oraine pas ses northward i nto a well -defined late ral moraine l n u on the s es west of the a e w e a m uc m a er yi g high p lop v ll y , hil h s ll latera m ra ne is seen low wn o n the s te eas tern s e In th l o i do oppo i lop s . e - . 8 G EO Lm lC A L S V E T WE T S EC D E T I U UR Y N Y ON ANNUAL R PO RT PAR I PL . L

A BO W I D E R M O R A IN E AT E L K H O R N

In th e m ud d le d istance m s e e n a e ll-m a r e d rm ina l m ain e v te or e xte ndm ac ro ss and com le te l dam m in th e alle . w k , g p y g y Sadd le be t e e n E rn P a n f r w lkh o e k l o le t ) and C ow Pe ak I o n right ) s e e n no background

T W AT N D D O L O M lT E - F O O T L EV E L E L KH O R N IN 8 BE D D IN G P LA N E B E E E N S L E A , , M E

r r w - ar d b dd in The illustration snow s diffe e nce in fractu e and e ll m k e e g of the s late . wa s ] G L A C IA TIO N . 455 granite area at the head of Turnley C reek the m oraine is less conspicu ous as the ac al w ers are all O f ran te and a te and res t u n , gl i bo ld g i pli , po a ran te s ur ace w ere t e c se resem e the r nar w ers g i f , h h y lo ly bl o di y bo ld

e e here Is a s tr n of dis i nt gration com m on to the granite ar a . T o g r h r h a e nd n a The cont as t in t e characte O f t e glaci t d a u glaciated are s . valleys which have not been glaciated s how s te ep s ides and rounded tt m s w t firm s and no swam s w e the ac ate arts bo o , i h oil p , hil gl i d p of the sam e valley are paved with h uge bowlde rs and s how a scanty dark e ta n d cas na s w T h soil cover d with rank vege tio an oc io l ampy areas . e XL ws the e tent O f the ice -c e r r e ca m a . V s e a eas g ologi l p, Pl , ho x ov d , th e well -m arked m oraines being i ndicated by a darke r tint than that he n i m arking the general extent of t bowlder heapi gs . It s evident t at ac at n not m ate r a affect the t o ra the re n h gl i io did i lly op g phy of gio , though many of the pictu resque as pects of the present scenery are ts er s e act n due to i o iv io . C H A P T E R V I .

G E N E R A L E F F E C T S O F T H E R M A L LIE T A M O R P H I S M O F T H E

R O C K S O F T H E D I S T R I C T .

T h e following acco unt of the m ore prom i ne nt features of the therm al or co ntact m etam orphism of the s edim e ntary rocks by the igneous i ntrusives presents only the m o re sali ent res ults of this action . Both the sedim entary rocks and the later igneous rocks have been h re t e O f m e n m m intru e in t m m etam orphosed by t e g a bodi s olt ag a d d he . s c an e ten ca le l ca or c ntact m etam or s m is here n Thi h g , of l d o l o phi , i es nate r t erm a m etam r sm ecaus e it is due to the eat d ig d hyd o h l o phi , b h

n r nce it IS cause the ne us and vapo rs of m olten i t us ion . S i d by ig o m asses the a terat n is natura reatest near t em and is ess and , l io lly g h , l r r T he c an es due to t erm a m eta less away from thei borde s . h g h l ' morphism difler ve ry m arkedly from the res ults of ordi nary rock s nte rati n or rock a terat n and are a s ffe re nt r m t se di i g o , l io , l o di f o ho e rm a m etam r sm changes which are due to m etas om atosis . Th l o phi em braces those changes prod uced in a rock m ass by the influe nce O f m m s high tem pe rature due to the i ntrusion of an igneo us ag a . The e Th c an es are O f two n s first s ca s ec n m nera ca . e h g ki d ; , phy i l ; o d , i logi l

r r In m esto nes the a terat n c ns sts physical changes a e very m a ked . li l io o i O f a bleachi ng or whiteni ng of the rock and its convers ion i nto coarsely In m ure m est nes the alterat n res u ts in crystalline m arble . i p li o io l m arked m i neralogical changes which give the rock an entirelv different r haracter and a earance and m a e it us ual c arser ra ne c c . y olo , pp k l o g i d T he san st nes are a te re to com and harder than the original rock . d o l d The c a s a es and the ass c ate roc s act a m s t flint uart tes . p , l o y q zi l y h l o i d k , w c c ns s t ar e y c ay t et er w t ca care us o r s ce us hi h o i l g l of l , og h i h l o ili o m atte r are nte nsely a te re and m st com m n are u n to rm , i l d o o ly fo d fo v erv e nse har r c s extrem e c m act te tu re w c rea w t d , d o k of ly o p x hi h b k i h a nc a racture and w c ecause t s c m actness and co hoid l f hi h , b of hi o p

sm o t ractu re are ten ca e rnst nes . es e rnsto nes av e o h f , of ll d ho o Th ho h e rea in lost alm os t all trace of the origi nal plane of bedding . Th y b k e t er icy or r o m a ts and th e r na strat ficat n can be i h d h boid l bi , o igi l i io d er O te n we e r e n e rm ne n a nt c r an s . e de t i d o ly by f i olo b V y f , ho v , v this crite rio n is difficult to dis tinguish unle ss weathering deve lops a surface on which the original bedding plane is i ndicate d by slight r n In n ne the a tere r c s at rn is a true s at idgi g . o of l d o k Elkho l y wa n . ) E F F E CT S O F T H E R M L M E T A M O R PH ISM 57 A . 4 c ea a e e e e e t er b y t erm a m etam r sm or t er l v g d v lop d , i h h l o phi by o h

he es s te r r s changes . In t l al ed ock a lami nation which approaches a slatv structure in e ne ra a earance m a v s t be bs erv e but in g l pp ill o d , t s cas e as fo r ns tance t at O f the an n - wa r c the rn hi , , i . h h gi g ll o k of Elkho

e the a are nt s at c ea a e c nc es w t the r na e n lod . pp l y l v g oi id i h o igi l b ddi g

O f the r o r is due o s i n s ock t lipp ng along beddi g plane . The m i neralogical changes are thos e due to a rearrangem ent of the m ate r al c m s n th e ro ck a c an e in w c new c em cal c m b i o po i g . h g hi h h i o i

ns are eff te nd n w m n r s rm e n r r natio ec d a e i e al fo d . I pu e calca eous rocks the lim es tone is sim ply recry s talli zed and forms a m arble whose coarse ness of cry stallizatio n is in a wav a m eas ure of the intens ity and

n he n o th duratio of t heati g f e rock .

The ar ace us r c s co m m onlv c nta n s ca and recrv stallize as gill o o k o i ili ,

T h re s r alum inum s ilicates . e pu iliceous rocks rec y stallize as quartz

It is a are nt t at w th h r ites . pp h i out t e addition of m aterial f om o ut s e as for e am e ne uma t t c act n the ure s ca and ure id , , x pl , by p oly i io , p ili p

m e r c s not c an e in m era m o ‘Vhe r h w n c c s t n . e o li o k do h g i logi o p i io , e er the ro c s are m i tu res as is so c m m n the case in se im entar v , k x , o o ly d y

ar tr k n r T h ii s s strata t e e s a te e . e s ce u ca care u s a es , h y i i gly l d l o l o h l c arse and atc a re ates m e n m n s ca s form o p h y gg g of li a d agnesia ili te . D s e and arnet the l m e -a um num arne t ros s u ar te are er iop id g ( i l i g , g l i ) v y

m m n and r n is res ent r i e c e te ccu s n a un anc . o o , , if i o p . pido o b d — A re/ m icem m ro cks S ands tones are but little affected by the rmal m tam r s m u n ess the c nta n s ure uart s an e o phi l v o i impuritie . A p q z d sto ne will recry s tallize as a quartzite if the tem pe rature be s ufficie ntly

d t i h s h n t m l in h r an a a e e a an aces t e str ct . A s a u e high , h s pp d y p di i l ,

we e r the a te rat o n c ns sts in a arden n the san s t ne and ho v , l i o i h i g of d o

e n r r r s lliz i n h n s — not an ti e ec v ta at o . If t e origi al rock wa im pure and im pure sandsto nes are es pecially com m o n in the M esozoic s trata com pos ing the long s pur of C ro w Peak l y i ng to the no rtheas t O f Elkho rn the res u t n ro uct m ay not a e any res em ance to the uart te l i g p d h v bl q zi ,

o ns rn m but m a c s t a et ca e tc . y i of g , i ,

' A rgzllacem zs rocks — T he s hal e s are com m only baked o r alte red to a ense flint ro uct n wn as h rns t ne w c rea s in an ular d , y p d k o o o , hi h b k g C hippy f ragm e nts and com m only has a well -de veloped cu bical fracture

s s s h r O ten brea n nt cu e . It i u ua ar c re as a e f ki g i o b lly d k olo d , v y

m o eneo us and e ry finel ranu ar te ture w c is so m et m es ho g v y g l x , hi h i ese ss nt t ose rm s in w c the m etam r s m has sugary . Th pa i o h fo hi h o phi

een reate r and the r c s are t c re wh te a e reen aven b g o k ligh olo d , i , p l g , l der o r n and a e the te xture and a earance a racture s ur ace , pi k , h v pp of f f o r l e r e t ure ar ace us r ck w u rm m i of p ce ain . A p f c ly p gill o o o ld fo ca ceous s c ists but m s t o f the s a es co nta n m ur ties w c deter h , o h l i i p i hi h m ne t e r a terat o n to co m act fine -te ture r c s c ns s t n i h i l i p , x d o k o i i g of

ine r ls n t is s r t th s t s late and various alteration m a . I h di t ic e po ty s flecked rocks see n in s im ilarlv m e ta m o rphosed areas e lsewhe re were 458 E L K H O R N M IN IN G D ISTR IC T , M O N T A N A .

r uc s ts are due s met m es to car naceous matter not obse ved . S h po o i bo w ere the r c s are s t m e tam r se or in m re a tere rm s h o k ligh ly o pho d , o l d fo A t rn e trem e m etam r sm has to areas free from m ica. Elkho x o phi resulte d in the form ation of extensive m as ses of com pact-textured rock which are bes t grouped as ho rns to ne . Calcurw us rocks — T he m os t striki ng effects of hydrothe rm al m eta morphis m are prod uced in the impure argillaceous lim es tones or

In t ese r s th e car n c c is c m ete calcareous s hales . h ock bo i a id o pl ly

- c A s elimi nated and the whole is converted i nto a lim e silica te ro k . the alte rnati ng strata frequently consis t of ve ry diff ere nt o rigi nal m ater a the res u t n r c v ar es reat y in a earance and m av con i l , l i g o k i g l pp s s t a re ates ten c arse but c m m n fine ra ne to c m act i of gg g , of o , o o ly g i d o p ,

- m e r c s cates w t uart t te etc. of li i h ili , i h q z , bio i ,

ol a t — The resen e t urm a ne and a n te s ws F um ar £0 l eration . p c of o li xi i ho that the re has been fum arolic action as a res ult of the vapors of the n n r r n r n cooli g ig eous rocks . That s u ch vapo s a e developed i la ge q ua tities by m agmas is well known from obs ervations on active volcanoes n n u as well as abu da t evidence about which there can be no doubt . S ch vapors would naturally penetrate along fissures or porous st rata and r duc m ar e lt r n p o e k d a e atio . T he impregnate d bed of garnetiferous horns tone which constitutes the o re the D olcoath m ne ma a e r nate as s u es te body of i y h v o igi d , gg d

D r. arre r m the s r n a e n u ce in a s tratum m ure by B ll , f o h i k g i d d of i p lim esto ne contai ned between beds of pure lim es to nes which have er l ttle n in m a or m n th m tam r sm th suff ed i cha ge ss v olu e . I e e o phi of e impure stratum th e carbo nic acid is drive n off and the rock loses u war 30 er cent its we t and 50 er cent its v um e p d of p of igh p of ol , and e n s te e l t te be re m etam r sm the resu t w u be a , b i g p y il d fo o phi , l o ld porous layer whos e fo rm ation m ay be likene d to that of terra cotta or ' r l r r w brick f om c av . Such a po ous laye ould aflord ready pas sage for m r i a rs or m nera n as Th ore tra i n fu a ol c v po i lizi g g es . e s t um s e v ide tly

an a te re m es tone bed . It is 1 5 to 1 8 nc es t c and s t l d li i h hi k dip E . a

T h cu r l r 5 e ore c s as s v e s ts r m 1 to 2 m m . acr ss and i o i y po f o o , s t n s ate w h ca c re he r I n of e as oci d it l ite scatte d through t ock . t co sists uri nd n h i of te ll de a s ulphide of bism uth carryi g gold . T e ore s appar ently confined to this narrow s tratum beca use its poros ity is greater t an t at the a acent and c n n n l m estone e s The r c h h of dj o fi i g i b d . o k hold ing the ore cons ists of nearly equal parts of diops ide and gross ularite t a l i h Th r r t w c c te as n te n t e a e n . e o e ccu s w s i h i , o d pp dix o i h diop ide as w w t ell as ith calci e . T he latter is e vidently a prod uct of later infil tratin s ut ns as it rm s e nlets and l s the cav t e ne w t g ol io , fo v i fi l i i s li d i h r w r ta a net s t . t l t te r l c s s e e c c ca c c s ta s . , , y y l of g diop id , i h poiki i i l i

S N A A 460 E L K H O R N M IN IN G D I TR IC T , M O T N .

scattered through the quartz and som ewhat resem bling the m ore com

m on tetrahedrite . — etrahedrite ra ca er ore amtim onial sul hide o co er . s T (g y pp , p f pp ) Thi m n ra ccurs a undant in the l rn m ne w ere it is i e l o b ly E kho i , h highly e It is a wa s c ars e ar ent er us and r a a s c nta ns a . y g if o , p ob bly l o o i l d l o ly cr sta ne and m as s e and ccurs in ra ns and m asses in the y lli iv , o g i - n It uart se ore w ere it is as s c ate w t a e r w en e . is q zo , h o i d i h p l b o bl d rare un in the t-wa es and is t en a s as s c ated w t ly fo d foo ll bodi , h l o o i i h It is r a the arent m nera the nat e s er and quartz . p ob bly p i l of iv ilv - h m s ilver rich oxidized quartzose ores of t e ine . i u /z d — s is c m m n ccurre nce not n in h P r te iron g l i e . t e y ( p ) Thi of o o o , o ly l rn m ne but in all the r s ects the s tr ct and is a s un E kho i , p o p of di i , l o fo d in ra ns scattere t r u the ne us r c s art cu ar the a r g i d h o gh ig o o k . p i l ly g bb o - r r In t ese r c s it is in art r m ry and the diorite po phy y . h o k p of p i a r n but m a n sec n ar rm ati n and was un u te e r e o igi , i ly o d y fo o , do b dly d iv d from the augite and other ferrom agnesian m i nerals by the action of h r i m ineralizing s olutions or vapors in t e o e bodies . It s usually m as l re d rm the u k h or in r s e er t c an s t e e se e a m ines . iv , v y ligh o o d , fo b l of v l In the kh rn m ne t u a un ant it es not rm a ar e art El o i , ho gh b d , do fo l g p In th r c n h the ore and is usua m as s e . e us a t es i t e m as of , lly iv d y vi i s ive pyritic ore well -crystallized specim ens have been found which show the us ual well -form ed pentagonal dodecahedron ; m ore rarely it r rm ssa m a the r t us or occu s in cubical fo . A ys de of py i o e S how it to c nta n S er but as th e m ass e m ater al us ua c nta ns a m e o i ilv , iv i lly o i d ix d n t r e t at the s r m r m particles of gal e a i is p obabl h ilve co es f o th em . — Y ielradym ite (s ulp hide and tellur ide of bism u th) . This m ineral occurs in grains up to one-eighth of an i nch a cross in the garnetifer It i m r i h ous rock of the D olcoath m i ne . s so ewhat alte ed n t e oxidized m ater a for fe w eet e w the s ur ace and then s ws m asses i l a f b lo f , ho of free gold having the u s ual rus ty or bronzy ti nt of gold derived f rom

The res m n ra i r t m eta l th a te ur e . e s c w t e us ual ll id f h i l b igh ly l i , i h r n w st n n s m a e in the s e us ter a d s s a ct at . e ts c e m ilv y l , ho di i foli io T d h i cal laborato ry show it to be a m ixture of bis m uth te llu ride and his In er n e n r l i n m ut su i e . a t th m e a ses ts am ate c aracter h lph d l i g , i l o l i d h m fi r It ur in m n te r and the fo lia beco e b ous . occ s i u g ains dissem i nated t r u the arnet and in ar er ra ns ass c ated wit ca c te h o gh g , l g g i o i h l i n t m n The r n th e r in the m ass ive gar e of this i e . o igi of o e deposit and the genes is of the m ineral is dis cussed in an account of the D 0 1 h n coat m i e . — B im u thite (s ulphide of bism u th) . This m ineral occurs intimately ass c ate w t and in the s ec m en is n st n u s a e r m tetrad o i d i h , p i i di i g i h bl f o , y i h D l oath m n nd i r n w m te . It is un n n t e o c e a ts ese ce as i fo d o ly i , p N t n h ur m a n D r. . e i t e determ i ned by che ic l a aly sis by H . S ok s S vey laborato ry .

- B lende s ha lerite zin e sul hide . s m ineral occurs as soc ate ( p , p ) Thi i d

P L A T E L I I .

S C E R U S IT E .

T his plate Shows a photograph of a specim e n of ce russite in which th e rosettes eon S is t of ates t at -are re ative sm a and are coated b a film of uartz so t at pl h l ly ll y q , h i m r h n n T h c ru it i in t s s eci en is not so a ie as t ose com m o see . e e ss e s art h p f g l ly p , at eas t orm ed u on nuc ea ra ments of ox idized uartz ore but cer ussite l , f p l l f g q , h r i c m ak es up th e greate r portion of th e Specim en . T e eversed s de S hows a oat n n h r h T ing of m inute ca lam ine eedles upo t e quartz that enc usts t e cerussite . h e n -fi h iz f m at r n I i i n crystals are about o e fth t e S e o so e th we e see . llustrat on s o e

half natural Size . 462

464 E L K H O R N M IN IN G D IS T R IC T , M O N T A N A .

c urs in s ix or e ht s te ate r u s the ra s e n rm e ates o c ig ll g o p , y b i g fo d by pl scarce t cker t an a s eet a er and s m et m es reac n dim en ly hi h h of p p , o i hi g is r m t ra s c r n e r s . a a e sions seve al i ch s ac o s . Pl LII f o pho og ph of p i m en showing relatively sm all crystal T he m ore com m on occurrence is as a m as s e mater a c at n a ena r m w c it is u te e entl iv i l o i g g l , f o hi h q i vid y m ter i us ua c ear and ass w t a ea us te r rm e . s a a s fo d Thi i l lly l gl y , i h l dy l , h and can be traced i nto the parent m ineral . T e galena alte rs along clea a e crac s and e er ra at n m av be ser e etween the v g k , v y g d io ob v d b i d e - res and una tere m ate r a an the c rus s te . s ra se f h l d l i A hy g y , loo b r undan In th u er te ture m as ses als ccur ut a e ess a t . e x d o o , l b pp workings of the Elkhorn m i ne large q uantities of carbonate o re were ext racte the ceruss te ccurr n in m as s e rm and ass c ate wit d , i o i g iv fo o i d h m various i puriti es . — s m i L ina rite (hydra ted s ulp fia te of lead and cupp er) . Thi ineral s rare but o s ec m e ns w t the us ua a ure - ue c r r m the , go d p i i h l z bl olo f o '

r n he. t n M r. ll lter . lle rn m ne we e seen i t c ec a e y . Elkho i , oll io of S K ' 7 — curs in h ol oath res and rare in the 0 mlcop yrzte. This oc t e D c o ly m ss nd urs s te t r rn m It is a e a cc a s c a w te . Elkho ine . iv o o i d i h py i G old — Nat e l was s ee n in the res the D olcoath m ne ar iv go d o of i , p ticularly those in which the tetradv m ite has bee n partially or wholly r n l w The r is ee e e . alte red . colo d p b o z y lo — Native silver was abunda nt in the thoroughly oxidiz ed por n Its m st nt rest n c r en e i n tions of the Elkhor m ine . o i e i g oc u r c s i ters r ew w re s wn on P I ranc n c us c sta s a f c a . . b hi g l of y l . of hi h ho LIII n e u r N 0 large crys tal s were obtai able at the tim e of visit . Th se cl ste s ccur atta che ten n the ase or s tem the ern- e c uster o d , of o ly by b of f lik l , to the i ron-stained siliceous decom position product which form s the e h r c r gangue of the upp r part of t e lode . Silve also o cu s as sheets or

ra s nd t n s ur ace ess a ter m t r In the scales in c ck a coa i g f s of l l ed a e ial . c usters the cr sta ine rm is er c ear rec n a e t u the l y ll fo v y l ly og iz bl , ho gh r m aterial is not especially adapte d fo crystallographic study . — P yrolus ite (black amide of This occurs in litt le spots c at n ca am ne and as e n r t c m ar n s o n the e w art a g y , y o i l i , d d i i ki g llo p i ll h r alte red dolom ite of t e Elkho n m ine . ‘ — h A z urite (blue ca rbonate of copp e r) . This is found in t e Elkhorn m ne in m nu te c usters or as a ue sta n als in num er us r s ect i i l bl i ; o o p o p s , r r h re th er n pa ticula ly t e o s of e C opp Ki g .

l re n carb n t — J ltz a c/aite (g e o a e of copp er) . This occurs as a green stain and in earthy m asses in the oxidized ores and as gree n crusts in botry i l rm as ate w t ca am ne in the rn It i n o da fo s s oci d i h l i Elkho m ine . s fou d It a ur in m n n n m m unt . o ly i s all a o s lso occ s a y prospects of the dis trict . — 1] em atiie and 71m /wtite. ese ccu r nt m ate ass c ate in the , y g Th o i i l o i d , r n res r m the m nes on the n rt s e h rn ea near the i o o f o i o h id of Elk o P k , mm t s ecu ar or cr sta e m ater wa en u a s se . s i . No p l y lliz d i l t it — s m n r L im on i e (brown he mat e) . Thi i e al occurs as earthy m asses P L A T E L III .

22 G E O L P T 2— 01 P L A T E L I I I .

N E IL E F L N A T IV S V R R O M E KH O R M IN E .

This plate shows a num ber of pieces of arborescent and dendritic native sil ver from T h im th e ores occurring in th e upper part of th e E lkh orn m ine . e S pec ens are very m uch sm aller than m any of those which were found in th e developm ent h T h i v r ur l a t n xidiz d ili of t e property . e s l e occ s oosely ttached o a o e s ceous n T h e cr sta are o te n flattened and e on ate d and orm com ex ga gue . y ls f l g f pl rou s or aborescent orm s usual attac ed to th e base of th e ern- ike mass g p f , ly h f l , h im n s w d ni r t rm T h e ar n although t e Spec e s ho efi te c ys al fo s. edges e ofte rounded and the faces du so t at th e m ate ria is not tted for cr sta m easure ll , h l fi y l h rm r n u t m m n na i m ent. T e o s owev e are u do bted ose co o to t ve siver f , h , ly h l n n hi r A t ou native si ver was of com m on and prese t no novelty i t s espect. l h gh l occurrence in th e u erm ost arts of th e oxidized o re bodies at th e m ine none pp p , of th e com m on wire si ver see n as an a te ration rod uct of the ric si ver l , l p h l n h h un t E k rn su ides at so m an m i es in t e State as been o d a o . Sca es lph y , f l h l and plate s of nativ e silver also occur in cracks in m asses of am orph ous white ii and red s l ca. - 8 . G EO LO G IC A L S RV E TWE T S EC D A N N UAL E T T II . lll U . U Y N Y ON R POR PAR PL L

N A T IV E S IL V E R F R O M E L K H O R N M IN E. m m . ) om: AN D G A N G U E MIN ER A L S . 467

n h r It is n in the r t utcr at many places i t e dis t ict . comm o us y o ops m n r e ns of i e al v i .

' i i dim l d — h ccur in t n D esclo z te vana te o ea a zin . s s ( f nd c) T i o hi , m ss ul e w- reen crus ts m nute cr sta ne sur ace c at o y , d l y llo g of i ly y lli f , o ing cavities in a deposit of greenis h and red-colored am o rphous s ilica from the hangi ng-wall ore body of the -foot level of the Elkhorn

m ine .

u it b i — s curs in A richa lc e ( as ic carbona te of z nc and copp er) . Thi oc unc es a e- reen s nee les l n ca t es one - a to t ree b h of p l g , ilky d , fi li g vi i h lf h r s in r th l n m ne fourths inch ac o s oxidized siliceous o e of e E khor i . — s a t l S m itbsonite (zinc carbona te) . This is found as oci te d wi h ca a r I m ine on dolom ite f agments of oxidized parts of ore bodies . t

rms ra crusts 1 mm . t c w t rus sur ace c vere a fo g y hi k , i h d y f , o d by second crust of pale apple-green botryoidal m aterial s howing both d e r s tr is latte r rus t r es r m ra ial and conc nt ic ucture . Th c va i f o 1 to in n h reen t nt is due to m l am unt 5 mm . t c ness a d t e a s a hi k , g i l o of

M a ac te crusts ten rest u n the s m t s n te and are in copper. l hi of po i h o i rn r n l ne tu coated by g ee ca am i . i t — m r P rrlwt te ma netic r i e iron s ul hide . s ne a ccurs in y ( g py , p ) Thi i l o m n s h r es he s trict n small a ou ts at everal of t e o e bodi of t di . I the h rn m ne it is un ass c ated w t r te as a m ass e c at n Elk o i fo d o i i h py i , iv o i g around cubes of py rite in the white dolomitic m arble of the botto m th m n lev els of e i e .

G A N G U E M IN E R A L S .

T he m inerals j ust enum erated occur ass ociate d with the valueless r he e r ew n num er em n uartz an ue m ne a s . s a e f i rac g g i l T b , b i g q ,

ca c te m te arnet and s e . l i , dolo i , g , diop id i — art is m st un e r l resent in Q ua rtz (cry stalline s il ca) . Qu z al o iv sa ly p re h r It rm s the an n wa the k rn the o bodies of t e dist ict . fo h gi g ll of El ho

- m ne and is a un ant lso . in th al r i a e o t w es . It ccu s n e er i , b d fo l bodi o v y rm r m coarse cr sta ne to e trem e fine cr s ta ne te tures fo f o ly y lli x ly y lli x , and is c earl a re acem ent uart in m an cases w e in t e rs it l y pl q z y , hil o h has the coarse com b structure charac teristic of the filling of open

s aces . G r u s lar e cr s ta s are rare un and the m nera p o p of g y l ly fo d , i l n has o especially interes ti ng feature as ide from its geneti c re lations . l im — is r n n t r ut h Ca cite (l e carbona te) . Th occu s abu da tly h ougho t e rn m ne w ere it rm s cr s ta ne c at n s wel -s a e Elkho i , h fo y lli o i g of l h p d . r m e ra in rus cav t es in the ea res ass c ate w t nc and ho boh d d y i i l d o o i d i h zi , a s ccas na l in sta act t rm s in sma t es and water c urse l o o io l y l i ic fo ll cavi i o s . olom it b n li i — s rm s the w te D e (car o ate of m e and m agnes a ) . Thi fo hi - is r m arbl e foot wall country rock of the Elkh orn lode . It v e y uni rm ih te tu re nd in n N o we -s a e cr s ta s a e een fo x a com positio . ll h p d y l h v b

seen .

— G arnet (r a r iety This lim e -alu mina garne t is com m on E L H O N N IST R IC N T A N A . 468 K R N MI I G D T , M O

r in the m etam orphic rocks of the district . It occu s as a ga ngue min . era in the D olcoath m ne w ere it is m ass e and is m e w t l i , h iv ix d i h h t I i he r u t the diopside in t e s ratum containing the ore . t s t p od c of rat n c n m m m r m The alte io by o tact eta orphism of an i pu e li estone . co r is us ua a c nnam n r wn and the m nera a ters to a reen lo lly i o b o , i l l g

i h r te . r n ra n t n r t s chlo i C ystals are seen pe et ti g the calci e i this o e . I occurs m ixed with m agnetite in th e ore from the i ron mines on the sum m t kh rn ea w ere it is ass and has a ul reen s i of El o P k , h gl y , d l g i h r yellow colo . -m — s rm r x n D iopside (lime agne sian pyroxene) . Thi fo of py o e e is un a un ant as a c ntact m nera t r u ut the s tr ct but its fo d b d ly o i l h o gho di i , n ccurrence as a an ue m nera is in the D olcoath m ne were o ly o g g i l i , h , i r It h w - as ust n te it s m e w t a net. as a a e e s reen j o d , ix d i h g p l y llo i h g d r e r c r an ccu s n m as s e no cr sta s be n s e . olo , o o ly iv , y l i g ob v d — m n r S erp e ntin e (hydrated silica te of m agnesia ) . This i e al occurs as an a terat n r uct r te in the rn m ne and a s in the l io p od of dio i Elkho i , l o r a cuts n r r rt It cu rs a s as an a te rat n r rail o d ea that p ope y . oc l o l io p od uet of garnet ( i) in shells surrounding fragm ents of lim estone i ncluded h n k n I i no str t s ea n a in t e a es tes r ea . t s t c an ue d i of El ho P k , i ly p ki g, g g m ine ral . A arin ite (bore -s ilicate of alum in um with rarying amo unts of iron and — s m ner ccurs t as a r c c nst tuent in h e manya n eee) . Thi i a l o bo h o k o i t e s di m ents altered by contact m etam orphism and as the filling of little veins

It i und a un ant in c arse cr s ta l ne m asse in in such rocks . s fo b d ly o y l i s such vei nlets at the prospect tunnel driven in from the cree k bed 1 1} l rn T h s c m ens are a u - ra s um r m es a o e . e e c il b v E kho p i of d ll g yi h pl olo , r a t r a e It c urs s in lam ellar aggregates of c ystals us u lly s i t d . o c as oci ated with calcite .

Tourm a line bas ic or s ubs ilicate o boron alum ina otas h lime m a ( f , , p , , g - s m nera ur s ar n in the c n ct-m eta m c r c nes ia) . Thi i l occ s p i gly o ta orphi o ks that i nclose som e of the sm aller ore bodies disclosed by s hallow pros n th e e rn s A l re to n u c n n t It a s peet pits o as te ide of p s G l h a d vici i y . l o ccu rs as st ut ac cr sta s te n a n a rus t c at n in the o o bl k y l , of h vi g y o i g, ce nter of alte red fragm ents of m arm orized lim es tone i ncl uded in the andes ite . ' E idote orthosilicate o lim e alum ina and iro n H Ca A l f e S i p ( f , , , ( ) 3 , — This occurs abundantly in the contact-m etam orphic rocks form ed I i m m n n ar c utte ere it r m m ure m es t nes . t s c e a w f o i p li o o o Bl k B , h r i n ater a i fo rm s fascicles whose fibe rs a e s everal nches lo g . This m i l s as s ociate d with ca lcite and axi nite .

s — s r st th liblla tonite (s ilicate of lim e) . Thi occu s as a con ituent of e nta ct-m etam r c r c s art cu ar the s t m ure m ar es co o phi o k , p i l ly ligh ly i p bl ,

re d e c ur It is se m in m as ses rec n a e to the in which o bo i s o c . ldo og iz bl e nak d eye . h r rs in w te ar e rm i icrocline. s m ne al ccu c s m as s ve M T i i o hi , o ly i fo n

A CH PTER VIII .

T H E E L K H O R N M I N E .

— H istory of the p rop erty The ore depo sit known as the Elkhorn m ne was cate as the ter e on anuar 2 1 8 5 an w y , 7 d as i lo d Hol lod J , w r e near c nt nu us r m that t m e unt l em r 9 y e 189 . o k d l o i o ly f o i i Nov b , The general history of the property has al ready been m entioned in h tr c Th N w rn n giving that of t e dis i t. e e Elkho M ini g Company stopped w r in th e m ne in the autum n 1 899 and as the um r o k i of , p ps we e wit rawn the s ur ace mac ner sm ant e and s d and the m n hd , f hi y di l d ol , i e ract ca a an ne it was enera un erst t at t r p i lly b do d , g lly d ood h his g eat r ucer was w r e out and the last c a ter its sto r c m te p od o k d h p of hi y o ple d . The ent re r ert nclu n suc s ur ace im r em ents as ere i p op y , i di g h f p ov w of alue was s to two success u m n n erators for e r v , old f l i i g op a ly in 1 900 and it was enera su se at t at t m e t at n the um , g lly ppo d h i h o ly d p

ea s w u be w r e . The new wners ec e we er to re en h p o ld o k d o d id d , ho v , op the u er e e s the m ne and the r ert is now M a 1 901 e n pp l v l of i , p op y ( y , ) b i g u e w t the necessar m ac ner for w r n and r m se eq ipp d i h y hi y o ki g, p o i s to T he n nm ent again becom e a producing property . aba do of the m i ne by the Englis h corporation was prim arily due to the exhaustion of the r c ore es for alt u m re or less ore rem a ne it was too i h bodi , ho gh o i d , of low ra e to a e e nses w n to the ea e e n tures necessar g d p y xp , o i g h vy xp di y in um n the m ne w c were er a wee t s and the p pi g i , hi h ov k ; hi , sm all num ber of m en scatte red over so large a m ine working the low ra e atc es and rem nants ore es le t in re us years ma e g d p h of bodi f p vio , d th the cost per ton excessive . Thus e actual cos t of m i ning exceeded l M n 1 5 er ton nc u n a l e enses . c st 9 w n to ase 8 p , i l di g xp illi g o $ , o i g b ness the ore and for a n t m e the re uct n w r s were ru n on of , lo g i d io o k

r fit n 50 cents e r ton t us neces s tat n er c se w r in . a p o of o ly p , h i i g v y lo o k g — G eneral f ea tu res of the ore dep os it T he ore de posit does not con stitute a true e n t u c m m n y s o en as a e n or l e . The v i , ho gh o o l p k of v i od ore occurs in two pri ncipal shoots lying on the under s ide of folds in th a e the s— and c n rm n to the of the e . e s of i . , ddl fold o fo i g dip r ese ore es are un n stratified beds in which they occu . Th bodi fo d alo g a beddi ng plane be twee n i ndurate d shale (hornstone) and dolom itic

s c ntact ane w as we in m n n and has c m m n m arble . Thi o pl follo d i i g o o ly t u t ere has een s m e s n an been s poken of as the lode . Al ho gh h b o lippi g d nt l n t s ane the c ntact is not m nera i e nor es it m ovem e a o g hi pl , o i l z d , do show any vein quartz or other lode m ate rial except in th e steeply pitch 470 wnw 7 ] E L xIIO R N M IN E . 4 1

in arc es un erneat w c r r The o re e os ts g h d h hi h the o e bodies occu . d p i

' consist of m ore or less irregular bodies of rudely lenticular cross section in a a nst the an n -wa rnst ne and as s ate m asses rm n ly g g i h gi g ll ho o , i ol d fo i g ” - h r l The an n so called cham ber deposits in t e unde lying do om ite . h gi g wall bodies consist of quartz carrying ore m inerals and are pra ctically n nu h h Th ea c am er co ti ous from t e top to the bottom of t e m ine . e l d h b s are isolated bodies connected by fissures or rusty stringers with quartz ore s ts but rre u ar s a e and s t n and ten lar e hoo , of i g l h p po i io , of of g ” h r c m er s e . e are not ca e e s ts and t e te m a a es iz Th y v d po i , h b ppli n to the ca erns e t the e tract n the ore no r na o ly v l f by x io of , o igi l c e t re m a n st u avernous open spaces occurri ng . Whil he y be ho e do bt concerning the formation of galena deposits in ope n caverns in lim e st ne at m an m erican ca ties the acts to be escr e ere n o y A lo li , f d ib d h i n preclude any such theory of formatio . To o ra hic relations — The ore e osits ccur as m a be seen p g p d p o , y by r XL in th re a e a and en a re e ence to the m a . V e t r e f p, Pl , l iv ly b o d op v ll y of h rn ree w ere the m re rea weather n l m est nes a e Elk o C k, h o dily i g i o h v been eroded into a bas in whose bottom is now filled and leveled by m n s m r m th ra a materia . s t n as a be seen e to o i l l Thi po i io , y f o po

ra ic m a . X is a re at e low one and rece es the ra n g p p, Pl LII, l iv ly iv d i age from a large area of imperv ious rocks form ing the high peaks It i r the a a nne c m e h s n w near . is as t we e e u s t e by , , p x of f l o po d of o y m r E r an r w ea s and t r a n n r e a phitheate of lkho n d C o p k hei djoi i g idg s . u the cree bed is in m ra na was so t at the stream e re Tho gh k o i l h , h b fo ers n ten carr e w n water as t the m ne a ar e art div io of i d flo i g p i , l g p of ' the precipitation and run-ofl of the northern part of the district finds its a nt the a tere m esto nes in w c it has rm e s u te rranean w y i o l d li , hi h fo d b r It s u w er rem ar e t at no ar e ca e wa wa s . e be s te y ho ld , ho v , k d h l g v a e een un and the sm a er wate rways n te in un er r un h v b fo d, ll o d d g o d exploration are evidently later and not in any way connected with the ore bodies . — Geological relations The ore deposit occurs in a dolom itic lim es tone The forming m assive beds and overlai n by altered shale . strata dip 0 tee the an e ar n r m 35 to t s att tu e e n due to s ply , gl v yi g f o hi i d b i g the uplift and folding of the entire Paleozoic and late r rocks into a series of para llel m ountain folds that extend eastward to the valley of

the M ss u r at wnsen . A s s wn in a re us c a ter all the i o i To d ho p vio h p , sedim entary strata seen in this dis trict form part of the eastern s ide of an arc the s ur rm er ccu e the west m t s ant c ne h , p fo ly o pi d by li b of hi i li .

e n now ccu e the ran te at t . s structu re as b i g o pi d by g i b holi h Thi , n erst r m the su r ace re at ns near the rn m ne is s wn u d ood f o f l io Elkho i , ho I T h r n . e a a c now rm in the cross sectio 2 of Pl . XLVI I h lf h fo s a m onoclinal fold composed of all the various beds seen in the Elkho rn The un rm t the m nocl ne is we er s tur e str ct . di i ifo i y of o i , ho v , di b d by sev eral m inor co rrugations which have been noted in the earlier chap H NI I I O N A N A . 472 E L K O R N MI N G D STR CT , M T

s Th ore e s t ccur in two t ese esser flexures w c ter . e d po i o s of h l , hi h coalesce near the surface and form one relatively broad arch . These t h n h r - r te ntrus n w c arc es tc stee o t e rt . T e a h pi h ply o h g bb o dio i i io , hi h has een escr e in a rev us c a ter rea s t r u the m e b d ib d p io h p , b k h o gh li stones and s a es and has s attere t em sen n out r s and h l , h d h , di g fo k n e rr r r u to gu s of diorite and allied rocks into these i egula b eaks . S ch intrus ions are seen on the surface to the northwest and southeas t O f the m ne and are enc untere in an e rat r r t u n er r un i , o d xplo o y d if d g o d , and t u t e a e no rect c nnect n w t the ore es and do , ho gh h y h v di o io i h bodi not m a e ore w ere t e ccur it is e e e t at the ma n a r k h h y o , b li v d h i g bb o m ass is the source from which the m etallic contents of the deposits n d ese s r r te ntru n are seen in the m ne have bee erived . Th le se dio i i sio s i h n t n n h - in t e lo g sou h exte sio of t e 650 foot level . — h n h Outcro Sout t e m ne u in s s wn i PI. t e p h of i b ild g , ho XLIII , outcrop of the ore body is obscured by m orainal débris and has not h n i h r t been found . It as not bee recognized n t e rail oad cut opposi e the t wn and in act at t at nt the r te brea s acr ss the con o , f h poi dio i k o h tact plane along which the O re body occurs farther north . To t e north of the m ine the outcrop is seen in the rail road cut back of the ° er use w ere the r c s a e a str e N . 50 W . and at boil ho , h o k h v ik of dip ° ° m e e h t-w an r or m te 30 to 50 N E . So 30 f et of t e foo all s d ock dolo i is exposed with 1 50 or 200 feet of thin-bedded argillaceous shales n n i h t h r n lyi g upo t. T e ou crop could not be traced on t e su face beyo d t s o nt but is sa to a een ene r s ect ts at a num hi p i , id h ve b op d by p o p pi b r n ili ifi d o mm he e of poi ts . S c e lim est nes occur i ediately south of t d m on th h Th r re rt the tailings a e south side of t e m ill . e ocks a pa of Keene lim estone and show brecciation and replacem ent by siliceous r I is t ere re nt t t th m n ra n s ti n wate s . t h fo evide ha e i e lizi g olu o s were not confined to the imm ediate channels in which the ores of the Elk rn m ne were e s te t u so far as n wn the ore es ho i d po i d , ho gh , k o , bodi r form ed near by a e not workable . — [f ringing-wall horns tone The ore bodies are sharply delim ited by

a an n wa ar i ace us r c . r c is ens and t u h gi g ll of g ll o o k This o k d e o gh , and c rres on s to w at s m e e sts ca halleflinta but w c in o p d h o g ologi ll , hi h t s re rt has een cal e h rnst ne w n the n m enc ature hi po b l d o o , follo i g o l of n T he r is wn i se usc . b R o c we e e as s n . B ut oes b h o k ll b dd d , ho Pl L , , d not ssess e t er a s a or a slat c ea a e but rea s nt nter po i h h ly y l v g , b k i o spli y m en s al n s m f fine nt n t u rm n ra t a ste o s cu ca m s . f g o g y joi i g , h fo i g bi l as es T he term ar te es not a to it as strict s ea n at n m gilli do pply , ly p ki g th a e or r T - M plies a slaty schistose ock . he characte r of this hanging wall rock is best seen at the -foot level where a cross cut drift has n r n 1 20 eet n r r ul tu bee d ive f i to this ock . A ca ef s dy of the rock was m a e sam es e n c e O ff e er t the stance a n t d , pl b i g pi k d v y foo of di lo g his b n hanging wall . It was found to e hard a d compact and to show no s at c ea a e or racture a t u the r na strat fi t n n l y l v g f , l ho gh o igi l i ca io li es are

wa s ] E L K H O R N M IN E .

a nl seen . It ar es r m a flint s ce us t e carr n 94 er pl i y v i f o y , ili o yp , yi g p cent s ca and 2 er cent m e to er ar ace us r c racti of ili p of li , v y gill o o k p e r m l In the m ne w r n s t s r c ws a a cally fre f o im e . i o ki g hi o k s ho pl ty

' racture w c m t be taken for s at c ea a e but w c is in act f , hi h igh l y l v g , hi h f m m h n due to slight ove ent along t e co tact . Th ree oth er hangi ng -wall crosscuts have been d rive n in the m ine ; one on the 300- t le e n rt a s rt one r m the - t s ut foo v l o h , ho f o foo o h - h r w sto e and one r m the ot s ut e e . T e fi st two ere not p , f o fo o h l v l extended far enough to pas s th rough the slate which form s the hang ing wall of the lode ; but the thi rd at a distance of 60 feet encounte red - n he n O f c nta t a a s a grayish blue lim es tone . O t li e o c soft cl y eam c rs w c n cates au t n m ement or Sl n on the e n oc u , hi h i di f l i g ov ippi g b ddi g

s c a carr es 2 unces s er er ton . m ew at er plane . Thi l y i o of ilv p So h high as sa s we re ta ne r m t s sam e c ntact at the su r ace w e re it is y ob i d f o hi o f , h e se in a ra r a cutt n and a s in a r s ect s a t recent xpo d il o d i g , l o p o p h f ly ese are the n nts at w c t is u er or sec n con sunk . Th o ly poi hi h h pp o d tact is e se but r er e l rat n m t e e som e new ore xpo d , p op xp o io igh d v lop n deposits on this horizo . - — h mi r m r e in he r F oot wall ch lom ite. T e dolo te o a bl which t o e occurs form s the uppermos t m em ber or stratum of the lim estone se ries he rein r m n T he t-wal cr sscuts s designate d the Cem ete y li esto e . foo l o how T h e n is rat this rock to be of very uniform character. e b ddi g her m s e and in a cr sscut r en for a s ta nce er 200 eet r as iv , o d iv di of ov f ho i r zontally a few O bscure partings o stratification planes are seen. They e m ar e m s ar ace us m atter and one t em ccurr n ar k d by fil of gill o , of h , o i g at 35 eet eneat the s ate an n wal the ore is er f b h l h gi g l of body , v y constant in characte r and is com m only called the foot wall of the ore The ar ace us mater a is art the m est ne we er body . gill o i l p of li o , ho v , w no or ken es or th er e ence m em en and sho s clay slic sid o vid of ov t. The roc is a cr sta ne m te m e um and e en ra n us ua k y lli dolo i of di v g i , lly - r e r It i w te but ary n to a u s ra o w c . s so uni hi , v i g bl i h g y y llo olo rt n l formly crystalline that it is properly called a m arble . A pa ial a a y i he em rat r th e sis ma e . . t es n t c ca a e G ca , d by H N S ok h i l l bo o y of ologi l ur e s we the am unt m a nes an car nate to be ract ca S v y , ho d o of g i bo p i lly ' r t e r m the sam e in the diflerent va ie i s f o the m i ne .

M residu ii F i n W ite do om ite contains CO e s ca e 0 etc. ro etc. h l g , , ( l , , , , ,

B ue do om ite contains M C O res idue siica. and iron l l g , , ( l ) ,

Y ow do om ite contains M C O res idue siica and iron etc. ell l g , , ( l , ,

T he yellow rock is the oxidized form found near the ore bodies in It mm n s ws m r r n and the upper part of the m ine . co o ly ho o e i o silica t an the te m te and the racture anes are m ar e b den h whi dolo i , f pl k d v n n dritic film s of m a ga ese . The rock has a very pronou nced granular saccharoidal texture and has en e nate the m ners a san r c S nce it is s m et mes be d sig d by i d o k , i o i o em en as to crum b e un er s t ressure to a w so lo sely c ted l , d ligh p , hite O 474 E LxR R N MIN IN G D IST R IC T , M O N T A N A .

o en wa un to w t at th m ti n sand . N evid ce s fo d sho h e dolo itiza o was the result either of m etamorphism or of th e mineralizing wate rs w ic r uce ore e s t n but it is n erre t at the c em ca h h p od d d po i io , i f d h h i l aracter th roc is r na s t erw se the s rin a e in um ch of e k o igi l , a o h i h k g vol e resulting from dolomitization would h ave left som e evidences of such t n ac io . S trike and dip — T he contact on which the main ore bodies occur

ar es in str e w n to the arc n the strata . etween th e ore v i ik , o i g hi g of B ° W n s i r r r s ts it is . 1 0 . a d t s e nea the st e art er s ut as hoo N hi v y ly ik f h o h , h r s n s c n rm near to th n r shown by t e p o pecti g level . This o fo s ly e ge e a l h r T h e s n h h str ke t e r c s nea . e e s w on e ma t e m ne i of o k by l v l ho t p of i , ll t s n ct rat er c se eav n it . w c ta at but few P y , l LVII , fo o hi o h lo l l i g l r e The e l r r aces and t en on v fo s rt stanc s . e s t e e e Sh w pl , h ho di l v , h fo , o ry a n the war n the s trata and that the c an e in str e is ve pl i ly pi g of , h g ik h not n a ri nta one but affects the tc t e s as we . o ly ho zo l , pi h of fold ll ° The c an es in are ess m ar e ar n r m 35 to The of h g dip l k d , v yi g f o dip the ore body is i ndicate d in the vertical transverse cross section of the d in 1. IV n n m a be n in I r re P L A a B a d a s see P . w e e o body , , y l o LI , h the rails on which the mi ne s kip rests are laid parallel to the dip of

- the hanging wall slate .

T H E O R E S .

— T e r the n r a m i n G eneral character. h o es of m i e a e classed s lli g ores e t n T he rst n ar uart se and c m e r m th and sm l i g ores . fi ki d e q zo o f o e hanging-wall bodies alm ost entirely and embrace only those ores that b r n th m In th ear histor the m ne the r uct can e t eated i e ill . e ly v of i p od was ent re ree m l n but as e t was ta ne the ore c an e i ly f il i g, d p h ob i d h g d in character and could be treated econom ically only by chloridizing h m n r n r in aseness w t t unt l t n . e o c ease e t roas i g T illi g e i d b i h d p h , i a the tim e the m ill shut down no other m ill of this character was known h atura r u t h treatmen treat res so as e . A s t e n es t e c st t ran to o b l l , o of up until at the tim e of closing down it closely approximated the cost m r of treatm ent by the s elte s . T he sm e t n ore c ns sts a ena t et er w t uart s me nc l i g o i of g l , og h i h q z , o zi h r t th en e and a esser am unt yr te . In t e u e ar e m ine bl d , l o of p i pp p of ese m nera s were x e and as a c nse uence the m ne e e th i l o idiz d , o q i yi ld d ar e am unts of car nate ore w c was sent to the sm e ter w en it l g o bo , hi h l h r In the e er e e s nd in the w r carried 20 pe cent of lead . d ep l v l a lo e rt ll he oot-wa ore es n s u e res ccur and t ese pa of a t f ll bodi o ly lphid o o , h ,

when brought to the s urface, contain m uch white lim estone and were d n to the m r N attem t w therefore cobbed by hand an se t s elte . O p as ma e at ore c ncentrat n and the was te ea s the m nes c nta n d o io , h p of i o i - n large bodies of low grade gale a ore .

- Value of the NO average. value can be given for the two kinds

re for the entire m rne . In 1 89 t n ore were mi e of o 7, o s of ll d .

m O R E S or E L KH O R N M IN E S . 475

which showed an av erage silv er content of ou nces and gold unc The r aste ore ha for th re ear a ra o e per ton . o d s e las t th e y s v e ged a t 9 Th n r n o bou 3 ounces of silver pe r ton . e shippi g o e se t t the sm e ters av ra d ur n 1 897 unces s er and l e ge , d i g , o of ilv un w t 2 r n t h e ce er ton 1 e ce t ea . A t e a ra e o of gold p , i h § p of l d v g l r n. se ling prices this gave a v alue of $67 . 78 pe to In the r r he m e r m u e r r tr t ea ly histo y of t in ve y ch rich r o es we e ea ed . h A ssa s s ec a sam es s w er a ues . us art y of p i l pl ho v y hig v l Th , p ly altered galena from the 1 70-foot level as sayed ounces of s ilver

er ton. x e ore m st m as s e ceruss te wit a tt e m a a p O idiz d , o ly iv i h li l l c te r m the ot -wa ore assa e er to n : er hi , f o fo ll body y d , p Silv ,

unces 8 enn we t ea 461 e r cent c er 51 o ; gold , p y igh ; l d, } p ; opp , } er cent p . r n n n Samples of the very zincky ore va y co siderably i sil ver conte ts . O ne r m the t-wa l w c was a r re res entat e f o foo l body , hi h f i ly p iv , n A m assayed 45 ounces of silver a d 2 per cent of lead . sa ple of the r n white blende contai ned 14 ounces of silver pe ton and no lead . O e of the brown zinc blende yielded 1 05 ounces of s ilver per to n and 4 pe r cent n t er s c m en n n n ar e am unts res n us en of lead . A o h pe i co tai i g l g o of i o bl de ” and the ar -c re en e or ac - ac a e 45 er cent of d k olo d bl d , bl k j k , g v p of

nc 3 er cent lea and 1 5 to 20 unces s er. In enera th zi , p of d , o of ilv g l e re s h ne has een s u se to ca rr s er a ues but in the ddi zi b ppo d y high ilv v l , specim ens whose as says are given above the silver val ues are probably

er e ar e not w r m nc u e ra ns a ena. The d iv d l g ly , if holly , f o i l d d g i of g l sm elter returns show that the ore shipped averaged over 20 pe r cent nc and as the ena t is 50 ce nts for eac e r cent er 1 2 in zi , p l y h p ov su e ore it will be seen t at the c st t reatm ent was m ate r a lphid , h o of i lly T h r t r w c n sm a uant t es a e een ncreas e . e c o e i d py i i , of hi h o ly ll q i i h v b lo m l w n n a m m ne is us ua w in s er. s ec a sa e s a i d , lly ilv A p i l p ho i g d ix ture a ena and r te el e on as sa e r cent lead of g l py i yi d d y p of , er cent r n 1 5 e r cent zinc w t one -tent unce p of i o , p of , i h h o of gold h r e r te n and 54 unces s er. A sam e r m t e te p , o of ilv pl f o py i t nat e r body contai ns 3 to 1 0 ounces pe r to n of silver. Al hough iv silve is m m n urrence in e ort ns the e s t no ree of co o occ oxidiz d p io of d po i , f h has so far as the wr ter n ws e er been un . T e rat gold , i k o , v fo d io of gold to silver in the ores is very consta nt and approxim ates n r un e ou ces of silve to 1 o c of gold . Th u n r m the m l is c mm n e twee n 850 and 900 fine the e b llio f o il o o ly b , e r F or s m e ears w e the m l im purity consis ting mos tly of copp . o y hil i l was runn n on ase o re w t ut an s ec a attem t at c se sa n i g b , i ho y p i l p lo vi g, 0 ne the bullion ran as low as 4 0 fi . T he fine particles of ore carried up in pum ping the m i ne waters

as s t r u sett n tan s in w c the s m es are cau t . ese p h o gh li g k , hi h li gh Th slim es have ave ra ged over 200 ounces of silver and 4 to 5 per ce nt of and the waste water r m the c n us e e s s m es carr lead , f o obbi g ho yi ld li y 476 E L KH O N N R N MI I G D ISTR IC T , M O N T A N A .

in 77 un r r g o ces of silve and 4 pe cent of lead . These results show the r tt e c ar r b i l h acte of the rich ore . C ar load lots shipped in 1 894 gave as say s as follows :

S id : Z inc i v r u e ore s e o d ead 14. lph , l , g l , l , O xidized si ver: Z inc siver o d ead l , l , g l , l ,

W R M IN E O K IN G S .

The m ine rk n r wn h r w s a e s on . w c t e w ter h as o i g ho Pl LVII , hi h i een erm tte to re r u e th n es e th r c y e n s M r. e e rme b p i d p od b ki d of K ll y , fo r manage . The m ain working shaft is an incline driven along the s ate - mi n t T he n ne s wn on h l dolo te co tac . i cli goe do t e dip a part of the wa an t n en n n y d he b ds so as to obtai a eas ier slope . It has an ° angle of 35 to the i nclination varying with the undulations of the n h s ate e n a e t at rm s t e r o . T he r is usua s l b ddi g pl h fo o f oof lly olid, and nee s n u r t t m ers for s u rt ca s and la n n d o ly p igh i b ppo , p ggi g bei g u e in he u e r r h m ne ere is no s n n s d t pp pa t of t e i . Th divi io i to com r h or - n a tm ents . T e e is ste in a two at rm s o w c th p hoi d pl fo kip , hi h e m n r n e ca s are ru rect r m the e e s as s wn in PI. . i di ly f o l v l , ho LI In the ater e e m ent the m ne e e s were r en at e er l d v lop of i , l v l d iv v y ' un re e m easure n th ese e els e run out n rt h d d f et d o e slope . Th l v wer o h and s ut r m the s a t but w n to the n rn the end n o h f o h f ; o i g ea ess of li e, the n rt e e s are re at e s rt and no r s ect n has een ne o h l v l l iv ly ho , p o p i g b do n t n n he n er a r en i hat directio . O t south a umb h ve been d iv beyond the ma n ore and one the for a stance near i body , ( di of ly

feet . T he regularity with which the m ain ore body m aintains its position has erm tte t s re at e sim e e rat n and as the uart ose p i d hi l iv ly m xplo io , q z ore bodies all com e to the slate hanging wall the levels are s ure to cut

- t em . O n the c ntrar the c am ers s er ea ore un in the h o y , h b of ilv l d fo d m te ccur w t ut r er or arran ement and as a c nse uence dolo i o i ho o d g , o q the t-wa r sscuts and e rat n w r a n er rre u foo ll c o xplo io o k h ve bee v y i g lar. A n instance the uc m nin and the ac t at the roc s in the of l k of i g , f t h k imm ediate proxim ity of the lead chambers s how no indication of their resence was the r in a e e as t a t -wa t u a p , d iv g of l v l p foo ll body , ho gh shell of lim estone but a little ove r a foot thick separated it f rom the e e l v l .

T he m ne s to es are c urse er rre u ar w n to the e trem e i p of o v y i g l , o i g x ly rre ar s h ar r m sm a l ca t es a i gul hape of t e ore bodies . They v y f o l vi i

few eet acr ss . B t r u w c one can scarce craw f o (Pl LV , ) . h o gh hi h ly l , to reat ca e rns 60 e In enera the an n -wa s ate nee s g v f et high . g l h gi g ll l d

su rt as it has a ten enc to s e off . im e s tu s and ca eces ppo , d y h ll S pl ll p pi

ere a e uate in m st cases . A a n e n er rare w q ( , ) d o Pl LVI , l ggi g b i g v y ly r W h e rs ecam er r e as a ens w en ne essa . en t h m e a c y h e c a b b v y l g , h pp h the t-wa o e lie a a nst the an n -wa uart th e t m ber n foo ll b di s g i h gi g ll q z , i i g f t ese r at ca erns ecam e a ser us m atter and the few stulls at o h g e v b io ,

wn nJ E LKH O R N M IN E . 477

first use had to be s u em ente w t t ers unt as s wn in d ppl d i h o h il , ho

P1. L V A the t m ers were so c se one c ul scarce ass etween , , i b lo o d ly p b t m he .

O R E S H O O T S A N D C H A M B E R S .

G E N E R A L C H A R AC T E R S .

The Elkhorn mi ne shows ore bodies of cont ras ted mineralogical char n The ma n ore c anne s w acter and different geological relatio s . i h l follo the s ate- m te c ntact and as the s ate n ar a rm s the an l dolo i o , l i v i bly fo h g $ ” e - ing wall th ey are co nveniently d s ignated hanging wall bodies . The ore is quartzose and is nearly all low enough in the baser m etals to $ ” h r n th o - e r r be m e . n t e t e a e a ca e a cham e s o t ill d O o h h d , ll d l d b foo wall bodies occur in the dolom ite and are usually separate d from the an n -wal s ts a reate r or ess t nes T h gi g l depo i by g l hick s of that rock . he ore can not be m e as it c ns sts s er- ear n a ena w t en ill d , o i of ilv b i g g l i h bl de d m e r s st nct n ccurrence and c r or an so py ite . Thi di i io of o haracte of e is not n ar a e s nce s m e m n ore ccurs in the t-wa o s i v i bl , i o illi g o foo ll b die d en h n -w r an som e gal a with t e ha ging all qua tz .

MA IN O R E S H O O T S .

The occurrence of the great m as ses of o re co ns tituting the hanging wall ore shoots is bes t appreciated by reference to the m ap of the under h r r un w r n s Pl. and t e trans ve se sect ns acr ss the g o d o ki g , LVII , io o d T h m ne m e n e os ts . A an B . e a a r nta ro d p i , Pl LIV, i p, b i g ho izo l p eet n es not e a e r ect true cture the s a e and e tent io , do giv p f ly pi of h p x the o re d es win to t e r but the ata for a r ect n or a of bo i , o g h i dip, d p oj io longitudinal section o n the s tratification plane of the rocks were not n obtai able . It is apparent from the m ap that the re are two m ain channels of ore e sit n one to the n rt the m ne s a t the t er to the sout d po io , o h of i h f , o h h , atte r n m r r e r in the e uc the a e . t are r r e u ar s a e l b i g h l g Bo h v y i g l h p , and their plan presents a striking resem blance to the m aps of the rn h h m h h fam ous cave s of t e wo rld . T e ap shows t at t e south ore shoot is m uc reater s e t an t at n rth the s a t and that it has of h g iz h h o of h f , r r r re n r som ewhat m o e regula bounda ies . Both s hoots a co tinuous f om the sur ace wn to the 50- t e e but ecrease in s e wnwar f do foo l v l , d iz do d nd na nc out T he s ts s w a str n er ore co nnect n a fi lly pi h . hoo ho i g of i g t em in the u er art the m ne n ta the 350 e e w ere the h pp p of i , o bly l v l , h centers or a es the two s ts are a ut 300 eet a art but as the x of hoo bo f p , s er e in e t h nn wa not se r e hoots div g d p h t e co ection s ob v d . — F orm s of th e ore shoots It is diffi cult to des cribe or even to s how by

fi ures the rm the ore sh ts s nce t e re er rre ular. r ss g fo of oo , i h y a v y i g C o

ect ns the two s ts are i en in . L IV A and B . ese were s io of hoo g v Pl , Th r are r m the m ne m a s t et e r w t s etc es and m eas ure p ep d f o i p , og h i h k h E L O I 478 KH R N M N IN G D IST R ICT , M O N T A N A . m ents es pecially m ade for the purpose and com piled under the direction W m D v e who ee r n er r un M r. a a v has n in c a e the u of illi , b h g of d g o d wo rk e ver s ince the property becam e a r A he is am ar w t p oduci ng m ine . s f ili i h every foot of the m i ne and knows all its peculiarities the sketches are believed to a r h ve a good deg ee of accura cy . T he cross sectio ns of the two ore s hoots are proj ecte d from the planes s how n on the m ne m a re l n T ese nes i p by d i es . h li change in direction at s everal poi nts in order to give a repres entative cross sec t n th e ore b and the nts w ere io of ody , poi h s uch change is m ade are shown on the sect n io s by vertical red li nes . 0 0 1 Th cross n r t - e sect s the s ut o e s , 0 io of o h hoo 5 6 w in 3 n e b tter ea s n . 7 c a e ho fig , o v y id of the s ize and s hape of the ore body at the two levels selected as representative of h r h t e upper and lowe r pa ts of t e deposit . ese sect ns are true cr ss sect ns Th io o io , si nce they are proj ected on planes cutting the deposit at exactly a right angle to the irr u . u r tes the esse r e dip Fig . 74 ill st a l g

larities of one of the s m aller ore s hoots . From a s tudy of thes e cross s ections and the horizontal proj ection of the ore odies it is a arent t at w e the ore b , pp h hil h t es are rre u ar in size and s a e u bodi i g l h p o s t e us ua y lie a ainst the tt m the e h y ll g bo o of h t s ate and ence a e a er re u ar r e l , h h v v y g l oof m or an n wal that t e r tt m is m re o h gi g l , h i bo o o s n rre u ar but c n rm s In the ar er o i g l , o fo l g bod l t c ies to a ane aral e to the an n wal . e pl p l l h gi g l 8- There IS a m arked tending to ward an ellip . 3 7 t ca cr ss sect n the ore es e n . i l o io , bodi b i g o l F er m uc w e r t an t e are t c so v y h id h h y hi k , that the greates t axis of the ellipse is parallel to the strike of the inclos ing T he s e the o re o es ar es s trata . iz of b di v i rapidly f rom place to place horizontally h In enera it is rea t and on t e dip . g l g tes h in the m iddle depths of t e m ine .

c — P e uliarities of tire [ranging walk The co ntact between the slate and lim estone is ver s ar and in the m ne w r n s the v h p , i o ki g

P L A T E L V I I I .

-W A L L E A H H A N G IN G B R C C I F R O M A P H O T O G R AP . ,

This s e cim en rom th e - oot ev e i ustrates th e rat e r rare slate breccia ound p , f f l l , ll h f r in m h h n n - r di Th i ra m n ove ly g so e parts of t e a gi g wall o e bo es . e l ght f g e ts are i - r T ir n in i in altered h ornstones of a warm p nkish g ay tinge . h e ba d g s part due n in f i h k to ori ina sedim entation a d art to secondar lm s o si ca . T e dar er g l , p y fi l ar t r u ri iz T h i t ra m nt ntain nsid ra pieces e ho o ghly py t ed . e l gh f g e s co co e ble h n f pyrite as a v ery fine dust scatt ered th rough th e rocks . T e arrange m e t o th e ra m ents is wit out de nite order but ad acent ieces fit eac ot er c ose f g h fi , j p h h l ly n m m r h f h i n Th and sh ow a m ov em e t fro th e botto to wa d t e top o t e spec m e . e inte rspaces are in part filled with gray crystalline quartz ca rrying pyrite and n T h r r aces t n th e r m ents are n sca tte red grains of gale a. e la ge Sp be wee f ag o ly partly filled and show open cavities lined with proj ecting crysta ls of lim pid r z T r i som d nse r r s in ua rtz ic is ev id nt ua t . e e s a so e e c toc ta e w e q h l yp y ll q , h h ly th e replacem ent m ate rial form ed by m etasom atic inte rchange with th e horn ‘ i n li stone ra m ents . T h e a ena rains w ic are b ack in th e i ustrat o e in f g g l g , h h l ll , r r n i or rit Th th e com b quartz and a e su rou ded b y a th n pellicle shell of py e . e d r in i r sta in I ur f v ra ner pyrite is ete m ed as such by ts c y ll e faces . t occ s o se e l ge a y n n T h tions th e atest or m bein ound on th e uartz cr sta s i i th e v u s . e , l f g f q l l g g s ecim en s ows no b ende in t is res ect dif erin rom m ate ria rom th e brec p h l , h p f g f l f P t-m in ra att rin i indi d b a cias form ed of lim estone fragm ents . os e l sh e g s cate y ittl v ein t of uartz w ic au ts th e s a e ra m ents and th e ore as ma be l e le q h h f l h l f g , y - seen on th e right hand side of th e specim en .

O N 482 E L KH R N M IN IN G D IST R IC T , M O N T A A . so uthern leg o r branch rapidly pinching to a narrow pipe of low-grade Th m n or n r rn r n nt n wnwar but als con ore . e a t e a c c u i o h b h o i es do d , o tracts in si e and ec m es low ra e in c aracter but ex an s a a n z b o g d h , p d g i betwee n the and levels into a cham ber-like body of lead r T he s t is nt n us nd m a nta ns its enera rect n o e . hoo co i uo a i i g l di io of and tc to a e t eet m easure on the s e w ere it dip pi h d p h of f , d lop , h

m a es an a ru t turn tc n to the s ut east . s wer art k b p , pi hi g o h Thi lo p of the s oo t is we er sm al cr ss sect n and the ore is e trem e h , ho v , of l o io x ly low in s er so t at the ore has not een st e e w the ilv , h body b op d b lo W er i r s the s t a ut 0 eet e w the e ust e e . t c ses a 8 e l v l h e o h f , bo f b lo l v l j m ent ne the s t is 3 eet w e and a few t ns ore ta en out io d , hoo } f id , o of k T he n in si nking the s haft averaged 50 ounces in s ilver per ton . i dica tions point to connection between this and the ore body s hown h h r nt nues wn to a int 25 e . T e atte c e t on t e m ap (Pl . LVII) l o i do po f e w the e e w ere it s u en c an es to a sm a m ass b lo l v l , h dd ly h g ll of low - ra e uart w c w ere it was cut on the e e was but g d q z , hi h , h l v l , 2 eet t c and 5 eet n and carr e nly a e er n ore f hi k f lo g, i d o p pp i g of m inerals . S outh ore show — This rem arkable ore shoot has yielded fully two In the u er e e s it c nnects thirds of the total product of the m ine . pp l v l o w t the n rt s t as a rea n te but assu m es its n ua i h o h hoo , l dy o d , i divid l h h n rt ore s t it term n ra te r e w t e e . L t e ates cha c b lo 650 lev l ike o h hoo , i

a a ns t a flexure the strata w c is m st m ar e on the s ut s e . g i of , hi h o k d o h id e n in the s ate m ar s its n rt ern m t a s t u t s flex ure A b d l k o h li i , l o , ho gh hi

is want n in the u er e e s w ere the two s ts c nnect . i g pp l v l , h hoo o The s outh ore s hoot is a continuous t runk channel from the surface wn to a e t eet m easure on the nc ine and t r u do d p h of f , d i l , h o gh n rm t cr ss e t n and out this length maintains its u ifo i y of o s c io dip . e w the e e the s t ar es in c aracter an ec m es sma er B lo l v l hoo v i h d b o ll , n to h but is conti nuously traceable and workable dow t e level . T he thi ckness of the shoot varies from a few feet in the upperm ost w r n s w ere it w l a era e a t c ness er a s 8 eet uart o ki g , h i l v g hi k of p h p f of q z ,

e r at ns are we s wn in the to 40 feet in the stope . Th se va i io ll ho

s e I . cros s ction B of P . LIV h nst the s ate e n an A s a rule th e top o f t e ore body lies agai l b ddi g pl e .

s is we s wn in the cr ss sect n ust m ent ne as we as in Thi ll ho o io j io d , ll - n It w be seen r m the sect n 7 7 a d 5 . t s e e n in fi s . 3 1 7 ho giv g , , ill f o io t at t s e n ane w c rm s the an n wal the ore b h hi b ddi g pl , hi h fo h gi g l of ody , ' s ws un u at ns in and that the ore bod 229 thickes t benea th 8 1z ht ho d l io dip, y g T h s n fi nce t is as s at n i m ent ne arches of the sla te. e ig i ca of h oci io s io d r h r later in discus s i ng the m anne of form ation of t e o e bodies . r c n B s ws t at the ore s t ea es the T he c ss se t . o io (Pl LIV , ) ho h hoo l v s ate etwee n the and ev e s the m ne but t e re is a s a l b l l of i , h l o h a n w r m th n connection along t e s l te co tact a ay f o e pla e of the section.

the n rt e rn ore s t th s one nc es u in o ne ace be w Like o h hoo . i pi h p pl ( lo wa v - E S O O T E L K O R N E 4 3 1 O R H S or H M IN . 8

the e el and is but 8 eet in reatest am eter but wi ens out l v ) f g di , d e w r t e r e Be w the e b lo i nto a ela iv ly b oad m ass 6 f et thick . lo l vel the ore is cut off a a ns t a s t au t in the an n -wa s ate a g i ligh f l h gi g ll l , branch of the ore body which was too low grade and too zincky to w r ass n off to the n rt and nt a low- ra e m as s rit c o k , p i g o h i o g d of py i or T r or - e r e and nearly barre n quartz . he floo foot wall sid of the o e s t t u h ess re u ar t an the r or an n wa is a r hoo , ho g l g l h oof h gi g ll , f i ly un rm and w ere the ore bod is t c es t c rres o nds to the o s i ifo . h v hi k o p p t n a - nc to 2 - nc ar ace us art n or be in ane s ee n io of é i h i h gill o p i g dd g pl , in the dolomite in the foot-wall crosscuts of the m ine to be of ve ry n ur co stant occ rence . — Intem zedia te ore bodies A n exte nsion of the connected o re s hoots runs a s hort dista nce downward as an interm ediate ore s h oot but plays w e r Is out and disappears in depth . Belo th level th e e another

nterm e ate o re bo w c e lde a c ns erab e am unt of o re i di dy , hi h yi d o id l o and w c is interes t n s ince at t s e t the ore is not x e hi h i g , hi d p h o idiz d

s s and its origi nal character m av be obs erved . T he hoo t lies again t the s ate fo r the m st art but in s m e aces is e the wer art o f t l o p , o pl lik lo p he s t se r t r m th s te b a m u s t a a e e a v s e te . s or o h hoo , p d f o l h ll of dolo i Thi e body is pro bably form ed by the com ing together o f the foot-wall ore s hoot (m arked Wilson s hoot on the m ap) and the deflected north

r s th b th is o ore shoot which c oss e e s haft elow e level . Th re shoo t as re us n te a s out e w the e e and as ses , p vio ly o d , pl y b lo l v l . p nt e ne r arr n rt 2 e t t c and ee t o n i o a pip of a ly b e qua z fe hi k 5 f l g .

' ' - r i h a o h m n s ho ots . s e r at G enw al characte f t e re f t e a i ore The, g e ore

d s n s r T he u rt w shoo ts are c m ose e se t a uart e o e . a en o p of i lly q zo q z , h not a te red the dat n the m eta ic s u e in it is us ua l by oxi io of ll lphid , lly

i t ra c ear or ass and rat er m e ium co ars e nes s l gh g y , l gl y , of h d of cr s tallization but ar -b uis h e r ens e cr t cry s t a ne s i ca v ; d k l , v y d yp o lli li

r A e cr ed ate r t r is er m arke n co a s ccu s . s s b e e a a d ns tant l o o d i l , h v y d association of quartzose m illing o re when the ore bodv ‘ lies agai ns t the s ate and o f s l er- ea s m e lt n ore w ere the ore es w l , i v l d i g h body li holly i m s di t n t n i not in ria e c ns er n the te . s c s a a e dolo i Thi i io v bl , o id bl masses uart se o re av n been und in the t -wa ore b es of q zo h i g fo foo ll odi , and of galena o r its decom positio n p rod ucts in the hanging-wall ore b h i r in d es ut t e s tatem ent s t ue a e nera wa . bo i , g l y

T he ore es c ns st ess ent al near so id uart m s t ree bodi o i i ly of ly l q z , o ly f

s s s s t n r l t ce t ns to t i from included m a e of lim e o e o of s a e . Ex p io h s

ccur in the 50 s t es w e re the uart cem ents m es t ne ra o op , h q z li o f g m ents and where the slate is brecciated and the fragm ents are cem ented s Th e T h firs t is es s c m m n t an o ne w u s u o e . s ec by ore . e l o o h o ld pp p i m l ‘ 1 s ws an e am e th s and the s ate recc a is en P . LX ho x pl of i , l b i h m ine was r ucin ea n th I I. W e t e e shown in P1. LV I hil p od g h vily o ly

o re was e tracte and m uc low- ra e o re was eft in ace good x d , h g d l pl which could have been m ined at a profit befo re the depreciation of 484 E L KH O R N M IN IN G D IS T R I C T , M O N T A N A . si er but c u not a the c st e t ra tion ur n h a lv , o ld p y o of x c d i g t e l st three ears re us to the c s n wn the r rt F or t s r a n y p vio lo i g do of p ope y . hi e so the extent of these lo w-grade portions of the ore shoots and their oss e nnect n wnwar t r u the m te is uncertain T h p ibl co io do d h o gh dolo i . e q uartz ore of the hanging-wall ore s hoots was m ore or less oxidized and ree m n wn to th - w i f illi g do e foot level . Belo this poi nt t stea ncreas e in aseness w n to the resence a e na and dily i d b , o i g p of g l r n th u r tet a hedrite . I e pper pa ts of the ore body thes e m inerals have ee n art a tere and eac e out r m th r It i ter b p ly l d l h d f o e o e . s of in est ” $ n t s he r r l t h to e a t u e sa s M . e e at t e actua a ue th o , l , y K l y , h l v l of e ore its elf did not dim inis h as depth was obtained and that the ore co ntai ns as m uch or m ore s ilver than that which was found at higher

e e s a t u it was ass c ate w t a ar er r rt n l v l , l ho gh o i d i h l g p opo io of base m etal . O xidation exte nded deeper along the hanging -wall bodies than in the t -wa ea cham ers as the s ut ns natura tra e e wn foo ll l d b , ol io lly v l d do t en on the - e r the slate contac . Ev foot lev l the f eshly cut quartz es s w vellow t har e s ts ab ut a ena ra ns and reen bodi ho li g po o g l g i , g in in e r arb na e s ta g of copp c o t about particles of tet rah ed rite . T he

r te is rus t and th uart is r us a ut the m n ra s ust n py i y e q z po o bo i e l j oted . s a te rat n was not o n a ar e sca e as it was in the u er e e s Thi l io l g l , pp l v l , b t it c ear h ws t at i in wa er n tr t u l ly s o h oxid z g t s pe e ated o this depth . The ore bodies above cons is t m ainly of oxidized o res carryi ng s om e

rite ccas na res ua nuc e a e na but s w no efina e y , o io lly id l l i of g l , ho d bl

n r es e res c an e in e t to t e in c r silver m i e al s . Th o h g d p h hos whi h tet a be r te a ears and t s is u n u te y the m nera r m w c the d i pp , hi do b dl i l f o hi h r a e e ee n er e in h u r r h r i silve v l u s hav b d iv d t e ppe pa t of t e o e bod es .

A s a ru e the ore s ws no wel -defined structure but c ns s ts l ho l , o i of r r r interlocking g rains without bandi ng or c ustification . Ra ely the e is a bandi ng due tothe replacem ent of lim es tone fragm ents by dark c re as e r - uart the nter en n s ace be n fi e w t w te olo d j p y q z , i v i g p i g ll d i h hi

z h - uart so t at t e ar e r c re v ariet is in str n c ntras t to it . q , h d k olo d v o g o t er n tes o n t ese c aracte rs are resente in an t er c a te r O h o h h p d o h h p , where the evidence s howing that the ore was deposited as a replace m is en In one ce er r t c re ccur m ent of dolo ite fully giv . pla wh e py i i o o s lying agains t th e slate the ore shows a well -m arked bandi ng parallel h n h to t e beddi g plane of t e s late . In the stopes galena was found in the q uartz lying agains t the s ate but as a ru e w ere a ena ccurs it is c nfined to the m te l , l h g l o o dolo i d i n n r an s not fou d i q ua tz .

ISO L AT E D O R E BO D IE S IN T H E D O L O M IT E O R F O O T -W A L L L E A D

C H A M B E R S .

— O ccurrence Besides th e well -defined and continuous ore s hoots of the m ine which have j us t been des cribe d there we re a num ber of detached o re bodies of highly argentife rous lead o re lying encas ed in

E L K H O 486 R N M IN I N G D IS T R IC T , M O N T A N A .

wall w c is a t n s eam ar l ace us m es to ne r m nc to 2 , hi h hi of gi l o li f o i h nc es in t c ness ccurr n at a stance ab ut 35 to 50 eet r m i h hi k , o i g di of o f f o n w in the ha ging all . This occurrence is shown the vertical cros s sec n re res e nt on d d i h n r t s e PI. A an B an n t e sect s ac ss the io p d LIV , , io o e ts s w n T he n n or s n on fi s . 73 74 a d 75 . s fica ce t s hoo ho g , , ig i of hi foot

— F m. 76. S ection across th e ore od of th e Elkh orn m1ne s howin t ical form s of re lacement b y , g yp p d si epo t . wa l es In the act t at w n to its ar ace us c aracter it w u l li f h , o i g gill o h , o ld lim it replace m e nt of the lim es tone and therefore form a som ewhat regu lar t w he r h -wa es foo all to thes e lead cham bers . T oof of t e foot ll bodi is rre ul r nd s In he te a a no efin te m t ha een un . t m i g , d i li i b fo d dolo i , p rongs and fingers exte nd o ut f rom the m ai n body in various direc t ns n an e t rem e rre u ar r fi e w en t e are seen in cr ss io , givi g x ly i g l p o l h h y o

P L A T E L I X .

SL A B O F O R E F R O M T H E W IL SO N S H O O T .

T is late s ows oto ra h s on a reduced sca e of bot sides of a s ab of ore rom h p h ph g p , l , h l f o T h w i r h i nd r k nd the Wilson ore sho t . e h te a eas are t e wh te dolom ite sa oc a th e dark areas are ore consistin of a e na e ow b ende and a v er itt e g g l , y ll l , y l l ri py te . A s h ows th e dolom ite fractured into s labs by parallel planes and th e slabs broken into re ctan u ar b ocks b cross ractures wh ic as a ru e do not e xte nd g l l y f , h , l , t rou m or t a on s a In th e ower rt of th im n m nt h gh e h n e l b . l pa e S pec e replace e h as one far enou to ob iterate th e rocks and on nuc ea m asses of im e g gh l , ly l l l

stone are seen. In th e u e r art th e ractures contain a ena and b ende pp p f g l l , which h as penetrate d along th e m inutest cracks and by replacem ent h as n ar h o nin and orm d r e l ged t e pe g f e th e laye s seen. s ows th e re versed side of th e sam s cim n w i i i k B e e e c s a s ab about an inc t c . h p , h h l h h In this specim en th e lower blocks of white dolom ite which are so cons picuous n h r t r a a m ost m r T h i o t e fi s figu e re l co pletely eplaced . e spec m en shows that itt e cracks e x tend into the b ocks of w ite do om ite a nd t at re acem ent takes l l l h l , h pl ace ra id in th e inte rior of th e m as s and not a one rom th e outs ide so pl p ly l f , that while th e corners are rounded and progress ive replacem ent is shown along th e bo rders th e inte rior is attacked until only a sponge is left of th e white do om ite l .

488

wm . ) O R E BonIE s or E L KH O R N M IN E . 489

h ns s wn i r s -wa ct i n T e s ect n . are t ca fo t e t l se o . io ho Pl LIV ypi l h e foo l

o es t u t e actua re res e nt the - e e an n -wa o re b di , ho gh h y lly p l v l h gi g ll

ot es e t -wa l bod es n ar a S w a c nnect n at s m e sho . Th foo l i i v i bly ho o io o

r - r c se nn t n r poi nt with the m ai n o e bod y o e hannels . The co ec io s a e str ngs o f a e na o re as s in nto a m ture a e na and q uart and i g l p g i ix of g l z , thi s into purclv q uart z orc co nnecting with the ha nging -wall quartz d bo y . ' ‘ — I he o re fo rm i ng thes e lea d cham be rs is no t a solid m ass a e na o r its a te rat n r ducts b ut co ns s ts o f a recc a w te of g l l io p o , i b i of hi

w i . r s b e o d o m te m n nt o Pl. m eti s or y ell ol f ag e t cem e ed y r ( LIX) . So m e the u ppe r pa rt is a lim es to ne breccia with the inte rs paces filled with b ue uartz in w ich cas e the uart carr es rom 50 to 1 0 0 o unces o f l q , h q z i f

ho h i m s w o r n b e ne r s o re c er t u t a v n eco a m a . S uc arr es S ilv , g ho g iz l i l h i

— l n - t F IG . 76 F a e of Wiso ore bod foo l l f lk m in . c y, eve stapes o E h orn e . no lead but when broken down is low grade and will run but 1 5 ounces si er r o n In the er h ts the of lv pe t . upp lead cham bers t e higher par of ore b es were e to lea car o nates s w n a s t c rat n odi oxidiz d d b , ho i g ligh olo io r m c er ca r nates . ery te n a s t ere was a uart crust f o opp bo V of , l o , h q z un as a s e a ut certa n arts the ore o us ua near the fo d h ll bo i p of b dy , lly

to . In enera we er the ea res are m te a ru t m e p g l , ho v , l d o li i d b p ly by li st ne o r low - ra e o re r m w c str n s and fin ers the a e na o , by g d f o hi h i g g of g l and inc e ten o ut n ra k m e t ne No t-wa z x d i to c c s of the white li s o . foo ll es a r 0- bodi h ve eve been found in the m i ne above the 55 foot le vel . A n expos ure s howing the tvpical characte r of the filli ng of these -w r m fl s h - ot a c am e s is s wn in P I. m a e r a a t fo ll h b ho LX , d f o ligh t T he tua ac ustrat w s n . Pe ri . ac e e a pho ograph take by M r. C . F a s l f ill d in the - l n rm at The ew s ws t at the l m e le ve i te edi e stope . vi ho h i 4 90 E L K H O R N M IN IN G D IS T R IC T , M O N T A N A . sto ne c s are in art an u ar and in art r un e and t u blo k p g l p o d d , ho gh positive proof that the ore is fo rm ed by the replacem ent of lim estone is aff r e un e r r un w r n s b num er us s ec m ens and o d d by d g o d o ki g , y o p i , t n sect ns the ore seen u er the m cr sc e art the ore by hi io of nd i o op , p of body appears to be due to the filli ng of the open spaces betwee n the n m en A n m r e t a t n r n lim esto e frag ts . y a k d solven c io would have ou ded off the s ar an es and estr e the n t n s ers m est ne h p gl d oy d lo g, hi liv of li o w n in h r r w c are s t e ctu e . n t e a e S n in fi 6 w i hi h ho pi A o h f c how g . 7 hich s a re t re ro u ti n an t er s h- t r di c p d c o of o h fla ligh photog aph taken by M r. P earis s ws r e n m est ne w c in the wer art the ew , ho b ok li o hi h lo p of vi is a breccia and in the upper part has been flexed and crushed s uffi cientl for the s ut ns to enetrate a n the ractures cem ent n y ol io p lo g f , i g r i ce in h r the pieces together . The e s eviden t e a rangem e nt of the m estone b c s not n the tear n a art the mest ne in li lo k , o ly of i g p of li o aral e ates but a s m em ent al n the ractures w c p l l pl , l o of ov o g f by hi h e twi t nd r r n n these blocks ar s ed a thei o ie tatio des troyed . ' — D escwp tion ( f a f oot-wall ore body M os t of the foot-wall bodi es ’ were ract ca m m ed out at the t m e the wr te r s is t the ore p i lly i of i v i , that was left be ing found along the sides of the ore bodies or in small e n r n s th m a n c m r rtun t f proj cti g p o g of e i ha be s . A good oppo i y or an e am nat n an o re b t s n was we er aff r e th y , y e x i io of od of hi ki d , ho v o d d b openi ng up and ext ractio n of an ore body lying between the two m ain

s r ha ee n ca he W n d ore s ts . o e s e t s S t an es hoo Thi body b ll d il o hoo , li in h l m te tween the s ut ore s t and th ma n t e do o i be o h hoo e i shaft . This un e t -wal bodies in eneral is not rect e n or body , lik foo l g , di ly b hi d un erneat the m a n ore s t but es off to one s e and at a reater d h i hoo , li id g n r m e s n an m s h - l dista ce f o th late co tact th o t of t e foot wa l bodies . Its enera c aracters we er are s m ar t u it c nta ns a ar er g l h , ho v , i il , ho gh o i l g T re ns proportionof quartzos e ore . he o body begi as a spur of the - r main south ore shoot near the foot level . This s pu passes off i nto the foot - wall dolom ite as a pipe or chim ney and opens out Into a r a and c am er r m w c ar e am unts ore were b o d high h b , f o hi h l g o of I rm a c nt nu us t rre u ar r r m e racte . t s u o e a xt d fo o i o , ho gh i g l , body f o point below the -foot level to a short distance below the e e its art e r e te ns n is uncerta n as the ore at t at nt was l v l ; f h x io i , h poi too ase to m ne t u it es rect er the u er art b i , ho gh li di ly ov pp p of r d nd r b nnects wit it l w- r r another o e bo v a p oba ly co h by o g ade o e . I t n c rres o n s to a ec e en in the strata w ic e e s ts loca io o p d d id d b d , h h d v lop into sm all cross faults throwing the slate hanging wall in the -650 and h u ts are a ar nt in th s but are i s ate r els . ese a e e ate s lowe le v T f l pp l , d ip d in t he lim es tone In a se ries of parallel fractures or in a sim ple crushi ng h T he c nnect n wit the ma n s ut ore s t S ws of t e rock . o io h i o h hoo ho d e uart w c c an es in the l m est ne to e porous an oxidiz d q z , hi h h g i o oxidiz d l ead ore co ns is ting of ceruss ite and oth er products of decom posi r ar ter or c nt nu tion carrying high values in silve . This ch ac of e o i es

wa s ] O R E BO D IE S O F E L KH O R N MIN E . 491

- wnwar to the - t e e the s er c nte nts be n er do d foo l v l , ilv o i g v y un rm e ce t on the r e rs the e s t w ere it ec m es wer ifo x p bo d of d po i , h b o lo

s ec s n m s O s h in grade . Thi d ompo ed ore contai ned ucleal as es f ulp ides in the upper part and changes to a solid body of galena with ble nde nd r te ab ut 200 eet e w the to T he sul e ore c nt nue a py i o f b lo p. phid o i d wnwar but ecreas e radua in s er c ntents c rres n n do d , d d g lly ilv o , o po di g to a decrease in amount of galena and a larger am ount of zi nc and r te unt na at a nt 1 0 to 1 5 eet e w the 650 e el the py i , il fi lly , poi f b lo l v , ore c ns ste ar e ac - ac e n e w t r n r te and carr e o i d l g ly of bl k j k (bl d ) i h i o py i , i d T h or er ns from 1 0 to 1 5 ounces of s ilver per ton. e big e chamb co is ted ma n s n ore the c aracter ust n ted the u er 6 eet i ly of hippi g of h j o , pp f , we er e n a r c e ore t at was ta en off for m n and ho v , b i g i h oxid h k illi g , rich bunches and s treaks of this ore were found along fracture planes n i n had a extendi g nto the lim estone above the ore body . O xidatio lso e ten e a s rt s tance wn the s es the ore a ter n the x d d ho di do id of body , l i g u rt or T he to o ter po ion of the e and the lim es tone adjacent to it . p of

' the m ain cham ber of the shoot difi ered som ewhat from this in S howing a c ean r w te m te san -r c ev an rust sta n or l oof of hi dolo i d o k d oid of y , i , t er e ence at n and the uart un near the to the o h vid of oxid io , q z fo d p of ore od w w te and re us In the n rt r n t s b y as hi vit o . o h p o g of hi big c am er t ere was s me at n th m te e n e w w t h b h o oxid io , e dolo i b i g y llo i h r m dend itic anganes e m arkings and the quartz carrying some oxides . T he r ch e ore wh c as ust n te was ta en off for m n i oxid , i h , j o d , k illi g , lay against the roof of the ore body and rested upon the shippi ng ore th o t- T h u rt a r wn and wa r l n and e wa s e . e a w s te of fo ll id q z b o y ooki g , a arent ca rr e no a ues t u it assa e 75 unces in s er and pp ly i d v l , ho gh y d o ilv U n rn t the a tere a r form ed a very good milling ore . de ea h l d le d o es t ere was us ua m re o r es s su e ore w c a era e er 200 h lly o l lphid , hi h v g d ov n s in r er to n The r tic r nt w c the o re s u ce s e . o e o t o ilv p py i , i o hi h h o in r in n as se e t a e reat a ue . s o u t e en e p d d p h , v i d g ly v l Thi do b d p d d somew at u n the r ort n a ena w ch it c nta ne but a s h po p op io of g l hi o i d , l o

s in n r n h consi ted a variatio in the silve co te nts of t e pyrite itself . T he size of thes e foot-wall workings is often somewhat larger than that of the actua ore s nce s tr n s r c alena e ten out nt the l body , i i g of i h g x d i o mes t ne art cu ar w ere the ore b we es out at the s es li o , p i l ly h ody dg id i e w in th l he t- and ends . This s w ll sho n e ower end of t foo wall sto e w ere the ore bo is 4 eet in the wer art w t ert p , h dy f high lo p , i h v i m t cal s ide walls showing no stringers of ore running i nto the . A the wer end w e re the ore en s in a c et the m est ne v e r n it , y lo , h d po k li o o l i g shows s treaks and film s running out i nto the yellow dolom ite formi ng mm n h r the o m te is an n cat n the roof . C o o ly t e yellow colo of d lo i i di io the r m t ore bodies as it is due to at n w n of p oxi i y of , oxid io follo i g d wn the o rders the ore s ots and it is sa to a e mar e the o b of ho , id h v k d

r r s n A s n t n to t s it s u ccu rence the c e t a e a . a e ce o of i h g l x p io hi , ho ld be noted that in the stopes the ore found in the white lim es tone 492 E L K H O R N M IN IN G D IS T R I T A C T , M O N N A . and as s c ate with the w ite art w he r t re u as t c es o . s ho o i d h q z i h Thi , w e e r is e a ne b the ccurre nce te rah r te in h I t e t e ore . n v , xpl i d y o of d i gene ral it m av be s tated that in all the ore bodies the proportion of

r te ncreas es in the wer art and in s m e cas es the lo w v a ues O f py i i lo p , o l t s r te re ente a t r u e rat n the ore b so t at hi py i p v d ho o gh xplo io of ody , h its actua m ts and wer e tens ns are not fin n w H ow l li i lo x io de itely k o n. e er a r y t r u r s ect n on the s ut e e s has e m ns v . f i l ho o gh p o p i g o h l v l d o trated that the o re bodies have pi nch ed out at that depth and that no

s s large m a es of payable ore occur. C o ns iderable s pace has been devoted to a description of the foot-wall

ie s as it is e ie e t at the v are m erel arts the m a n o re bod . b l v d h y p of i s hoots and s how in the s t ructure evidence of the m anner of fo rm atio n

h d In the am be rs the m es t ne has een re of t e epos it . lead ch li o b placed m ainlv b a ena in the m ain o re s ts uart carr n tet v g l ; hoo , by q z y i g ra bedrite .

N A T U R E O F T H E O R E D E P O S IT .

ST R U C T U R A L R E L A T IO N S O F T H E O R E S H O O T S .

A n exam i nation of the m ine map shows a very i nteresting co nnec tion betwee n the occurrence of the ore shoots and the presence of flex ures in h r t W t r r t e s t a a . i h a e excep tio ns ' the m i ne levels are driven a n the s late -l m t ne c n lo g i es o o tact. T he m ap there fore S hows ho rizontal

sect ns the c ntact anes and io of o pl , the curves in the level s correspond r n I i to flexu es i the rocks . t s ap parent from th e m ine m ap that the s trata are bent by two flexures whose

— f r F G 7 . D i m sh in occurre nce o o e I . agra ow g r I n i th r a c S lo gest n e deepe levels . 0 h mm or rows or th e T h flexures re art est art in $3: fifk filfifigg e a f h ap the deep levels and approach each Th t er in the u er eve s until t ey rm ract ca y one l . e o h pp l l , h fo p i ll fo d w i m e r m the m ne m a s ws t at the ore a ram fi . c s a di g ( g 77) hi h d f o i p, ho h ur on the un er s e s tee tc n arc es t at is un er occ s d id of ply pi hi g h ; h , d - m b re r as neath flexures in the hangi ng wall slate . This ay e ga ded definite proof that the o re-beari ng s ol utions cam e from below and followed up channels form ed by the crus hed lim estone and confined - s r is the m re nter bv th e imperv ious hangi ng wall slate . Thi p oof o i esting because of the gradual im poveris hm ent of the ore shoots in i h r w r n The m ater a depth and thei r pinching out n t e deepe o ki gs . i l for t he ore shoots m ust have com e th rough fissures now filled with low- ra e or arren q uart s uc as the s m a ore s t the g d b z , h ll hoo of T h ore the an n -wa es was in art a brecc a and level . e of h gi g ll bodi p i v r fr m the ccurrence the in art a s ore . It is certa n we e p olid i , ho , o o of

494 E L KH O R N M IN IN G D IS T R IC T , M O N T A N A . latte r seem s to im ply horizo ntality and poss ibly sy nchronous fortna s r In th tu r e r ta h tio n with that of the i nclosi ng t ata . e dis b d st a of t e Elkhorn district the folding has been accom panied bv m ovem ent or s n a n the e n anes and t s m em ent has r duced lippi g lo g b ddi g pl , hi ov p o s b recciation and m i no r faulti ng alo ng the axes of the lesser fold . The im pervious s late acts as a cove r that confined the circulating waters i r m to the pipes of crushed rock n the saddle of the a ches . Si ilar con tact depos its be twee n slate and lim estone a re relativ elv comm on in the

R c M unta ns and m a be as cri e to the m er us and ns lu e o k y o i , y b d i p vio i o bl nature of the slates and perhaps to the fact that p recipitation was he retar at on the current a n th c nfin n wa induced by t d i of lo g e o i g ll . A s a rea state it is onlv w en the s ate is recc ate t at ore e l dy d . h l b i d h d po s t u has ta en ace m t s r c the e s ts ene rall e n i io k pl hi o k , d po i g v b i g confined to the dolom ite . That actual m ovem ent has occurred along the s late and lim estone bedding plane is proved by the platy nature of the strata at thei r con tact as we as the recc at n t s ate and m e st ne in the , ll by b i io of bo h l li o , at th m em nt w s not en ra and en e that th s . s e a e c e fold Th i ov g l , h e n ane was not the s te a stri e ault or fis s ure is s wn b ddi g pl i of k f , ho by the localization of the brecci ated m aterial and of the slips coi n h r n the r nn s i e ent wit the s . O n t e t e a o e c a e ccur n id h fold o h h d , h l o the sadd es th e s and it is e ent t at l s rm e un er a l of fold , vid h fo d fo d d en e n s d relatively light load would t d to dev lop ope space in the sa dles . W ere a s t st ratum s uc as s a e ccurs etween ar es t ere h of , h h l , o b h d bodi h is a ten enc fo r the s t r c to flow nt and fill s uc a ca t d y of o k i o h vi y , rm n a sa le -s a e m ass t c on to the and e r t in fo i g dd h p d , hi k p of fold v y h h l e er h on the s an s . In t e rn e s its w t e rnst ne h k E kho d po , ho v , ho o (called slate bv mi ners) yielded to flexu re and form ed a ge nerally un r e n im er i us r t u m etam r s e it res s te crus b ok , p v o oof ; ho gh o pho d , i d h in ette r t an the un e r n r tt e and r ab e m te so t at g b h d lyi g b i l f i l dolo i , h the latte r was crushed and s hatte red and form ed a loose breccia under s A s the enera r c n h sedim en ar s tra the arche . g l o k foldi g of t e t v ta curre e re the ne us r c s we re intru e and ence e re oc d b fo ig o o k d d , h b fo ro m e tam r sm to k lace it is necessar to s u se a later ck o phi o p , y ppo period or periods of folding in which the convol utio ns in which the ore deposits occurred are supposed to be due to the th rus t of the later

igneous int rus ions . In its brecciated characte r the deposit is analogous to the s ilver s i h r m en l a e s ts ure a N ev . nter t ces etween t e a ts e d d po i of E k , I b f g of crushed lim esto ne wo uld adm it of a relatively free passage of s ur r un n waters and w u urnis s ace for the rec tat n o di g , o ld f h p p ipi io of and the ore m nerals w e the rea s u e m te ra quartz i , hil dily ol bl dolo i f g t r The rre u ar rm f m ents would be replaced by vein m a e ial . i g l fo s o s uch ore bodies are to be explained bv the occurrence of crushed m as ses of rock and by the dissolving action of the mine ralizi ng

solutions . wa n. ) N A T UR E or O R E D EP O SITS or EL KH O R N M IN E . 495

T he Elkhorn deposit is noteworthy for several reas ons I r di r r s m e r e F irs t . t occu s in plicate d se m enta v ock tamo phos d by a

s succession of igneous i ntru ions .

Second . It occurs in the crus hed rock found in the saddle of m inor s and ws the e n ane an er n a tere s a e fold , follo b ddi g pl of ov lyi g l d h l rn e T h crus e r rm e the r mar re c anne fo r (ho ston ) . e h d ock fo d p i y o h l uprising s iliceous wate rs .

r T h e s ts re no h r t h fi n re e s t n Thi d . e d po i a t t e es ul of t e lli g of p xi i g ca es in the im es tone but are m a n due to ra ua re acem ent v l , i ly g d l pl of h b - The dolomite by t e s ubs ta nces brought in v ore bearing solutions . ore is c n ne a m st ent re to the m re readilv s u e m te o fi d l o i ly o ol bl dolo i , when found in the hornstone being only a cem enti ng m aterial fo r m n r frag e ts of that ock . he r i l be ri m w e u e t urt . T o e s e e e to e e r e ca s i Fo h b i v d d v d f o b lo , b

ccurs on the un ers e an m e r us s tratum ent in tc n o d id of i p vio , b pi hi g n anticli al folds . h r e n m n and ri n s n e t i n Fifth . That t e ore dec eas s i a ou t ch es i d p h s o ect n to the t e r r se s nce it is now we n wn t at in obj io h o y p opo d , i ll k o h $ ” the ac s the arren or but sl t m nera e ertica s Bl k Hill b , igh ly i liz d, v l (m ere cracks travers ing the rock) are the feeding channels for im po r tant ore d es t at ccur in rea re acea e r c s art cu ar y at bo i h o dily pl bl o k , p i l l n - n ts t e nt m rm e b st ta beddi g plane co tac wi h adj ac i pe a le ra .

A L T E R AT IO N O F C O U N T R Y B O O K .

T he rock adj ac ent to the ore deposits s hows none of that m arked T he alteration characte ristic of ore deposits in igneous rocks . dolo mite is not the res ult of the dolom itization of a lim es tone by solutions as cen n t r u the ore c anne ls s nce it m a nta ns its c aracter di g h o gh h , i i i h

T h n n -wa s at is us ua s rem ote from the ore . e ha gi g ll l e lly lightly altere for a few nc es r m the ore es and w e re the rock is d i h f o bodi , h brecciated and the f ra gm ents are cem ented by m etallic s ulphides the T he m te e rock has been decomposed and even py ritized . dolo i do s not r - s is a n show alteration from the o e depos iting sol utions . Thi l cki g because the action was one of replacem ent of dolom it e by s ilica and

s he r c i altere fo r near the ore o es it s u es . In t s sen e t s lphid hi o k d , b di is shattered or cra ckled and the fractures are filled by silica . These silica film s are due prim arily to a pene tration of the s olution along the ractures but t e are m re r er re acem ent e o s ts s nce f , h y o p op ly pl d p i , i their sections studied under the m icroscope show that the dolom ite is attac e a n c ea a e nes etc n and e s t n s i ca n on k d lo g l v g li , hi g d po i io of li goi g

h is urs e m ere a as e the rocess s m u tane us . s c i l o ly T i , of o , ly ph of p of ore e o s t n fo r the uart film s te n carr s u i es and the d p i io , q z of y lph d , if

l m te be ss e awa by lute ac a s e eto n is e t c m se do o i di olv d y di id k l l f , o po d

s ca and m eta c s u es w c c nst tut e s a ce u ar m ass se u of ili lli lphid . hi h o i ll l p s r nt W er at th dom orphic afte r the di s olved f agm e s . h e oxid ion of e 496 E L KH O R N MIN IN G D IST R IC T , M C N T A N A . sulphides has produced acid wate rs this sam e action has taken place in W nature . ere s u st tut n has ne st urt er and the r na h b i io go ill f h , o igi l s ut ns a e re ace a ar e r rti n the m est ne the r ol io h v pl d l g p opo o of li o , o ig nal recc ate c aracter is st and a m as s uart or ore carr n , y i b i d h lo of q z , , i g i ncluded fragm ents of lim es tone is all that is left to indicate that the r w r p ocess as one of eplacem ent .

M O D E O F F O R M A T IO N O F O R E S .

O R IG IN BY R E P L A C E M E N T .

That the ores are deposited from aqueous solutions there can be no u t t e r enera ccurre nce m nera c c aracter and a stu do b ; h i g l o , i logi h , dy un er the m cr sc e t n sect ns all c n rm t is c nc us n and d i o op of hi io o fi h o l io , as s uch origin is now gene rally accepte d for m os t o re depos its no rt er c m m n n b m n h r r fu h o e t eed e ade o this s ubj ect . That t e o es we e e s te bv ascen n waters is r v e as has a rea een state d po i d di g p o d , l dy b d , t e r c u r n n n r ha th r r by h i o c re ce u der pitchi g a ches . T t e o es a e e s te re lacem ent c untry r c and n to a m n r e ree d po i d by p of o o k , o ly i o d g y the fil n the nters aces etween m es t ne ra m ents is r e b li g of i p b li o f g , p ov d

the e ence affor e a s tu the ore e s ts the res by vid d d by dy of d po i , by o them se es as seen in an s ec m ens and t n sect ns e am ne lv h d p i , by hi io x i d n e r h m r m n m n u d t e icroscope . By eplace e t is ea t what is tech nica lly

e m etas m t m t a is n er n u n ffe rs r m ca a s t t c a e s s ta ce . It ll d o i ; h , i h g of b di f o se u m r sm as the atte r m ies reservat n the rm of the p do o phi , l i pl p io of fo m atter re ace wh le m eta s m at sm es no t n v e th e res e r a pl d , i o i do i vol p v ti n t r r or s n o of ei he fo m s ub ta ce . T he pos itive evide nce of m etasom atism is very striking and com let rn In th - l o i s s i r e at . e t wa re e e ec a the s t uc p Elkho foo l bod p lly , tures— which are represented in the plates m ade from photographs of actual ore faces— show that m asses of crus hed dolom ite res ulted f rom f n and the en s ace etween the ra m ents if s uc e ste oldi g , op p b f g , h xi d ,

w u fi e ore r uce the o re recc a no w s ee n . ca re u o ld , if ll d by , p od b i A f l s tudy of all the cavities left by the extraction of o re s howed that no r e i t n r r M re e r th e s ca es se e as c for o e e s t n . e p x i g v v d lo i d po i io o ov , r s m n n r h r an o e bodies a i ed eve s howed open cavities abo ve t e o e . H d s ec m ens as for nstance the one w c t ra s are re r p i , i of hi h pho og ph p o d d i l in s uce n PI. s w tt e m te c s v ar u s ta es LXI , ho li l dolo i b o k io g of r a m en In h r r m be en r m e e ce t . t e ua t es e er s ta e a se tt pl q z o v y g y , f o li l film s of s ilica penetrating the shatte red lim esto ne along capillary f rac

It s tures to a m as s of quartz co ntaini ng nuclei of lim es to ne . hould e n r i b oted als o that the ore shows no c ustificat o n . T he dolom ite adj ace nt to the ore bodies is netted with film s of silica wh c are te n t r m e n The si ca occu es fis sures wh c i h of ex e ly thi . li pi i h are e v identl the res u t a s atte r n the r c s in lac W t ut v l of h i g of o k p e, i ho

s tu rbanc th ra W er the s ca carr e no m eta di e of e f gm e nts . h e ili i s llic

P L A T E L X I .

L IME ST O N E F RA G ME N T S C E M E N T E D BY Q U A R TZ .

Th e oto ra s ows a S ecim en A natura size of im estone ra m ents cem ented ph g ph h p ( ) , l , l f g b dark- u rtz Th r n y bl e qua . e f agm e ts fit togeth er so closely that it is ev ident t at no m ovem ent h as taken ace but t at th e brecciation is th e resu t of a h pl , h l sh atte ring of th e rock with th e infiltration of siliceous wate rs and replacem ent

a on th e cracks . Th e uartz carries ood siv er va ues t ou th e ore arti l g q g l l , h gh p m in f r cles are too ute o determ ination of species . T h e quartz is ex trem ely finely ranu ar s owin th e texture and co or c aracteristic of m etas om atic re ace g l , h g l h pl m nt and n t th com stru t r h T h m e o e b c u e of t e in of O en cav ities . e s i en , fill g p pec illus trates an earlier stage in th e form ation of th e quartzose ore of th e hanging

wall ore shoots . O ther specim ens ( B and 0) show progressiv e stages in which th e sm all fragm ents are re aced and th e uartz seam s are en ar ed b th e roundin off of corners pl q l g y g , and still others in which only residual masses of lim estone occur scattered z through a m assive quart . Y WENT - ND ANN E T T II 8 . G EO LO G lCAL S RVE T S EC O O R LXI U. U Y UAL R P PAR

I T N F R A M E N T M E N T Y ART L M ES O E G S C E E D B Q U Z . O I G sT IC O N T A N A 500 E L K H R N M IN N D i R T , M . s om etim es py rite g rains occu r abundantly do tting the do lo m ite par a e to the fis s u re wa and in s uch cas es the uart carries no ll l ll . q z

r te or e s e is s im l dark co o red w n to a dus t m nute py i , l p y l , o i g of i

iro s u d M r om m on the uartz is s een particles o f the n lphi e . o e c ly q r acin the do o m ite bo th as we - m ar ed fis s ures and as ra ns epl g l , ll k g i w ich are a are ntl inters titial re s e ntin the a earance o f an h pp y , p g pp

s e o f do o i b rt In t his cas e ho we v e r e n o n m te fi ed ua . op p g l ll y q z , , the relatio n o f the cry s ta lli ne o utlines of the quartz to the do lom ite grains and the pe ne tration o f the latter by the q uartz s how clearlv e In ne ra th that the s ilica has replaced the form r m ine ral. ge l e

c h o s ho r q uartz re pla ing t e d lom ite ws a fine ly cry stalli ne characte . O ftentim es f ragm e nts o f the carbo nate a re s u rrounded by a band

ens e cr tocr s ta lline of d . yp y q uartz which is of the char acte r s upposed to be typical ‘ o r f eplace m e nt . M ore often th is cryptocry stalline quartz is s ubo rdinate in am ount o r enti rely want in and the m ain bod of t at g , v h m ine ral p res ents the s o m e what coarsely crys talline char acte r which is s upposed to be typical of the filli ng of open

It s h u be n ted s a e . p c s o ld o , howeve r th a t t h e q u a rt z grains do not exceed a m illi te r or two In W i th and In m e d , w $ h m i lacem ent 0 , 78 , T sh ow n re p , m um of ore g p

dolom ite b alena a fte r M a d en . a le na $ h rm th r . . no cas e t e v o e e y g ( m ) g. g I do f v y M a nifi d it ( i m ite ondar . e o ; q . q uartz , sec y g coars e ( ‘ O illb uart seen in pgzism itqgrql q z s e T h n r m anv fis u r v ei ns . e ea es t approach to this s tructure is s ee n where q uartz crys tals partially filled the d rus y cavities res ulti ng from the dis s olutio n of lim es tone

s fragm e nt . Atte ntio n has already bee n called to a replacement that is m o re in the nature of ps eudom o rph is m than that which has j us t bee n m e n

i n d s en m e n n has bee n es cribed in scus s n the c ar t o e . Thi ph o o d di i g h acter of the res and it has bee n s wn that the s hatte re l m est ne o . ho d i o has bee n penetrated by s olutio ns which depos ited film s of silica ; that s e uent the lim es t ne has been isso l e awa ea n the s i ca ubs q ly o d v d y , l vi g li as he iv s n wa s of a e r s e ce llu ar m ate ria and as t t at t d i io ll v y loo l l ; , l ly . h secondary waters have depos ited s ilica upon thes e thin walls until in m any cas es the cavities have been com pletely filled by som e of the

l - s i b W ld m r ind n v n nth A nn R t . G old uartz v eins o f G ras alle C aliforn a a e a L re : Se e tee . e q V y , , y g p

] v e P rt II . 391 . . S . G eo . S ur a U y , , p

502 E L KH O N I N I T R T A N R M N G D IS ICT , M O N A . ditions favoring a shatteri ng of the gabbro and a leaching of deep seated portions of the m as s by uprisi ng waters following the granite n I i i trus ion. t s evident also that the pneumatolitic action accom panying the cooli ng down of so large a m ass m ust have greatly s tim ulated the circulation of hot waters and vapors ; although no definite evidence of rm er hot s r n act n is now rec n ab e an sur ace e s ts fo p i g io og iz l by y f d po i , th s r e ob e ved facts all point to the form er prese nce of hot wate rs . It is true that th ere is a certain amount of evidence provi ng a differentia t n the ran te a ut its r ers and t at t ese asic fferent at n io of g i bo bo d , h h b di i io r ucts m a a e ee n eac e the c rcu at n hot water but it p od y h v b l h d by i l i g , seems reas na le to su se t at the a r a m re as c r c o b ppo h g bb o , o b i o k , afforded som e favorable phy sical conditions for the leaching and e trac n its re u -m nten The a r now e se x tio of p cio s etal co t . g bb o xpo d in the s tr ct is er res or s ws nl s ur ace a terat n and di i v y f h , ho o y f l io , t ere re it c ul not a e ee n eac e as suc act n w u ea e h fo o d h v b l h d , h io o ld l v

n ns s r t T h ea n m ust a een in defi ite sig of olfata ic ac ion. e l chi g h ve b m re ee ur e rt ns and the u ris n m nera e waters o d ply b i d po io , p i g i liz d , l w n nes east res stance were c ncentrate b the m er us fol o i g li of l i , o d y i p vio a in he r n w m r T h n n sh les t cou se o a ked by ore shoots . e pi chi g out of the deposit in depth and its impoverishm ent in values in the deeper workings are at first sight m ore favorable to the theory of precipitatio n from descendi ng solutions than of precipitation out of ascendi ng r w er h h waters . A s a ea s wn e t e e s t n t ace o n t e l dy ho , ho v , d po i io ook pl u n er s e a tc in ant cl na and t s is s t e e ence d id of pi h g i i l fold , hi po i iv vid h r w re n n urr n M re er the w r n that t e w te s e asce c e ts . s a di g o ov , o ki g s w that the ore c anne s not sa ear in e t as uart is seen ho h l do di pp d p h , q z in the - o t e e the tt m e e the m ne w ere it st f o l v l , bo o l v l of i , h ill r l e h s h l l ca r es m eta c su s . es es t e e rt ca t e ac s i li lphid B id , v i l of B k Hil region— of whose exis tence and character we now have v erv definite ata— are e am es e treme sm a and ten arren fissures d x pl of x ly ll of b , s m et m es m ere crac s in the r c wh c se r e as ee n c anne s o i k o k , i h v f di g h l for the siliceous ores form ed by replacem ent along the bedding planes tw en m st nes and es s s u e s a es or rt s be e li e o l ol bl h l q ua zite . It u be rem em bere a s that in s ut ns s t ar n con sho ld d , l o , ol io ligh ly v yi g ditions m a c an e or e en re erse the c emica acti n s ut n and y h g v v h l o , ol io l r n e r deposition being really re ative term s . Va yi g te mp ratu e and pres b A s ut n sure are the factors whi ch disturb chem ical equili rium . ol io m ay dissolve m etallic s ulphide under the pres s ure and tem perature prevaili ng during the cooli ng down of the granite batholith and carry i r n In suc c s s n t n them upward w thout p ecipitatio . h ase phy ical co di io s n t r rec tat n and the su es were e s te n w en did o favo p ipi io , lphid d po i d o ly h r d n relief of pressure and cooling have pe m itte thei r precipitation. O e can conceive that under stable co nditions uprising hot waters flo wing in t runk channels would s how a lower zone in which the waters wo uld be capable of taki ng the metals i nto solution; above this another zone W EED - l M O D E O F F O R M AT IO N O F O R E S O F E LK H O R N M IN E . where the waters would hold metallic s ulphides in s olutio n but would not rec tate t em and a t r ne a e the as t in w c re p ipi h ; hi d zo , bov l , hi h p i N e r he ur ce the t s ub e m eta c c pitation would occur . a t s fa leas ol l lli sulphides would have all been precipitate d and the hot waters issuing d he m r a u as springs would depos it only silica an t o e re dily sol ble salts . It m us t c urse be un e rsto t at in t s t e ret ca arran em ent , of o , d od h hi h o i l g t ere w ul be a ra ation r m one ne nt an t er and t at the h o d g d f o zo i o o h , h r tr is in m etal s should s how a fai rly orde ly vertical dis ibutio n. This act w at we find in the rn e s t w ere the rcs s w a ena f h Elkho d po i , h o ho g l r b r T h unde lai n y zinc and pas s ing i nto pyritife ous ores in depth . e occurrence of this depos it on the border of the granite batholith is T h analogous to that of m any of the great ore deposits of M ontana. e G ran te M unta n m ne the D rumlumm on the m nes at W nston and i o i i , , i i , the W t atc U n n o e near e ena are all am ar e am es of hi l h io l d , H l , f ili x pl ' r ts rm in e n tra er n m ar on rich o e deposi fo ed v i s v si g si il c tact rocks .

SE C O N D A R Y SU L P H ID E E N R IC H M E N T .

The e tens e at n ore es c nv ert n t em in th u r x iv oxid io of bodi , o i g h e ppe art the m ne nt car nates and es has een acc m s e p of i i o bo oxid , b o pli h d by a art a eac n out the s er c nte nt r m lar m s se r p i l l hi g of ilv o s f o ge a s of o e . s n act n w t x v at n is ess r m nent in e t and Thi oxidizi g io i h li i i io l p o i d p h , , in the l wer ore bo es has ne no urt er t an to rm a ca o di , go f h h fo p of

e ore a o e the ore o . ere is we er abun ant e n oxid b v b dy Th , ho v , d vide ce that oxidizing wate rs have penetrated to the greatest depth yet reached

in the m ne eet on the cl . T he res r m t s e e s i , f ip o f o hi l v l how art a a te re a ena w t e c at n s and tetra e r te p i lly l d g l i h oxid o i g , h d i partly c n erte into so t ac s u es m re er en water c r o v d o y bl k lphid ; o ov , op ou ses are un in the m te and the uart se ore ta en r m th fo d dolo i , q zo , k f o e res y ene ace the uart o s we the su e f hl op d f of q z b dy , ho d lphid m nera s art al e w le the uartz was r us and e i l p i ly oxidiz d , hi q po o h ld little

- rus t stai ned cavities . It has been s hown that where ore bodies are leached by oxidizing waters the des cending solutions may precipitate their m etallic conte nts r by reactio n with lean prim ary sulphide ores . This eaction is prom i nent we er nl w ere the r na su es c nta n m uc r , ho v , o y h o igi l lphid o i h py ite T h a s n e s u res in he or other iron s ulphides . e b e c of ch o t shoots worked in the Elkho rn m ine would explain the absence from the ore bodies t us far w r e but it es not e a n wh t e not ccur in th h o k d , do xpl i y h y do o e

r tic res un in the wer e e s . ere is it is true s m y , e p i o fo d lo l v l Th , o evidence indicati ng the secondary nature of tetrahedrite and of local unc es of sec n ar su i es but it a ears r a e t at the a b h o d y lph d , pp p ob bl h f il ure to form large bodies of such secondary ore is due to the m as sive

1 n h m m iner al v eins b la m et allic sul h id b W H W d B ll o E ric ent of te es . . e : u l m e . Ge Soc. A . y p , y , l 17 -2 6 nrich m e n f l n ilv in X1 1 9 7 . E t o o d er s b W e d n V o . 900 a d s ve W. H . e : Tra s . A m . Ins , , pp g , y t. l 1 24-44 n XXX 900 . 4 8 . M i . En . V o . g , , , pp 0 O 5 E N N A N . 4 L KH R M I IN G D IST R IC T , M O N T A and unbroken co nditio n of the o re bodies which were not freely pene d s i h w r r n trate e ce n n wate rs . M o re er t e m ne ate s a e ot ac by d d g ov . i id and not c nta in err c s u ate s o t at it is e ent t at a rab e do o f i lph , h vid h f vo l conditions fo r secondary enrichm ent did not generally exist . T he impoverishm ent of the ores in depth may perhaps be due to a slight r m n h l W it is n b e t at a ena en ich e t of t e upper leve s . hile ot eli ved h g l e erc ses any am unt act n u n escen n s ut ns it m ust be x i o of io po d di g ol io , a m tte t at s m e the er - ra e a ena un in the m ne d i d h o of v y high g d g l fo d i , that was partly altered and showed sooty film s alo ng the cleavage r in i r e w r h i ates m a a e ee n c s e s u s . e e t e m ov pl , y h v b i h lv lphid Ho v , p erishm ent ea -s er e osits in e t and t e r assa e nto low of l d ilv d p d p h , h i p g i ra e nc res is a c mm n en m en n in the m nes the R c g d zi ky o , o o ph o o i of o ky

n r i n . I ha n es e iall n te ar M ountai eg o t s bee p c v o d by R ick d .

A G E O F T H E O R E D E P O S IT .

T he geological e vidence which has been presented in the foregoing pages proves that the rock m etamorphism of the strata containing the ore e os t is m a n the resu t the ntrus n the ran te at t d p i i ly l of i io of g i b holi h , and it follows from this that the ore deposit is younger and has not It i h t a rm suffered m etamorphism . s believed that t e deposi w s fo ed s rt a ter and is a se ue to the ran t c ntrus n and ence is ho ly f q l g i i i io , h of n r r n T r nt ns late Eoce e o ea ly M ioce e age . he hyolite i rus io and out burs ts which to ok place at neighboring localities in m iddle and late

M cene t me were it is true we ore e s t n at s me io i , , follo d by d po i io o

aces but t ere is reas n to e e e t at the l rn e s ts were pl , h o b li v h E kho d po i not the result t s er nam c act t but t at e t se of hi p iod of dy i ivi y , h , lik ho h r r utte t e rece e t e t c nt us ns . of B , h y p d d hyoli i i io

L S T 506 E KH O R N M IN IN G D I R ICT , M O N T A N A . stratum and that the ho rizon m av be expected to yield workable ore b es w ere the c ntact rm s a tc n ant c na erm tt n odi h o fo pi hi g i li l fold , p i i g o T he m ne st es s w s m e e ence s n or saddle dep sits . i op ho o vid of lippi g acc m m at n a n the contact ane but as the s a t had ca e in o od io lo g pl , h f v d at the tim e the m ine was visited and the workings were not safe no detailed study was m ade . r lie n ion m ine is s tuate at the ea A l reston G ulch in the T i d h d of p , r n Th r rt h e se r lim estones near the g a ite contact . e p ope y as yield d ve al ra r a car a s si ver- ea ore but the ore b es are not c nt nu il o d lo d of l l d , odi o i ous and the average value of the ore is said to be too low to warrant T h r r was w r e in the ear e ties and the w r n . e e t o ki g p op y o k d ly igh ,

- - developme nt in 1 884 cons isted of an 80 foot shaft and a 240 foot tunnel . T he o re ext racted at that time is reported to have carried 80 to 350 ‘ ounces of silver per ton .

Th e D olcoath mine difi ers r m t se rev us m ent ne and f o ho p io ly io d ,

n e so far as n wn r m all t er r ert n wn in the tate . i de d , k o , f o o h p op y k o S T h ore s ws ree in the u er weat ere arts the and e ho f gold pp h d p of body , the u nalte red portion consists of glittering m etallic spots of bism uth d n t T h ore - ar sulphide an tell uride (bism uthi ite and te radym ite) . e be ing s tratum is from 1 5 to 1 8 i nch es thick and dips north eas t at an angle ° ° r 5 The n ha e n n of 45 to the st ike bei ng N . W . m i e s b e ope ed an nc ne sha t 1 70 eet ee w t a e e at 1 00 eet and an t er by i li f f d p, i h l v l f , o h in Th or - a r n at th e bottom of the shaft driven both di rections . e e be i g stratum is clearly an impure limestone altered by contact m eta morphism and now consisting of garnet (gross ularite) with les ser T h r nu amounts of diops ide and spots of calcite . e ock is coarsely gra lar the arnet e n the c arser m nera and the s e n . g b i g o i l , diop id filli g nters aces the ca c te e sts in str n s e n ets and unc es w c i p ; l i xi i g , v i l , b h , hi h are evidently the filli ng of d rusy cavities also s howi ng garnet crys tals he rne h r and ractures in t a t tse . T e ca c te m asses are we e of f g i lf l i , ho v , often similarly oriented and appear to be si ngular individuals which o to e r e t n n n The o m nera s s eem s opp s ed th i d posi io i ope spaces . re i l m n have been exa i ned i the chemical laborato ry of the S urvey by D r. h N . t es w o re o rts t at t n s sm t . e c s t u s u e H S ok , p h h y o i of bi h lphid r n m i e n r e e u te r D . t es c t s t u e B T er ce t . p , bi h ll id ( 2 3 ) p S ok at m uch he su w n o e r m l r says th of t lphide as fou d t b f ee fro the te lu ide . Whether the latte r is tetradym ite with o r without sulphide could not be s t e ete rm ne nor c u it be ete rm ne w ic m nera po i iv ly d i d , o ld d i d h h i l h carr es t e . sm ut s u h e is the m neral sm ut n te and it i gold Bi h lp id i bi hi i , has altered in the upper portio n of the ore depos it to the carbonate

bis m utos hte rite w c ccurs as a ellow w er m ater a and p , hi h o v , po d y i l , r - r n endr m r n T he etr m c nt ns s m e has da k b ow d itic a ki gs . t ady ite o ai o c er and in t s res ect acc r s w t the ana s s m a e G enth opp , hi p o d i h ly i d by , ' r in D In m r which is reco ded ana s M i ne ralogy . the altered ate ial this

1 o f irector o f M in n P rod uctio n o f o ld n i v W R e rt o D t o G a d s l er ash in to n 1885 . 304. p , g . , p wnzn ] U N IO N A N D D O L C O A T H M IN E S . 507

er h s ine the r c w t m alac t T he b sm ut m ne ra s copp a sta d o k i h hi e . i h i l occur sca tte red th rough the ore in m i nute grains disse m i nated t r u ut the arnet in lar er ra ns in the s de and as art h o gho g , g g i diop i , p i c es ar n u to a uarte r of an nc acr ss w c a ear re ue nt l v yi g p q i h o , hi h pp f q ly b w c to e as sociated ith the cal ite . A study of the partially alte red ore leaves no doubt that the gold occurs as a cons titue nt of the bism uth m inerals and that it has be e n T h is s m e freed and left as native gold by their oxidation. e gold o what dull and rus ty and has the habit often observed by the writer s T h whe n it is undoubtedly derived by oxidation from telluride . e ore st ratum is overlain by im pure marble and underlain by a similar lime s r r sscu r en nto h o t wa s we th s ilicate rock . A ho t c o t d iv i t e f o ll ho d e pres ence of an intrus ive sheet of gabbro-diorite at a dis tance of about h h r t ra u is a h 1 8 fee t from t e ore body . T e o e in his s t t m prob bly t e res u t neumat t c acti n due e t er to c rcu at n va ors or m re l of p oli i o , i h i l i g p , o

r a to c rcu at n h ot waters r s n u war t r u it . A s s u p ob bly , i l i g i i g p d h o gh g l h m m l m to es te D r. arre t e m eta r s m an ure es ne g d by B l , o phi of i p i w u r duce a s m e w at r us roc s nce the m nera s r uce o ld p o o h po o k , i i l p od d r by contact m etam orphism occupy less volum e than the o riginal ock . T he act n is s m ew at ana us to t at the a n r c in io o h logo h of b ki g of b i k , d A n in n t n each case the action being ue to h eat . exam atio of hi sec n th wa r ha n m r arre and s ws t at tio s of e ll ock s bee ade by D . B ll ho h they are p ure r lim esto nes and that the res ultant prod uct is far less per h r -b r tr um I i i e t ere r ns to water t an t e o e ea n s at . t s e e e vio h i g b l v d , h fo , that as the rocks are tilted and s lightly folded the pressure was not s uffic ent to c se u the o res in the r c and t at e n c nfine i lo p p o k , h , b i g o d by th re at e m er us s tratum a ve and e w the u r s n waters e l iv ly i p vio bo b lo , p i i g

we t s r n and e os te t e r m eta c ntents to et er w t follo d hi ho izo d p i d h i l o , g h i h h r ca c te in the nte rst ces t e c . l i , i i of o k It has been imposs ible to obtai n any data concerning the v arious t er r e rt es t is s tr ct as all the s a ts are fi e w t water o h p op i of h di i , h f ll d i h h e h re rt the D irect r th or t e tunn ls are i naccess i ble . T e po of o of e M int upon the productio n of precious m etals f or 1 884 co ntains a short account of a num be r of m i nes upon which co ns iderable developm ent rtee n r erties are m ent n work had at that tim e bee n done . Thi p op io ed — d . h n n . an D es es the rn m ne . ree t ese t e U b id Elkho i Th of h io , C , — and Kee ne m ines have al ready been m entioned ; the othe rs i nclude a num er ore es s tuate c s e to the rn m ne n ne b of bodi i d lo Elkho i , o of w c ave so far as it has bee n os s ble to as ce rta n een w r e hi h h , p i i , b o k d n T h n n ne a tt e n rm a withi the las t ten years . e otes m e tio d give li l i fo n n r n o r e t tio co ce ni g the geological character of the e d pos i .

' Tbe R elet m én e cate a uarter a m e n rt eas t the f , lo d q of il o h of Elk rn is sa to a e s wn at a e t 20 eet a 1 2 - o t e n ho , id h v ho d p h of f f o v i of

r s s 0 n silve ore givi ng a ay s of 50 to 1 0 ou ces per to n.

T/wP a /N a d e r ocate 41N) ar s no rth O f the rn m s we y , l d y d Elkho ill, ho d

- n s - a 4 foot face of ore alo g a late dolom ite co nta ct . 50 O 8 E L KH R N M IN IN G D IsT R IC T , M O N T A N A .

— Th x/f l cci nd s Th m s ta t re - urt s e [fi a d ? a M e D u n to e. e e e e n Ho k , h fo h of a m le n rt west the rn and the D unst ne 600 ar s n rth i o h of Elkho , o , y d o w st h rn o in e e t e s w re es m t . of Elkho , ho bodi dolo i

' Tbe M m te C ris ta the L umm bm ' and the Ilflm n tazn Vie w — T he , g, M nte r st ar s west O f the rn c nta ns a 1 0- t e n o C i o, y d Elkho , o i foo v i r o n r Th u ne - c e re n i s ate and an te . e L xenbur o a of opp lyi g l g i g, h lf m e s o ut th e rn m ne s we at a e t 35 eet a 6- o t il h of Elkho i , ho d d p h of f f o e n ore carr n c er and ea as we as the rec us m eta s v i of yi g opp l d , ll p io l , h ntr r e n he r h h n m i h t e c u c ran te . t n t t e r t e o y o k b i g g i To o of Elkho ll , M unta n ew r ert s tuate m es u the u c is sa to y , H o i Vi p op i d il p g l h , id l x - r- or have shown a 3 l foot vei n of s ilve lead e in s late and syenite .

IR O N M IN E S O F E L K H O R N P E A K .

O n the n rt s u er rn ea a ut 700 eet e w the o h ho ld of Elkho P k , bo f b lo s um m t t ere are two e s ts r n ore t at are e ose en i , h d po i of i o h xp d by op cuts and un er r un ri ts and r m w c c ns era e ore has een d g o d d f , f o hi h o id bl b

m s in h n ear T he r e rt i r shipped at various ti e t e last te y s . p op y s eached a stee but a r wa n r a ea n r m h rn u by p f i ly good go o d , l di g f o Elk o p A l resto n G u c and er a s ur k rn ea a stance a ut p l h ov p of El ho P k , di of bo

T he ore es ccur enea t the reat m ar e nc us ns in 4 m iles . bodi o b h g bl i l io ' the an es tes rn ea and rm the w te clifis so r m nent d i of Elkho P k , fo hi p o i

m n s e nc n are un er a n in aces in all views of th is ou ntai . The i lus io s d l i pl n n ne bv thin seam s or lenses of m agnetite and gar et . I ge ra l these seam s are too t n to be ec n m c v alue but on the n rt west s e hi of o o i , o h id h r t c en u h to c nstitute w r ore e of the peak t ev a e hi k o g o o kable d posits . T he o re is a wa s un a n the tt m the m ar e m ass and is l y fo d lo g bo o of bl , i I r o i n u te a re lacem ent t . t es ts n an es te w c s a u do b dly p of d i , hi h b dly n in r n w r e r s n Th decom posed a d rich i o he ev it wa see exposed . e r T h m arble above the o re is not ove 1 5 feet thick . e ore its elf is a m ass e fine - ra ned m a net te s win rare ra ns c a c r te iv g i g i , ho g g i of h l opy i r r e reen sta n r ar n te and an adm ixtu e of ga n t. G i s of coppe c bo a occur T h m s rn r nt r e n on the weathered crust . e as of ga et p es e va i s co s ider a and it is ss b e to ta n er - ra e ore s rt n the bly , po i l ob i v y high g d by o i g is sai o carr sm a a ues in ut no as sa m ate r a . s ore t b s i l Thi d y ll v l gold , y r rt T h n u r r were m ade to pro ve the acc uracy of this epo . e o ly se fo i on

res in M ntana as et has een in the sm e t n esta s m ents as a o o v b l i g bli h , s r T he E rn m ater a has all ne to the flux for siliceou o es . lkho i l go East r T he r e rt is re orte to a e een n e in Helena s m elte . p op y p d h v b bo d d u 1 90 0 to the e lena and n st ne Sm e t n and Re uct n J ly , , H Livi g o l i g d io Com pany for as a s o urce of flux for the sm elter j ust men D e s ts s m ar to t s e at rn a e een n te the tioned. po i i il ho Elkho h v b o d by

u n us ntrus ns in t er arts the tate n ta in writer abo t ig eo i io o h p of S , o bly r mm r r he resu the Little Belt M ountai ns . They a e co only ega ded as t lt r r s s and mm ns a e of the alteration of contact deposits of py ite . C o E o h v

1 - e of th e D irector of th e M int [or 188 4 . 304 305. R port , pp

51 0 E L KH O R N M I N IN G sr c'r N A N m m , M O T A .

o e th or In he we - n wn s e r e s A ab v e big e s hoots . t ll k o addl e f of us tralia w c are e s ts rm e in tc n ant clines ore es are , hi h d po i fo d pi hi g i bodi

n in su i n a an r T h e rt c n fou d ccess ve saddles o e bove othe . e v i al sectio s com m only given in text-books and the illustrations in descripti ve reports do not dis close the close analogy to Elkhorn which the descrip te t s ws to x It is certa at t rn t e re w re at tive x ho e is t . i n th a Elkho h e

east three m e r us s trata— the l rn s a e the c ntact cut at 60 l i p vio E kho h l , o i n h th n m n h e o eet a e t a d t e s ale e t e ee e e . ese t re c n f bov , h b l of K i Th tacts are all m ar e y a tt e ore and the c ntacts s ul be care k d b li l , o ho d fully searched where pitc hing arches would afford favorable conditions for ore shoots . llICROSCOPlCAL PETROGRAPHY OF THE ELKHORN MINING I EF RS N OUN Y M ANA D STRICT , J F E O C T , ONT .

By JO S E P H BA R R E L L .

IN T R O D U C T I O N .

In this report the re will be pre s e nte d an account of the petrographical h Th features o f the various rock types of t e Elkhorn m ining district . e wo rk is the result of a careful s tudy of the rocks and their sections in the la rat r the S effie S c ent c c Y a e o e e un er bo o y of h ld i ifi S hool of l C ll g , d

r . r he r th s u er s n . . Pirsso n t s a e on t et r e p vi io of P of L V , hi p p p og aphy forming a chapter in a thesis entitled The G eology of the Elkhorn M n n D st r ct resente for the e ree octor s in i i g i i , p d d g of d of philo ophy

M a 1 900. s re ort treats rst the una tere ne us r c s y , Thi p fi of l d ig o o k , and s e ara te the a te re se m entar rocks the m etam r s m p ly of l d di y , o phi of a r duce he eat and a r the n n r which w s p o d by t h v po s of ig eous i t us ions .

U N A L T E R E D I G N E O U S R O C K S .

U nder this heading an account will be give n of the igneous rocks of re r r r the district that a unalte ed by contact o late weathering . As they form part ( if the series of rocks form ed from the earlier Tertiary n ous act t th s art M ntana the res u ts the stu ig e ivi y of i p of o , l of dy apply re ar wel to th r to a widesp ad ea as l as e Elkho n district. A general as n r W i h account of these h bee given by M . eed n t e paper precedi ng t s and it is to be n te t at w e no unus ua t es are nd hi , o d h , hil l yp fo u , c ns er n the m te area a reat man ar et es are re resente o id i g li i d g y v i i p d , l n n to c ns ecut e e c s eru t be o gi g o iv po h of p ive activity . R eferred with probability to the oldest period are a num ber of dikes eman ating from a syenitic magma but showing extrem e differentiation a n from acid to bas ic types . Thes e h ve bee observed in the Elkhorn mine and also in the southern part of the district within a mass of Starmount lim e-silicate rocks isolated from the other sedim entaries t o w n es by late r erup ives . F llo i g th e at an unknown inte rval came a 511 51 2 M IC R O S C O P IC A L P E T R O G R A P H Y o r E L K O R H N D IS T R IC T .

er of reat e ru t ns rom dio rit ic and abbr c m a m as o rm n p iod g p io f g oi g . f i g the B ac utte s t ck o f abbro the lacco t c ntrus n of ior te l k B o g , li hi i io d i r h r num e ro us dio r t c and abbr ic d es the nt rus i e and po p y y , i i g o ik , i v ex trus ive areas o f ande s ite found in the no rthe rn and s outhe rn parts of the s t r ct and erha s the C em eter R i e st ck t u the di i , p p y dg o , ho gh re at ns of the la te r the o t e r e r i s ar no l w A er l io t to h upt ve e t we l s ho n. ft this there was an i rregular and b ranching i ntrus ion of quartz - dio rite

or h rv acro s s the s ut e rn a rt of the d s t r ct and finall cam e the p p y o h p i i , v breaki ng o f the quartz - m o nzo nite of the batholith across the we s tern

r r T h n rus n ite es t r - bo de . e i t io of apl dik h ough the quartz m onzo nite and the adj ace nt s edim entaries m arks t he close within this area o f the

r c n e uptive a tio s . T he plan o f th is report is to des cribe the m icros copica l features of t es e r c s o int n ou t an ecu arities to v e the c em ica and h o k . p i g y p li , gi h l m ne ra o cal co m os t o ns and to ndicate w e re o s s b e w at re a i l gi p i i , i , h p i l , h l ‘ m x s o n hem i n l tions av e i t am g them . C cal a a y ses of the pri ncipal r c t e s are ve n but in the des cri tio ns t es e are s u e m e nted o k yp gi , p h ppl th ro ugho u t by es tim ated m ine ad o r che mical com pos itio ns as dete r m ned h i r s i by t e m c o cope . In those clas s es o f rocks whe re the m ine rals are clearly s eparated f rom one anothe r an es tim ate of the chem ical com pos ition s ufficie ntly accurate fo r m any purpos es m ay be o btained by the use of the m icro s T er e he e r r cope . he p ce nta g s of t s ve al m ine als and their com pos ition

r rm n In m n r s s h r n r are fi s t ete e . t s e e a uc as t e e es o d i d ho i l , h py ox ho rnble ndes , where certain elem ents m ay replace each other in un no wn uant t es the ene ra c aracte r the r c w l ena e one k q i i , g l h of o k il bl to de c de a r m ate w at com s t n to as s n to t em but as i pp oxi ly h po i io ig h , the y us ually fo rm but a s m all part of the rock the final erro r res ulti ng

o s o r n n l r d f r m th is urce is cor es po di g y e uced . T he com p ositio n of the felds pars is us ually s usceptible of accurate

W h m t o M ic e Lé i s e e term inat o n. ere ss e t e e v s u d i h po ibl h d of h l y d , dependi ng upo n the extinctio n angles obs erved in cry s tals twi nned acco rding to bo th the albi te and C arlsbad laws and cut nearly pe rpe n l W r th s e t o n i no t r b fo r dicular to the c i nopi nacoid . he e e c i s favo a le the applicatio n of this m ethod a varie ty of other m ethods m ay be em o y e s uch as the re latio n the v brat n irect o n to the cleav pl d . of i io d i ages s ho wn in cli nopi nac o idal s ections and the po s itions of the optic

s axe . F or es tim ating the pe rce ntage of the cons tit ue nt m inerals of the s ti n an o ne o f a num be r m et s m a be us ed that w c is ec o y of hod y , hi h

l h es e anal s e s w re m d e for m e b D r H N S to k es in th e la ora tor of th e United S ta te s G eo T s e a y . . . b y v y h c r t r h k r S o kes is w ll k n wn nd is a l s u r T a e ul natu f t wor of D . t e o a t ea e lo ica l S ur e . e e o e g . i p for m e to ca ll a t te ntio n to t h e close corres po nd ence b e t a een th e m ineralogical and ch em ical com

h c y s s c — W t) i s o f e ro k d e rib ed h e r i H Wh a . posit on t t pe e n .

s i o ( s it is h s 3 m s e nd s n h rm in ur l d t rm na i n e tc t 1 S a e co fa i 1896 . O t e d ete a Etud e . a e e t i pa , 94 . . c cle ,

s c s V o tio n o f la io c la e fc l i a rs in ro c k s e c t io ns b G eo r e F . Becke r: A m . J our. S ci 4ih se ries l. p g p . y g . V . 4 M a ) , lose , p . 3 9.

51 4 m o c aos orroxL PE TR O G R A P H Y or E L K H O R N D ISTR IC T .

an rt s tes in t ers towar the r en tes and ass n in the out o ho i , o h d py ox i , p i g n es nto r In th n th i lyi g dik i dio ites . e followi g table e composit on of the re resentat e r c is i en and is c m ared with t at the p iv o k g v , o p h of a r s of the s ut western r n ack re i n to w c t s r c g bb o o h Adi o d g o , hi h hi o k ar a tron re n be s s g sembla ce .

A nal se o ab r m a N ew Y or y s f g b os fro M ontana nd k.

C onst ituent.

° H ,O at 1 10 C

° above 1 10 C

a U nd rmined wh eth er sul h ide or sul h ete p p ate .

A N orm a abbro rom west side of Black Butte not far rom th e im estone con . l g f , f l

k . A na sis b H . N . Sto tact E kh orn M ont . es , l , ly y - rst en rom th ut wes rn A ir n k N w Y rk . 0 . H . B . H e e abbro e so te d o dac s e o yp h g f h , rn f i n u 1 94 Sm t r. A m erican J ou a o Sc e ce J 8 . y h , i , l , ly,

The r c is seen to be a nrm a a r but ffers s mew at r m o k o l g bb o , di o h f o any from M ontana yet described by the U nited States G eological Survey in having a rather high percentage of alum ina compared to n s e resses ts e in the m nera c m s t n the amou t of silica. Thi xp i lf i l o po i io ri by an abundance of labrado te . To show the variations in compos ition within the stock and also in l T h rce nta s are the adj acent dikes the following tab e is given . e pe ge The ma n estim ated by the microscope and are given by volum e . g e n am e ] lil. o BUT T E S T O C K AN D A S S O C IA T E D B IK E S . 51 5

e n the m ne ra reate st s ec fic ra t the m st n t cea e tite b i g i l of g p i g vi y , o o i bl change in converting the se i nto percentages by weight would be a ‘ slight raising of the amount of m agnetite and a lowering of the labra F or u m r n the m s t on lum e orite . r ses c a s c e n d p po of o p i o o po i i by vo , b i g l ffi w t at rect ta ne the m cr sco e w su ce e ua el . h di ly ob i d by i o p , il q lly l

i i ks r lack Butt stock and iated di es Mineral compos tion of gneous roc f om B e assoc k .

M ineral.

1 G ab ro rom n ar n rt i in . b f e o hern s de of ma stock b ide of B B t i 2 . a b wes t G ro s ack u te near m estone contact . See ana sis on a e , l , l ly p g k 1 H . S to es 55 14 b N . 5 , y ( r -di ri m m 3 . G abb o o te fro s all intrus ion h alf m ile north of Black Butte

Diorit . Inc usion in at r iv 4 . e l l e erupt e feet south of Black Butte - m i r r i . 5. A ndes te o rom sa e inc us on as N o 4 a di erent ortion of sam p phy y, f l ; ff p e mass — N orm Th e num bers in parentheses afte r all ta bles refer to field numbers given i i nd t s e im ns ken r m t to local t es a o p c e ta f o hem .

The m r N . nd is to n o s a t os 2 3 a 4 be te N . 2 com n r i il i y of , , o d , i g f om the s to c w e 3 and 4 are r m ut n ntrus ns eet a ar k , hil f o o lyi g i io f p t . The latte r two closely resemble each other in microscopic appearance as we as in c m s ti n and all are ac s - r h r n ll o po i o , bl ki h g ay oloc ystalli e r c s n te un er the m cr sc e for t e r ar e ercenta e a r o k , o d d i o op h i l g p g of l b a dorite w ic runs in com os t n r m A b A n to A n the ent re , h h p i io f o l , Ab, i er es ossess n s a n ta e am unt s i p i g al o o bl o of biotite .

2 m ar to N o . 1 ws th ar in No . co p ed sho e v iation com position withi n

the sam e st c and Nos . 4 and 5 e t a s m ar c ntrast for an out o k , xhibi i il o n ra m ent of the sam e m a ma r na an ntrus n but now lyi g f g g , o igi lly i io , e as an nc us n w t n the uart - r te - r r w c ccu es h ld i l io i hi q z dio i po phy y , hi h o pi a s e con id rable area south of Black Butte . In re ar to the r n r t n N and 5 to th strata w g d o igi al ela io s of os . 4 e hich nce surr un e t em not muc can be sa s nce t e are now arts o o d d h , h id , i h y p n n u n h n a e r ff r n in m n of a i cl s io wit i a l ter eruptive . Th i di e e ce i eral 51 6 M IC R O S C O P IC A L P E T R O G R A P Y or E L K O N T H H R D IS R IC T .

c m s t n etween t em se es m ust be n te we er as n cat n o po i io b h lv o d, ho v , i di i g the ra c an es w c m a ta e lace w t n the sam e n rus n pid h g hi h y k p i hi i t io . T he as s c at n m n ra s is e n n c m ar n h r o i io of i e l s e o o p i g t e seve al sections . ree uartz e s ts in t ose w t m uc t te and rn en e e en i F q xi h i h h bio i ho bl d , v n the resence th se m nera s c aracter stic the m re as c r c s p of o i l h i of o b i o k , b ut avoids those parts of the stock in which the m agnetite and augite r nd he t and rn en l It is to b e a e high a t bio ite ho bl de ow. e not d that where the quartz is present the labradorite feldspars are zonally built in the ute r arts n cat n a reater c nte nt s a and s l ca nd o p , i di i g g o of od i i a a trans t n t war the a te m ecu e a eature u te a sent in th i io o d lbi ol l , f q i b e m s r o t bas ic gabb os . T he reater ercenta e a ra r te in certa n sect ns the s toc g p g of l b do i i io of k , and its m atte m anne r ccurrence w en ta en in c nnect n wit d of o , h k o io h the act t at in the a r s it is the first m nera to cr sta e su est f h g bb o i l y lliz , gg the explanation that the cause of the segregation has been the cooli ng T he m n nd se t n the m a m a w t n he t of the wal ls . ovi g a t li g of g i hi t s ock while undergoing crystallizatio n would afford opportunity for the a ra r te to cr stal e a a ns t the c ntacts e b t n a ara e sm l b do i y liz g i o , xhi i i g p ll li in r entat n w e the t er c nst tuents were s t in the flu con o i io , hil o h o i ill id t n resu t n in a c ncentrat n the la ra r te at the s es and di io , l i g o io of b do i id h r t of the i ron ores and augite to ward t e cente . If his is the true nter retat n it is w rt n te s nce in the m re ac ma m as it i p io o hy of o , i o id g is the i ron ores and ferrom agnesian m inerals which are the first to crystallize and which by that m eans are often concent rated near th e n borders of an i ntrusio . The thatched or m atted m anner in which the labradorite crystals lie u n one an t er is not str ct c nfine to the m ar in and the sam e po o h i ly o d g , r cess m a ha e een c nt nu us for s m e sta nce nwar r e p o y v b o i o o di i d , p ovid d there was a slow m ovem ent of m aterial withi n the throat as the crys n ee tallizatio proc ded .

R O C KS O F T H E C E M E T E R Y R ID G E S T O C K .

s area eru t e r c a a m le in widt t ree- urt s Thi of p iv o k , h lf i h by h fo h of

a m e in en t es m m e ate eas t the t wn rn . e il l g h , li i di ly of o of Elkho Lik the o ne re us stu e it has r en t r u the a e c me p vio ly di d , b ok h o gh P l ozoi li r rr r m n A t the s ur ce it is s to nes and possesses a ve y i egula argi . fa enti rely isolate d from contact with all of the other erupti ves of the istr ct and t s in a t n to its a tere c n t n and the d i , hi , ddi io highly l d o di io a sence res m ater a for stu ren ers its re at ns s s m e b of f h i l dy , d l io hip o T h r r r s t is seen to ssess a what do ubtful . e ock whe e f eshe po holo n I r n crystalline texture and the appearance of a diorite . I ts p ese t condition it shows the influence of both exte nsive subaerial and sub rr n n t n T he ur ac eat rin has ro uce rust te a ea al eratio . s f e w he g p d d c r w er nd eneral eca for ns i erab e e t but the olo ed bo ld s a a g d y co d l d p h ,

51 8 M IC R O S C O P IC A L P E T R O G R A P H Y or E LKH O R N D ISTR IC T .

t m a nta n n t e r r er r entat n n cate that the resent s ill i i i g h i p op o i io , i di p

sur ace is near the rm er r . In t is ca t t en as c se re f fo oof h lo li y , h , b i g ga t ns w u be ex ecte to ccur and ee e r wn the st c ma io o ld p d o , d p do o k y be r m o e siliceous . T he e un in the m ne and w se c m s t n is en un dik fo d i , ho o po i io giv der is so near to the m a n s toc t at the r ab t its re at ns B , i k h p ob ili y of l io hip

to the atter is str n t u no m eans r e . T he r c is res l o g, ho gh by p ov d o k f h nd ranu r in a earance a t- ra c r and w t a g la pp , of ligh g y olo , i h m any It i essent a an r s ar n nts r te . s t c ase r c an p kli g poi of py i i lly o ho l o k , d shows affinities with the syenite dikes found in the extrem e southern n s A is an t e r n art the str ct. Its un e es to cat n t at p of di i lik o h i di io h , t u in r im t to the em eter R e stoc it m a no ho gh p ox i y C y idg k , y t be It furth en discus sion is reser e for the related to it. s v d section on

syenites . T he rock given under C has the com position of an acid diorite or uart -m n n te fferin c ns cu us r m in ssess n m q z o zo i , di g o pi o ly f o B po i g uch

a c ase and uart . It was c ecte r m the sam e r t as B bu pl gio l q z oll d f o d if , t owing to its unlikeness can not be regarded as belonging to th e sam e n h at r intrusio as t e l te . T he rock whose m ineral com position is given under D is a m icro te urrin in a e 50 eet in t c ness com form bl e e diori occ g dik f hi k , a y b dd d h m es t nes but e se for n a s rt d s I w t t e tance . i h li o , xpo d o ly ho i t is u te ecause it is the ar e ntrus e nearest to the stoc but d q o d b l g i iv k , oes n r n i not have any ev ide t elatio to t . In e au t urt er m ater a r m the st c tse its n d f l of f h i l f o o k i lf , ge eral ra ter and affin t es rem a n in u t the r a t e n cha c i i i do b , p ob bili y b i g that It is a r te as s n in aces nto a a r er a s in t er ace dio i , p i g pl i g bb o , p h p o h pl s - into a quartz m onzonite .

S Y E N IT E D IK E S .

There are a number of thin dikes and s heets varying greatly among them selves but poss essing the com mon characteris tic of being with r nd nta n n an a un ance a ne out qua tz a co i i g b d of lkali felds par. O wi ng t e r t nness and a s to the act t at t e are us ua e e w to h i hi , l o f h h y lly b dd d ith the strata t e are er a e to esca e ser at n e ce t re , h y v y li bl p ob v io x p whe T he n r exposed by fresh cuttings . o ly exposu es noted have been in the Elkhorn mine workings and in a railroad cut in the southern part F or t ese reas ns t e m a be m re un of the district . h o h y y o ab dant within h str ct t an is su se es ec a l as w ere ccurr n near h t e di i h ppo d , p i l y , h o i g ot er r m t m e not re im n n eru t es t e a e s e s a scr ate i the el . p iv , h y o i dily di i d fi d Those found in the extrem e southern part of the district are in a mass m etam r se tarm unt ar il ace us mestones 400 eet of o pho d S o g l o li , f n t c ness ent re iso ate r m the ne r n sedim entar es i hi k , i ly l d f o ighbo i g i , being underlain by intrusions of quartz -diorite -porphyry and andesite m . m u ) BY KH IT E D IxES . 51 9

- a t W t n t ese i t and capped by the diorite porphyry l ccoli h . i hi h l gh c re careous m etam r c r c s th e c cur as s eets and es olo d , cal , o phi o k y o h dik r n e r from a fraction of an inch t o 1 8 i nches in width . They a g f om w te c arse ranular r c s c ns stin a m st ent re m cr c ne hi , o ly g o k , o i g l o i ly of i o li ar to b t n u s d r m the nc s n strata to s n n and h dly e dis i g i he f o i lo i g , ho ki tes ar in c r and c arse ranu ar in w c a f the r c co n i , d k olo o ly g l , hi h h l of o k s is ts of augite and biotite . Be re escr n t em in eta a s n s s wi be en the r fo d ibi g h d il , y op i ll giv of i m nera com s t n um e as est m ate the m crosco e. i l po i io , by vol , i d by i p

Mineral composition of the E lkhor nsyenites .

M ineral.

Titanite

G arnet

Pyrite Secondary products

100. 2

A . G arnetierous s enite rom th e 1050 sout drift of the E k orn mine a dike f y f h l h , cutting th e foot-wall m arbles

B . Bostonite . A dike 5 inc es in widt er endicu ar to th e strata in th e iso ated h h , p p l , l mass of Starm ount m etam orphosed lim estones ( 1 70) A i - ni A t 18 in i u u e te . S ee c n id iso C . g te y h hes w th in th e lated mas s of Starm ount

m etam or osed im estones . D ikes B and C inters ect but the oint of inte rsection ph l , p is concea ed and t ere ore t eir re ations at t at oint are undete rm inab e l , h f h l h p l

D . S onkinite . A s eet of irre u ar widt and occurrence in the is o ated m ass h h g l h , l of Starm ount m etam or osed im estones 75 eet sout of C ph l , f h

The orthoclas e or m icrocline in all of these s pecim ens contains co n S iderable s a as is n cate the m tt e e tinct n w e n r od , i di d by o l d x io h otated n r s n arl tw nni i betwee c o sed icols . C sbad i ng s common and the m icro n s fin wa eff t Th l n r h cli e show e tered ec s . e a kali e felds pa s we re t e last m n r r s nd l th r i e als to c y tallize a hold a l e othe s enm eshed . Where they form the bulk of the rock the individual crystals are from 2 to 3

- mm . in en t and one m in t n a to 1 m . c ess but in certa n l g h h lf hi k , i laces t e ec m e muc c arser s m et m es n ea a e f es p h y b o h o , o i giv i g cl v g ac an 520 M ICR O S C O P IC A L P E T R O G R A P H Y or E L K H O R N D IS T R IC T .

. n The cr sta s s w a ta u ar de e m ent n the inch in le gth . y l ho b l v lop alo g th t ric in ka n ane 01 0 the cl n nac . A t u in e y es a e pl , i opi oid l ho gh p h l li e s ars the atte r t us assum e s m et n t e r cr sta ne rm f ld p l h o hi g of h i y lli fo , the na unct n ne w t a acent cr sta s the sam e s ec es is er fi l j io li , i h dj y l of p i , v y rre u ar as is to be e ecte s nce t ese a e een the fina r ucts i g l , xp d , i h h v b l p od of crystallization and h ave necessarily g rown u ntil they have m et us th r s ws the an m r c structure each other. Th e ock ho p idio o phi of Rosen usc eac cr sta rt clase ass um n s m ew at its r er b h , h y l of o ho i g o h p op r sta l ra ic rm but ex t n n ne the s u ar- ranu ar c y l og ph fo , hibi i g o of g g l aplitic structu re . In the augite-syenite and shonkinite a certain amount of plagioclas e occurs in scattered and sharply idiomorphic crystals from one-half to

m in en . 1 m . l gth h r r i T e err m a nesia n c m nents a e e t e m ca or au te or ot . f o g o po i h i g , b h T h te urs in m r r m s r m one-fifth to 2 m e augi occ idio o phic p is f o m . in en t and s ws str n eoc r sm the c r a n the se era l g h ho o g pl h oi , olo lo g v l

n r e n s T he r s axes e r t e n ree ue and t e w . c ta s b i g b igh g , g i h bl , ligh y llo y l are scattere t r u ut the sect n t u m re a un ant in s m d h o gho io , ho gh o b d o e r n in t er The ver str k n m tt n the a e pa ts tha o h s . y i i g o li g of C of bov ta e the au te-s en te as seen in the an s ec m en w c a bl , gi y i , h d p i , of hi h i due th un e tr bu to ra is en . A s to e e n s t n pho g ph giv (Pl LXII , ) , v di i io of the almost m icroscopic grains of augite . In certa n s ma nes the au tes S w ess e c r sm resum i ll zo gi ho l pl o h oi , p ably from possess ing a s malle r percentage of i ron . In the s honkinite the aegirine exists as an occasional narrow border zone to the augite s . T he m ica in the syenite from the Elkh orn m ine is pale green in h r wn of th n n c r u te un i e t e r c e s te t te . olo , q i l k i h b o ho ki i bio i T he titanite in these rocks is abundant and occurs in the usual

- l en e s a e cr sta s a era n mm . in en t . In A how oz g h p d y l , v gi g l g h , ever the rn s en te it a s ccurs in an e ra ra ns and t ese , Elkho y i , l o o h d l g i , h are so closely inte rgrown with a brown garnet that it is only under n b ear se r polarized light that they ca e cl ly pa ated . This close association of the crystals suggests that before crystalli zation the two m ay have exis ted in segregations of som ewhat s im ilar m ecu es fferin r m eac t er in t at one n s sess r n ol l , di g f o h o h h ki d po ed i o , th t er t tanium the two su stances on cr sta n e n r e e o h i , b y llizi g b i g fo c d w n to r iff r n a r l to separate o i g thei d e e t h bits of c ysta lization. Titanium is known to occas ionallv replace i ron and silica in andra te and w ere it r ses to a ercenta e and a um na is a sent di , h i high p g l i b n n the m i neral becom es schorlom ite . Tita iferous a drad ite contains r m 1 to 1 0 er cent TiO and has een n te in ra and at f o p of , , b o d Colo do M a net e r ansas as we as at a num er re n ca t es g Cov , A k , ll b of fo ig lo li i . r m te c ntains r m 1 2 to 22 er cent TiO and occurs at Scho lo i o f o p of , ,

m am a ] M IN ER AL C O MP O S IT IO N O F T H E S Y E N IT E S . 52 1

The relat ns s t es e m nera s are see n o n com M agnet C ove . io hip of h i l paring the form ulas :

‘ i ni 3 ( a0 3 T lO 3 8 10 T ta te . ,

d i 3 C at) 3 SiO A ndra te garnet , , T i i rous and radite 3 03 0 3 S iT i 0 tan fe , ( ) , ‘ it 3 ( a0 F eT i O 3 SiTi S m . ch orlo e , ( ) 2 , , T ( ) O 2

These re lations suggest that in the garnets of the Elkhorn sy enites T i I e e is ro a l ns e ra e a m i t ure O . n the s ec m e n th r p b b y a co id bl d x of , p i n h rnet ws an inte r r wt a s w t he m from the m i e t e ga sho g o h l o i h t ica . T he arnet ccu rr n in the s t n te is c r ess in t n s ect n g o i g bo o i olo l hi io ,

n ns f brous nc us i ns w c m a be w as t n te as wel as and co tai i i l o , hi h y oll o i , l l In t ace s it s ws nte r r wt s wit a very little ca cite . bo h pl ho i g o h h other

h bos to n te is te n t c in the rt l s m nera s and in t e c a e . i l , i of poikili i o ho G arnet has been f req ue ntly noted in granites and sv enites as well as in ne isses sch s ts and rn e s and t e re re no reat we t need g , i , ho f l , h fo g igh be attached to its occurrence h ere ; but the absence of its prope r cry s

li is ce t na at eas t for t is str ct . tal ne fo rm ex p io l . l h di i n e am n n the m nera co m os t n the s e n te s as en in O x i i g i l p i io of y i , giv th ta e the ecreas e in the alka ne e ds ar in C and D is seen to be e bl , d li f l p he ri in r n m e and m a nes a w c rm n l du to t s e . a e i o , li g i , hi h , by fo i g p gio

e a en u certa n am unts the s Th se and au te a t a a e . e cla gi , h v k p i o of lk li m arked drop in D is s een to be due to a p roportionate fo rm ation of

tite w n to the c m nat n a c ns era e am unt tas bio , o i g o bi io of o id bl o of po h i with i ron and m agnes a . These dikes withi n the is olated m as s of m etam o rphic rocks s how no relation to the e ruptives which bo rder it on all s ides and are ev ide ntly nt to s m e rent res e r r more ancient than the latter. They poi o pa voi of a syenitic m agm a which has not bee n detected and m av still lie co n e led e neat the s u r ace o r w c has oss b ee n rem v e bv th c a b h f , hi h p i ly b o d e ns at cu m nate in the rm at n the at later intrus io th l i d fo io of b holith .

D IF F E R E N T IA T IO N IN P LA C E .

Th e aria e c aracter o f t ese d es in c ose u ta s t n and v bl h h ik l j x po i io ,

c a the s o tte and m tt e sur aces s wn in the au te -s enite espe i lly p d o l d f ho gi y ,

w are re ro uce in . A n cate t at e trem e and hich p d d Pl LXII . , i di h x dif l T he low er e n c ferentiation readily takes p ace . p c tage of sili a indi cates a cons ide ra ble liq uidity and the coarsenes s of te xture pres um ably a le s ure cr s ta at n two c n t ns w c m a be ta en i ly y lliz io , o di io hi h y k as

a ra e for fferent at on . O v er certa n atc es w c m av be f vo bl di i i i p h , hi h a

t or m re in en t no au te is res e nt and the r c s ws n foo o l g h , gi p . o k ho o ly

n w ite e s r r large and prom i ne t cleavage faces of h f ld pa . S u roundi ng s uch a white nucleus the m i crocline is so thickly charged with the

i r s ta s t at the c r bec m es a lue - ac and th c ea aug te c y l h olo o b bl k . e l vage

D ana S stem of M ine ralo 1892 . 413 . U . cit . . . 44 7 . , y gy , . p f p p 522 ICR O S C O PIC A L P E T R O R A P H Y r K O M G o E L H R N D IS T R ICT . faces of feldspar on a close exam ination are seen to be thickly speckled h n u r t n h r with t e i cl ded c ys a ls . I t e parts whe e the dark augites are a un ant are man sm a er s ts a an nc in am ete r ree b d y ll po , h lf i h di , f r m t em n a m tt e a e r n h T h n a a ce to t e w e . e m tt of f o h , givi g o l d pp hol o li g the augite-syenite does not appear to be due to the intim ate adm ixtu re re us fferent ate ac and as c rm s the ma m a but of p vio ly di i d id b i fo of g , to a differentiation in place . This is j udged from the character of i w in n I A . the m tt and s s n . o li g , ho Pl LXI , T he contact of the white and blue - black varieties does not show the rounded and drawn-out form s which an adm ixture of viscous m agm as w u r uce but on the c ntrar it is an nte rw en o nta t in , y , c c o ld p od , o i ov which the white nuclei pass out in m any knotted s tream ers and form on the surface spots showi ng a certain parallelism for a width of seve ral inches . ere is a t r and nterm e ate rm the r c ccu rr n as Th hi d i di fo of o k , o i g nes r m an e t to a uarter an nc in t c ness betwee n the zo f o igh h q of i h hi k , r s r us n n In the an s e im en the in r two va ietie p evio ly m e tio ed . h d p c te m e ate ne is seen to be t ree n in c l r and un er the m cr di zo ligh g o o , d i o scope the reason is found to lie in the lighter color of the augite h r en cr sta s n ca t n t e es ce a ess am unt r n. y l , i di i g p of l o of i o T he se re at n has ri nate t ere re in ace and e re the g g io o gi d , h fo , pl b fo nn n r l a n a it r nate in ce is n begi i g of c ystal iz tio . Th t o igi d pla show by the form of the two kinds of areas ; that it occurred befo re the e nn n cr sta at n and was not r uce m ere the s et b gi i g of y lliz io , p od d ly by t n to et er the au te cr sta s a rea rm ed is s wn the li g g h of gi y l l dy fo , ho by r h diffe re nt characte r of the augites in those two zones whe e t ey occur. T he feldspars in growi ng have shown no power to exclude or even to o rient the num ero us small crystals of augite and titanite which e nc u e and t us the sam e cr stal rt c as e ma c nta n th y i l d , h y of o ho l y o i n r It is c nse uentl seen t at the cr stalli portio s of all th ee zo nes . o q y h y zatio n of all the m inerals has taken place after the segregation and

In w th act t at th au te cr s s i ndependently of it . V ie of e f h e gi y tal were co nsiderably heavier than the liquid feldspar withi n which they rm ed it is fficu t to un erstan wh a s ett n out not ta e fo , di l d d y li g did k place ; but that it did not is shown by the white nuclei bei ng bo rdered ‘ F r by the dark mate rial on the upper as well as the lower s ide . o s uch a diffe re ntiation to have taken place within the dike before cr s ta at n the ma m a m ust ave resumbabl bee n ° held for s m e y lliz io , g h p y o tt e t m e ui a c n t n w c m es a enera i tem era li l i liq d , o di io hi h i pli g l h gh p ture of the rocks and the near pres ence at that tim e of a co ns iderable um e ne us m ater a the sam e c n t n e n n cate the vol of ig o i l , o di io b i g i di d by r lin coars ely c ystal e rock struct ure . S uch a segregation withi n a narrow di ke is in contrast to what has bee n O bse rved in the C razy M ou ntai ns and which is to be described in

52 4 M IC R O S C O P IC A L P E T R O G R A P Y or E L K H O N m s 'rm o'r H R . close exam ination of the larger blocks exhi bits their original char ' acter w et er a as recc as or tufi s t u the natures the , h h l v , b i , , ho gh of latter are scure r m a n een su se uent m etam r e ob d , f o h vi g b b q ly o phos d h l - and ar ene . T e a as are ense fine ra ne r c s w t but h d d v d , g i d o k , i h tt e e ence e n s m et m es ss e ss n m an la ra r te li l vid of b ddi g , o i po i g y b do i en r sts but a wa w t n a un an r m T h latte r c s a t un ass . e ph o y , l y i h b d g o d is never c arse cr sta l ne and in the first s ficat n was r o ly y l i , olidi io p ob a te n ass t u su se uent re eat n has in all cases en bly of gl y , ho gh b q h i g giv r it a m ic ocrystalline texture . A ll trans t ns r m a s to recc as are un t u care u i io f o l va b i fo d , ho gh f l e am nat n is te n necessar to scr m nate the atter w n to the x i io of y di i i l , o i g fact that fragments and gro undmass have ass um ed the sam e finely r ne te ure in lat r r s i n c ystalli xt e c y tall zatio .

The tuffs t u e art n w e r m the c m m n c r ter , ho gh d p i g id ly f o o o ha ac of ' tufls m a be etecte e n s m ew at s te r and te r t an the , y d d by b i g o h of ligh h t er an es t c r c s and us ua sh w n fine- ra ne e n o h d i i o k , by lly o i g g i d b ddi g n r m r m r h and u a es a us a tu es as st . pl , f o v io d ix of d In c ntra st nct n to the ure e trus e rm s ntrus e s eets o di i io p ly x iv fo , i iv h m m m h in of andesite ay be found i nterbedded with them . So eti es t e tru sive natu re is demons trated by the structural geology ; at other tim es it is m a e r a e w e re the s eet was t c the reater c arse d p ob bl , h h hi k , by g o s r s t n en we er th two m es ccu rren nes c ta a . t e ce of y lliz io Of , ho v , od of o

an not be s e arate s nce a t n ntrus e s eet m ay not ffer in an c p d , i hi i iv h di y r h n s particular f om t e deeper parts of a e xt ru ive flow.

T T E D E L K H O R N M O U N A IN IN R U SIV A N E SIT E S .

In taki ng up the petrography of the andes ites in detail the r T h t r undoubted intrus iv e form s will fi st be dealt with . e wo g eat s a s m ar e r m the M a s n rmat n c nta ne w t n the l b of bl f o di o fo io , o i d i hi an es tes rn M unta n and escr e in eta e sew ere are d i of Elkho o i , d ib d d il l h , e separated by an i ntrus ive sheet of andesite 80 to 1 00 fe t in thickness . In utcr n t s rm s a r un e s um m t eet a e the sea o oppi g , hi fo o d d i f bov d e t th e trem e a E rn M unta n an f e wes t of e x pe k of lkho o i . m ann a ‘ 52 5 ] CH EMIC AL C O MP O S ITIO N 0 1 A N D ES IT E S .

A n na e t ntrus e s et is v en and a s s . . t s s e ly i , by H N S ok , of hi i iv h gi is compared with a couple of closely sim ilar andesites :

s i rom na and Y ellowsto rk A nalyse of andes tes f Monta ne P a .

Constituent.

1 1 ° H ,O (at 0 C . ) — ° H O abov e 1 10 C . . , i ( )

T race .

1 2 1 100. 7 00. 99. 68 5

a. Und term ined wh eth er sul h id e or sul h a te e p p .

E rn i N 8 . k rn un in k o M ontana . A 1 A ndes it rom E o M o ta na s s o . 85 A . e f l h . l h , ly

k t. 2 H N . Sto es ana s F ie d N o. 4 . . l , ly rn nde- rox ene-andesite rom Se u c re M oun tain Y e owstone N ationa B . H o b e l py f p l h , ll l r n i n nd ti D cri P rk ntains au ite e st e e a oc ase or b e e m a ne te . es bed a . C o g , hyp h , pl g l , h l , g i U . . G . Su . 633 n Bul P ios b Iddin s Twe t A nn . R e t . S eo rv e a so . . Soc. y g , lf h p l y , p ; l l h l V I i n . . . . l. I II I 210 and n M o U S G S urv o Pt . . W ol I . eo e as in ton V . h g , , p ; l y, XXX ,

a rd . T M . ta A nalysis by . C h - i n R uf M on n n in a i H t ne andes te ort west of ed B ta a. C o ta s oc ase . ers e C yp h , h l f, pl g l and roxene with an am or ous as s base and som e tim es o iv ine a te red to py , ph gl y l l P d r Bu . . . A a G . P . M er il U S . G e te d b . C . e e an o riti m tte r. C o ec chlo c a ll y l l , ll l 4 Surv ev N o. 1 68 . 1 1 . , p

T he analys is shows the Elkhorn M ounta in andes ite to be of normal c a r w t the e ce t n th e tas in t s res ect s w h racte i h x p io of high po h , hi p ho

. n h n in affi n t es w t the latite s . R ans m e a d er a s a g i i i h of F L o , p h p vi g a - In h som e significance to the later intrusion of qu rtz monzonite . t e 6 S P C P T O 52 M IC R O C O I A L E R G R A P H Y or E LKH O R N D IS TR ICT .

an s ec m en it a ears as a ar - ra a m st ac m cr cr sta h d p i pp d k g y , l o bl k , i o y l n r un m ass in w c is s r n e a sma am unt a c e li e g o d , hi h p i kl d ll o of pl gio las phenocrysts that never exceed a m illim eter in length . U nder the m cr sc e t ese are un to be er as c r a c rres o n n i o op h fo d v y b i , p ob bly o p di g on the a era e to a t wn te and to ssess o n the uter r ers a v g by o i , po o bo d n at r i T h en c r s v ery narrow zo e of a r he acid plag oclase . e ph o y ts are r na but the rem a n n structure the roc ears m ar s the o igi l , i i g of k b k of cr sta at n a n een m se y c ntact m etam r s m and y lliz io h vi g b i po d b o o phi , this is readily explainable whe n the nearby pres ence of the batholith

- and its far reaching contact effects are recalled . Th e groundm ass consists for the m os t part of a m as s of feldspar in n e ri at s r m to m m . e t w s n e e a e r l h f o l g h , hil p kl d lik v il ove all the r c th en cr sts and r un m ass are flakes t te o k , bo ph o y g o d , of bio i r ese n to be n n to str n n and ho nblende . Th te d co fi ed i gs a d patches er the a c ase en cr sts and are ta en to s ow a recr stalli ov pl gio l ph o y , k h y zation ur n a er intense m etam r sm acc m an e a d i g p iod of o phi , o p i d by dissemination of the elements of the hornblende and biotite i nto the cracks of the feldspars . C rv stals r e ne are resent but are rare and a tt e ma of py ox p , li l g m am n n netite occurs associated with biotite . S all ou ts of a alcite and s n r n natrolite are fo und a alteration products . A o igi al faintly r e flo structure t s ws t u t r c h r m a k d w s ill ho . Al ho gh his o k as s uffe ed r m m etam r sm t at act n has res u te in no c em ca c an es f o o phi , h io l d h i l h g , and the sm all amounts of water present indicate that it has not bee n ur n altered appreciably by s face weatheri g . Beds of this character c m se the weste rn s es rn ea n strat ra ca o po lop of Elkho P k , lyi g ig phi lly h ar a w en th m m above t e m bles s ell as betwe them . A t e s u it the grain cr sta l at n ec m es c arse and the r c is rea ly a m icrodiorite of y liz io b o o o k l , h m though still of t e sa e series . Lim e-rich augite-andesites occur in a num ber of beds of unusual c aracter on rn M unta n c ns ist n a fine- r n - r h Elkho o i , o i g of g ai ed light g ay r un m s w c m a or m a not c nta n a c a en as se cr s . g o d , hi h y y o i pl gio l ph o y ts The t c r is due to the a sence all ar m nera s the r c ligh olo b of d k i l , o k c ns st n la c ase e s ars rt c ase e tes and c r es o i i g of p gio l f ld p , o ho l , z oli , olo l s ea s of au te the atter n cat n a ert in r n and r n s b d gi , l i di i g pov y i o a ich es in m and he rth c ase a m lar r n i l e t s c ess n a a e . h h li , o o l i i i h lk i s T e c ief significance of these beds is in regard to the poss ibility of an addition ime r m a se m entar s urce and for t at reas n t e w l b of l f o di y o , h o h y i l e fully considered in another pape r in co nnection with that s ubj ect and w er b n ri e r e ec m ens r ta wo ill h e e o ly b efly d sc ib d . Sp i we e ken from t

' r ns the wes t 1 5 eet in t c ness n but a few ee e ho izo , lo , f hi k , lyi g f t abov the M esozoic m arbles and lim e- silicate rocks at the head of Elkhorn G u c t us e n near the ase the an es te e trus er l h , h b i g b of d i x ive s ies ; the second is from a bed 30 feet in thickness lying som e dis tance above r r the fo m e .

528 m C R osooP IC A L P ET R O G R A P H Y or E L KH O R N D IS T R IC T .

n n n ru t n th e cr ta s re m m m olte a des ite . U po e p io ys l we so eti es blown nt th e air bv the e s e rces the esca n as es a on w t i o xplo iv fo of pi g g , l g i h bits of glassy and newly s olidified andesite and also of hornsto nes w c had een t rn r m the strata t ese w t the acc m an n hi h b o f o , h , i h o p yi g

ust rm n e s tuff and recc a. A t t er t m es the en cr sts d , fo i g b d of b i o h i ph o y a d ass ra m ents ate out in the a as the atte r c n so n gl y f g flo d l v , l ooli g u c t at t e ecam e e t e r ass or m cr cr sta l ne t us q i kly h h y b i h gl y i o y l i , h preventing the formation of other m inerals of s ufi cient size to be identified .

Q U E E N G U L C H R E SID U A L A R EA .

s sm a area an es te n 400 eet W e 500 eet n es Thi ll of d i , o ly f id by f lo g, li on a s teep hill slope at a co nsiderable distance f rom any other occu r

s rence and 1 x m es rect ea t rn . ere are a num er , } il di ly of Elkho Th b ar e ties resent but all a ree in be n r c s w t an a un ant of v i p , g i g o k i h b d , r n e th o s ructure i finely crys talline groundmass . In ce tai b ds e fl w t s ° n i in 5 W . t an an e str n m ar e the flow a es S . 5 a o gly k d , pl d pp g gl of ‘ O ne abundant variety of this rock is so charged with tabular cry stals of labrado rite ave raging three -eighths of an i nch in diam eter that the waterwo rn float bowlders are conspicuous in the s tream beds r r r r r n nd tt w t for a m ile below . Othe v a ieties a e da k g ee a spo ed i h rn en nd r r t Th rms the s mall crv stals of ho bl de a lab ado i e . e fo of andes ite are the sam e as those noted on Elkhorn M ountain and dis $ ” n r the ea rn M unta n ntrus e an es tes cussed u de h d of Elkho o i i iv d i , t e ffer n on t he w e we er in e n so c ns cu us h s e di i g hol , ho v , b i g o pi o ly t c po rphy ri i .

T U R N L E Y G U L C H A N D E SIT E S .

T he edge of this area occupies the southwest corner of the m apped nd m r s l s a n urn e rea . a r s stee as s e a (Pl XLV) , fo p , g y hill op lo g T l y

sam le for ana s s was ta en in the ra r a cut a uarter G ulch . A p ly i k il o d q of a m ile s outh of the lim its of the m ap and 70 feet from th e contact r - l Th anal with the upper surface of the dio ite porphy ry accolith . e v i s ws s m ew at unus ua r rt ns and is c m are w t two s s ho o h l p opo io , o p d i h

r w c resem e it in s m e res ects but e art r m it in t ers . othe s hi h bl o p , d p f o o h m anu a l A N D ESIT ES . 529

t ll w to r Analyses of andesite and related rocks from Mon ana and Y e o s ne P a k.

Constituent.

° at 1 10 C

° H 0 abo ve 1 10 C

T race . T race

b 100. 80 100. 33 100. 70

a C ontains indete rm ina le a m ounts of oth rite and rrh otite th e actual am ounts l in b b py py , y g

e e tween Pe s and Pcs and F 08 and Pc s . F cO can not b d irectl d eterm ined b n , g 5. ; g y , n in lud in h F uival n f sul h d T h e actual F eO lies the per ce t c oa c g t e 930; eq e t o F eO and p i es . etwee n and and th e actua l F e o etween and b g ,, b b S umm ation too h i h ecause F e in F eO and sul h ides is stated as F e 0 th e actual excess ein g b p , ,, b g l indeterm inab e .

H m blend — ndes ite- r h r a ter rom an au ite-andesite and a roac A . o e a o ed p p y y, l f g pp h

in a basa t in com osition rom sout western art of th e E k orn district . A na sis g l p , f h p l h ly

N . n . F i N . 3 . H . Stokes a al st 185 . e d o 5 No . 8 l , y - r in i as au i i b r H urri n R id A ok Ran . onta a oc e te B . M ca a b o ca e e bsa a e C s g , g , g pl g l g ,

e rsth ene biotite m a netite wit som e ort oc ase and a litt e uartz and o iv ine . hyp , , g , h h l , l q l 1 n V l. II Pt . 1 . A a i M n U . S . G o . ur o s s D escribed b Iddin s in o . e S ve y g l y, XXX , ly

in B U . . . S ur N o 1 . Se u . S G o v . 68 by Eak s . e ll e l ey

Basa t dik nort s ur of M ount Was h bum Y e owstone Nationa Park . C . e l , h p , ll l - i i on ain R eporte d by Iddings as approaching pyroxene andesite in com pos t on . l t s labradorite au ite se r entinized o ivine and m a netite in a roundm ass of obu itic , g , p l , g g gl l

r N . 1 . 1 1 . l i Bu . . o . u e o 68 0 and Inicro it c brown ass . . U S G e S v gl ll l y , p

T he hornblende-andesite -porphyry in A has s uff ered greatly from a terat n r uce in ar e art c ntact m etam r sm but as l io , p od d l g p by o o phi ; , shown by the abse nce of ca rbo n dioxide and the s mall am ount of wate r h resent its c m s t n m us t close y re resent the r na . T e p , o po i io l p o igi l

22 G E O L PT 2— 01 — 34 , 530 C R S C P R E L K D I IC M I O O PIC A L E T R O G A P H Y or H O R N S T R T .

r rn n T he a l phenoc ysts consist of plagioclase and ho ble de . pl gioc ases are a labradorite and exist in two forms : (1) T he crystals unite d in

c us ters u to 2 mm . in ameter w c s m et m es s w rad at n l p di , hi h o i ho i i g arms rm n ra e and c m le cr sta n a r ates and at t er , fo i g f gil o p x y lli e gg eg , o h t m es are r u e in c usters w t ut ranc es in tota am unt t ese i g o p d l i ho b h , l o h rm r n n un nt crystals fo about 1 0 pe ce t of the com positio . (2) Ab da -l r s l w r r n m r at e c ta s c a e m to mm . c l h ik y , hi h f o lo g, idio o phi , and which exhibit a strong zonal structure ; these comprise about half of the bulk of the rock . Th e phenocrysts conta in many i nclusions filled with granular horn T h th in u n s is n he blende . e form of e cl s io cavitie determi ed by t feld

s ar the cav t es e n r ente to the cr s ta ne s stem the atter. p , i i b i g o i d y lli y of l Incl us ions of such form in feldspars are of comm on occurrence in rocks w t a as s s i h gl y ba e . Phenocry s ts of hornblende or augite have once existed and were of a later r wt t an the e s ars but t e r aces are now s m et m es g o h h f ld p , h i pl o i l e w t ranu ar rn en e no r entat n s u st n eca fi l d i h g l ho bl d of o i io , gge i g d y of h e r na m n r and r r a n t t r t m s th t o igi l i e al then ec ystalliz tio . A o he i e e rn en e rem a ns in its r na s stem but the rm the cr sta s ho bl d i o igi l y , fo of y l and the character of th e rock make it appear clear that the hornblende i r uct u t n r m r n T h r un m ss s s a p od of rali izatio f o py oxe e . e g o d a al o c ns sts ent re the sam e ranu ar rn en e t r wn nd o i i ly of g l ho bl d , bo h b o a

reen in c r. t es e ccurrences t s m nera c m r n g olo All of h o of hi i l, o p isi g ab ut 40 er cent the r c c n rm the m ress n t at the r na o p of o k, o fi i p io h o igi l cr s ta ine c aracter w t the e ce t n the e s ars has een y ll h , i h x p io of f ld p , b n r l m n n he n rm n r m h e ti ely a tered . Co bi i g t i fo atio derived f o t e micro sc c stu w t t at r m the c em ca ana s s it ma be c nc u e opi dy i h h f o h i l ly i , y o l d d that the rock was origi nally a glas sy pyroxene -andesite-porphyry r m h r n n w c a ac e a asa t in c s t n. T e ore c ns st hi h pp o h d b l o po i io i o , o i i g r te and rr t te w t er a s m a net te is scatt re in of py i py ho i , i h p h p g i , e d r granules th ough the groundmass . This andesite is q uite different in composition f rom that of Elkhorn M unta n w c the ana s s was en but the ar a e c aracter o i , of hi h ly i giv , v i bl h t ese a as has a rea een nte out and the t e un er con of h l v l dy b poi d , yp d s derat n is e ent one the m re as c rm s but ust to w at i io vid ly of o b i fo , j h n i t ll i e xte t it s ypical of a the andesites of th s area is difficult to state . Its alteration from contact m etam orphism is plainly due to the lacco t r lith which lies imm ediately below it and of which i fo rm ed the cove . T he an es tes t s area resent on the w e a m uc m re n rm d i of hi p , hol , h o u ifo a earance t an t se rn a recc a are na pp h ho of Elkho Pe k . B i s occas io lly ser e but the re m nant t e c ns sts an an es te - r r ob v d , p do i yp o i of d i po phy y , in w c the a ra r te en cr sts are us ua smal and few but hi h l b do i ph o y lly l , a l bec m e a c ns cu us eature th r T h r un occ sional y o o pi o f of e ock . e g o d mas s is finely crystalli ne and in som e cases has the appearance under the m icroscope of a dev itrified glas s .

53 2 MIC R O S C O P IO A L P E T R O G R A P H Y or E L KH O R N D IST R IC T .

i r ti r k o iori - h r it M ne al composi on of oc f d te pom y y laccol h .

H om blende

G roundm ass

A lkaline feldspar

100. 6

Th e com position of the feldspars of the groundmass is necessarily m r n es r e the cu arit s th estim ated with so e unce tai ty . To d c ib pe li ie of e in r n r r m ine rals m ore detail : The feldspa phe oc y sts a e from 1 to 4 mm . r c r Th r r in diam eter and S how a v ery strong zonal st u tu e . e greate pa t i a rat er as c a ra r te ar n r m A b A n to A b A n nter s h b i l b do i , v yi g f o 1 , , u i l n r h rt grown perthitica ly with an a desine . Ove t e pa s which are n h n n n n A er s r n m e in th intergrow t e exti ctio is co sta t . ft ce tai ti e period r wt we er the res ua u c u not or not su an of g o h , ho v , id l liq id o ld did pply y n er the m re as c e s ar m ecu es and t en the er n l lo g o b i f ld p ol l , h p iod of zo a r wt e an the c m s t n the a e s e ls e s ar as n g o h b g , o po i io of dd d h l of f ld p p si g r m an es ne to c ase and r m the atte r to a te the e tens n f o d i oligo l , f o l lbi , x io of the li nes of albite twinning to the extrem e boundaries pro ving that m n n s r s t n h n no o ocli ic feld pa exi s i t e oute r zo es . The noticeable fea ture the na r wt is its w e ran e and ar e am unt runn n of zo l g o h id g l g o , i g s it es t r u near the w e ser es the a c s a do h o gh ly hol i of pl gio lase feld pars .

T he trans t n is not a wa s re u ar a m re as c ne s m et m es sur i io l y g l , o b i zo o i n n l r roundi g o e s ightly m o e acid . Some of the felds pars conta in m any sma nclus ns w t n the a ra r te nter r c ns s t n an ac ll i io i hi l b do i i io , o i i g of id a c ase w t a nuc eus rn en e r a res u t n r m th pl gio l i h l of ho bl d , p ob bly l i g f o e rapidly growi ng crys ta l inclos ing small portio ns of the m agma which

na cr s ta e nt the e s ar and rn en fi lly y lliz d i o f ld p ho bl de . The t te is l er in rm at n t an the rn en e W t h bio i o d fo io h ho bl d . i h t e latter it is collected i nto i rregular clus ters containing the iron ore and nd n the ar s tt a r n a at te a s a e ea a ce to th r c . p i l o , givi g d k , po d pp e o k In one instance a well -bounded prism atic crystal of hornblende is

ser e 5 m m . n and u te different in ut ne r m the ra e ob v d, lo g q i o li f o gg d n r m cry stals norm ally prese t . This p is has nuclei of brown hornblende t r u ut s urr un e reen rn len e ssess n the a earance h o gho , o d d by g ho b d po i g pp r uc fferen e n of an alteration p od uct. S h a di c i habit in the same thin section between crystals of the sam e species m ay be ta ke n as ev idence r wt un er fferent s ca c n t ns and t s m a be n of g o h d di phy i l o di io , hi y a nstance a rn en e hen cryst rm e e re ntrus n in con i of ho bl d p o fo d b fo i io , dis tin tion th t ers rm r r s tra c to e o h fo ed afte wa d . m u m 33 T U R N L E Y R ID G E ST oox . 5

r en i s arse resent ccurr n in sm a c r ess an edra Py ox e s p ly p , o i g ll , olo l , h l

r sta s as s c ate w t the a at te and s met m es res ent as nc us ns c y l o i d i h p i , o i p i l io h rs within t e feldspa . T he feldspar of the groundmass em braces about 50 pe r cent of the ent ire r In so far as it can be se arate it c ns sts a ut 40 ock . p d o i of bo e r cent s ma la c ase cr sta s s w n e t er a rod - e rm p of ll p gio l y l , ho i g i h lik fo b n er h unky crv sta ls with lines of al ite twi nning . A oth 40 per cent t s r un mas s e s ar c ns s ts s m ew at ust c un of hi g o d f ld p o i of o h d y , h ky T he crystals showi ng no albite twinning and taken to be orthoclase . other 20 per cent is orthoclase i nte rgrown with quartz and form i ng a

- well developed Inicrographic structure . Th i racte r the en cr sts in t s r c is to be n te e bas c cha of ph o y hi o k o d , standi ng in contradistinction to the rather acid characte r of the r un m ass and r m a c ns e rat n the rmer a ne a er g o d , f o o id io of fo lo v y s t n w e ta ne erroneous idea of the compo i io ould b ob i d .

T U R N L E Y R ID G E S T O C K .

T he Turnley R idge s tock is a very i rregular and branchi ng i ntrusion running for about two m iles across the so uthwestern corner of the

s dist rict and v aryi ng in width from everal hundred feet to a half m ile .

In its n rt ern art it c ns s ts two arms w c a se r es o h p o i of , hi h hold i of altered shales and quartzites between them . T he s ur ace r s w ic it m a be rea rec n e c ns sts f déb i , by h h y dily og iz d , o i an u ar c s a o t o r m re in iam eter a cha c r w ere of g l blo k f o o d , of lky olo h b i r not stai ned light rown and red from fe rric oxide . It s a ock readily susce t e to weat er n w t ut is nte rat n e d n s ur ace p ibl h i g i ho d i g io , yi l i g f m aterial rather light in weight and giving a dull so und when struck t r w the r m a b rec n with the ham m er. A fea u e by hich ock y e og ized c ns s ts num er us cr s ta s uart a tent to a twent et an o i of o y l of q z , h i h of m eter t n s e n t n nc in a sca tere t r u the r u m as . s e o e i h di , d h o gh g o d Th b i g su ect to the s ubaeriel ec w c attac s the t e r m nera s s w bj d ay hi h k o h i l , ho the greasy q uartz l uste r and i rregular f racture and are readilv noted

s d s th a upon a close exam inatio n o f the hand s pecim en. Be i e e qu rtz cr s ta s w c rm ab ut 1 5 er cent the v ume the r c con y l , hi h fo o p of ol , o k ta ns ab ut 40 e r cent o f rectan u ar cr sta s O i clas e weathe re i o p g l y l of l go , d h w In a r e tuat ns as in the ee rair a ut to a c te . v a s c alky hi f o bl i io , d p l o d on rn G u c m ate r a m a be taine w c has not een Elkho l h , i l v ob d hi h b

r n s u a res unweat ere s ur ace h affected by weathe ing . O ch f h h d f t e

r un m as s w ic c m os es a ut 40 er ce nt o f the e nt re um e g o d , h h o p bo p , i vol , - e n w is seen to be de ns e and of a light gray color. Ev here unweath

ered we er the e l s ar en cr s ts are u and us ter ess . A , ho v , f d p ph o y d ll l l m icros copic exam inatio n s hows that the rock has been greatly alte red m ine raliz n waters w c ha e not o n en t is a earance to by i g , hi h v ly giv h pp

the e s ars but a e utter es tr e the s e era e r cent ark f ld p , h v ly d oy d v l p of d 53 M IC R O C L P O F L O R I T I 4 S C O P I A P E T R O G R A H Y E KH N D S R C T .

m nera s w c we re r na resent and a e fina res u te in i l hi h o igi lly p , h v lly l d

e rm ati n o f n r r A rt a ana s a s c m en th fo o co s ide able py ite . pa i l lys i of pe i as near u na tere as c u be un ta en r m the ra r ad cut ly l d o ld fo d , k f o il o eet s ut the rn sta t n a e the w n resu ts f o h of Elkho io , g v follo i g l

r - i i - P artial analysis of qrw lz d or wporphyry.

k s s R N ld H . N . Sto anal t ec . o. 1858 . F ie N o . [ e . y .

A 1 0 inc udin P 0 2 , ( l g 2 5 ) F e 0 to ta iron ,, 3 ( l )

N a, 0

96 . 20

Allowing for the destruction of the ferrom agnes ian m inerals and ta n the a era e a num er sect ns the r na m nera com ki g v g of b of io , o igi l i l pos ition has been estimated by volum e as follows

in al ti t - t - r M er composi on of quar z diorie po phyry.

O ligoclase

H ornblende

G roundm ass (quartz and feldspar)

There is us ually a s harp distinction between the phenocrysts and r un mass the atter e n a m cr ran t c m tu re uart and g o d , l b i g i og i i ix of q z

e s ar the rm er ssess n s ar m r c un ar es se era f ld p , fo po i g h p idio o phi bo d i v l m l m eters in ameter t u certa n t ese m a be c ea a e aces il i di , ho gh i of h y l v g f

n n rm nat n the r s l T he s ar n r and ot those o ce te i i g c y ta s . feld p phe oc ysts

ssess an a era e am eter 2 m m . us ua s w a te tw nn n po v g di of , lly ho lbi i i g, n ran e in em m s t n r m a s a r r te i a d g ch ical co po i io f o odic l b ado i to alb te . O rthoclas e is present in s om e parts of the i ntrusive and m ay form a h m n r art in t ers . oc ase we er is t e c m m n t e the i o p o h Olig l , ho v , o o yp of n r feldspar phe oc ysts . in T he uart en cr sts a era e 1 to 2 m m . am eter e t n q z ph o y v g di , xhibi i g,

w en un r en stra t cr sta ne s es r un e on the c rners . h b ok , igh y lli id o d d o The ir pri ncipal growth took place before the intrusion i nto the present ca t s nce m an ra m ents s w a cr sta ne ut ne on one s e lo li y , i y f g ho y lli o li id nd an rre u ar racture on the t er and s ar ra m ents the r s a i g l f o h , h p f g , déb i

f r en and dra e cr s ta s are c m m n. T he e tinct n is s ar o b ok gg d y l , o o x io h p nd not un u at r s w n t at the ract u re has not ee n accom a a d l o y , ho i g h f b p i b s n in c m r at s tresses . s s to e e ecte ce a s me u nied by g e Thi xp d , i vi id di

o 53 6 M IC R O S C O P IC A L P E T R O G R A PH Y r E L KH O R N D IS T R ICT .

Th e analyses f rom those localities are brought together for purposes r of com pa is on .

nal s s o r ks rom th B o ulder batholith A y e f oc f e .

a 518 98 ( ) .

° at 1 10 C

° H ,O above 1 10 C

T ot l 9 99 88 0 . a 99. 7 10 . 05 100. 18

G ni ks u on nd i ini w W ou o l v —D ec « ra te roc of B tte M t a v c t b H . e d : J G o 1 99 737 e r. e . N . 8 . . , y , y . , , p W h r r O n 5 eth e S o S , und eterm i ed .

T f h a i N . 1 12 . t e b o t E k 1 o e o t at orn m ie rom th e contact . yp h l h l h , l f o B ut r nit N 6 . te t of a e B di r k n . i t H . t l ut st c N . s e te S o es a a t . yp g , ; , y o H f n r k 98 . d o a ce e n r S N . b o ea C C e o t rn art of t li II N . k s n e a o t . t e a a y st . l y , h p h h ; , l 5 P r i f o 1 . i n t r ni N . 8 eva e o t f it B d r H . S a e o bat o at ou e N . t kes anal o st . l g yp g h l h l ; , v

T he rock whos e analv s is is given unde r 1 1 2 res e m bles the Butte

ran te as c s e in a earance as it es in c e m ca m it n g i lo ly pp do h i l co pos io .

It is a rat er cr s s - ra ne roc sh w n ab ut e ua am un s of h o g i d k , o i g o q l o t

rnblen e and ot te uart ex s t n in s m o rains nd h ho d bi i , q z i i g ky g a t e feld

ars w t ra to n c ea a e s ur ce U n er th sp i h g y pi k l v g fa s . d e m icros cope the a oc as e is s ee n to ran e r m an andes ne A b A to a s c pl gi l g f o i , s m, odi labrad r te A b A n exis t n in m o r c but not we - o rm e o i , l i g idio phi ll f d crv s s w am . 7 ) B A T H O L IT H AT E L KH O R N . 53

h r n r r r m to 3 mm in am te r. T e t c ase a d ua t a e a ta s 1 . e s l f o di o ho l q z , us ual the na r ucts cr stal ati n the rm er c nta n n s m e , fi l p od of y liz o , fo o i i g o h n e u h rn n and t te ar s a. T e a at te ma et t a te e e e od p i , g i , gi , o bl d , bio i ass c ate t et er the a at te be n the ear est m nera to cr sta o i d og h , p i i g li i l y l e t en the au ite the atter rm n er rre u ar nuc e in the liz , h g , l fo i g v y i g l l i n r i nterior of many hornblende crystals . T he hornble de p eceded the t te in cr sta zat n and the rmat n t ese t ree min bio i y lli io , fo io of h h T he erals res ulted in the sim ultaneous separation of m agnetite . latte r exis ts in grains averaging a third of a m illimeter in diam eter and als o as a dust spri nkled th rough certai n of the hornblendes . The small amount of titanite p resent belongs also to the earlies t m i neral e ner n g atio . ’ ’ ’ zo —T he r the bat t ar es t in araat ns in tartare. ock of holi h v i bo h s T t i na e i rather texture and in compo ition . he ype wh ch was a lyz d s coarse grai ned and without any evidence of gneissoid or flow strue

ture . A t t er aces not far stant we er and rm n a t e o h pl di , ho v , fo i g yp er a s e ua a un ant the r c w th ut c an n n t cea in p h p q lly b d , o k , i o h gi g o i bly c m os t n s ws a a ntl ara e arra n em ent the ar m nera s o p i io , ho f i y p ll l g of d k i l , s r n which gives it a som ewhat gneissoid structure. Thi va iety does ot s w an nt m ate re at on to the c ntact th u er a s m re ho y i i l i o , o gh p h p o

a undant w t n a m e it . In the c n t the c ntact and b i hi il of vi i i y of o , rm n a ne ar a e w t the r c ec mes ner ra ned fo i g zo of v i bl id h , o k b o fi g i , t u no case has een ser e w t in the s tr ct or e sew ere at ho gh b ob v d i h di i , l h the mar ns the ath t w ere a st nct o r r t c structure is gi of b oli h , h di i p phy i i um ass ed . T he principal effect of the contact has been to cause a more or less s m u tane us r wt the m nera s w c es an a r ach to the i l o g o h of i l , hi h giv pp o s u ar- ranu ar s tructure and the uart and rt c as e are a s s m e g g l , q z o ho l l o o i n n tim es po ikil tically i terwove . ' 7 ' f a — T h m r r s ar no o 17 1atlmzs in cox/7p s zti0 n . e la p ophy ic facie e t s str n m ar e as is re uent the case but st are n t cea o gly k d f q ly , ill o i bly r r u ut the ent re c ntact and for sta nces a uarter p es ent. Th o gho i o di of q of a m ile from it the rock usually s hows a greater predom inance of l a clas e and ten err m a nes an m nera s . es e c ntact pl gio , of of f o g i i Th o

acie s m a be ar l n t ceab e to o ne tra e n e r the c untr but f y h d y o i l v li g ov o y ,

ecom e e ent u n c m ar n s ec m ens r m fferent nts the b v id po o p i g p i f o di poi , trans ition bei ng by a s hading of one type i nto another and not by

r s m a ked contras t . T he lim its of the variatio ns are b rought out in the table of compari s ns on the nex t a e the r c r m ur nts a n the c ntact e n o p g , o k f o fo poi lo g o b i g na h e n T he r ck compared to that o f which the a lysis as be n give . o ne er eco m es m re as ic t an a i r te and the am unt ark v b o b h d o i , o of d

r S t ffer n e in c m s ti n c m o nents is not reat nc ease . e c s o p g ly i d ligh di o po i o , h wev e r m a in s o m e cas es caus e rn e n e to becom e the ru in o , y ho bl d l g 53 8 M IC R O I E S C O P C A L P T R O G R A P H Y or E L KH O R N D IS T R IC T .

nd t r s ec es a a t e t m es t te . T h m st ns nt p i , o h i bio i e o co ta differences on approaching the contact lie in the i ncrease of plagioclase and the ecreas e uart n cat n a r se in a um na an a in i d of q z , i di i g i l i d fall s il ca. A t t er laces on the m ar ns t s at t w ere ntact o h p gi of hi b holi h , h co ac es a r ac n the a r s a e een s er e t e a e een f i pp o hi g g bb o h v b ob v d , h y h v b ten un in a s ente r n nto the se m entar es and the a sence of fo d b y i g i di i , b of s uc w t n t s str ct at east in e ent c nnect n w t h rent h i hi hi di i , l vid o io i h t e pa 1 bat t ma be the reas n for t r r r T h an e a u e to u . s s holi h , y o h i f il occ e aly e two s uc rm s r m R ed M unta n 1 0 m es s ut ut s of h fo f o o i , il o h of B te, how d an er cent s ca res ect e . p of ili , p iv ly T he tabl e shows the m ineral composition by volum e as estim ated the m cr sco e a m et suffic entl accurate to r n out h by i o p , hod i y b i g t e enera ar at ns w c are nder scu n g l v i io hi h u di ssio .

Varieties o ranitic ro ks o the Boulder batholith within th n f g c f e E lkhor district.

rad orite A b A n Lab , , l

A ndesine A b A n , 5 it O rthoclase w h N a,0 Q uartz

Biotite

H om blende

u it A g e .

100. 8 100. 1 99. 7 100. 1 100. 1

- i . T n rn 12 . m onz n te f ba h it 1 m i m on t o weste N o. 1 uartz o e o t o e ro c tac Q yp l h , l f , i m i S a a s s on a e 536 . li t of m ap. ee n ly p g F r m i ntact h th . ri n m c N 1 D iorite . o bat o i a l r a 1 eet ro o o . 38 . O e s 00 l g l y, p h p , f f r s n r t i F rom oint with lim estones ; at p e e t sepa a e d by apl te dike 400 feet wide. p n B k Butte tri n ti n in fee t orth of lac a gula o po t. N - z ni ric in r 1 r t i an i 56 . uar z m n te e ds 00 t m ntac w t es te o . t o o a ee o co d Q , h f l p ; f f h ; r rn m no theas t co er of ap. - i — ri r m n it rn n n i n al 164 . uartz m ea dio te 20 e t o co t t w o sto i c us N o . Q ; f e f ac h h e l o ; h f i ut t B k B ri ati n oint m le so hwes of lac utte t angul o p . i m h a z-di ri - r R id 38 . D orite 50 eet ro t e u rt o te r stock of T urn e e N o . ; f f q po phy y l y g f m r on weste rn lim it of m ap 1 m ile north o southeas t co e .

1 - d i init b W . Weed J o r. l N 1 r ks f Bu te an v c . H : u G eo . ov . D ec. 899 . 789. G ranite oc o t . y. y , , , p

540 M IC R O S C O P IC A L P E T R O G R A P H Y or E L H O R I T R K N D S IC T .

1 the erras has een o nt W n of Si b p i ed out by eed . A analysis of one of the erra ranu te s a tes is a e for r s s m ar n Si g li ( pli ) dd d pu po e of co p iso .

A nal ses o a lites rom Montana and ali rnia y f p f C fo .

° H 20 at 1 10 C

° H , O above 1 10 C

’‘ l i0 2

T race .

T ota 100. 1 2 1 44 93 99. 8 00. l 3 99.

W s a T otal S . 5 h eth er sul h ide or ul h ate und eterm in p p ed .

it r m m idd e of ar e intrusion t w st of bat o i n N o . 1 09. A e o ee e t co pl f l l g , f h l h B n l i tact nort of B ack utte . A a s s N o. 1858 . F ie d N o . 109. H . N . Stokes , h l y l , analyst. 142 A i rom rn id of m intru ion near inc usions f uartz N o . . te easte S e sa e s o pl f , l q

i N 42 H . m onzonite eet nort of B ack Butte . A na sis N o. 1858 . F e d o. 1 . . N , f h l ly l

Stokes ana st. , ly i f B ni Bu n 4 . A o utte ra te tt M o t. No . 6 0 te e pl g , , nuli it dik ast f M iton S i r un l d 2 G ra te a e e e o e ra t C a . o ecte N o. 19 . C o C ( pl ) l , y, ll

W ur r W . i n n l F . H bra d a a t . . T n . S B . . and described b H . . e e s . ee u U S G eo y ll , y ll l

2 . S urv e v N o . 168 . 19 , p

In ge neral the aplites of the Elkhorn district show the s ugar-graun lar te ture and cons s t s a rt c ase and uart w t a fe w e r x i of od o ho l q z , i h p n n l i i ce nt of plagioclase a d occas io al crvsta s of biot te and m agnet te . The plagioclas e is an andes ine and is idiomorphic; the q uartz and o rtho

1 - in i W d u l D . n i t b W. 11 . ee : J o r. eo . N o ee 1 7 . r ks f utte M ont a d v c G v . 899 . 40 G ra nite oc o B , y , y . , , p m ann a P L IT 54l ] A E S .

c ase we er a e r wn s m u tane us and ne t er has een a e l , ho v , h v g o i l o ly i h b bl s to as um e its proper crystallographic form . T he t te us ua rms r m one- a to 2 e r cent and ccu rs in bio i lly fo f o h lf p , o i T he u t r i t n s es er n m m n am eter. ac c s e n n hi cal av agi g . di l k of l i g - the biotite is characteris tic and gives a finely speckled appearance to

the rock .

E L K H O R N M O U N T A IN A PL IT E .

T he above petrographic description applies entirelv to the northern m st ntrus n a t c s eet n etwee n the uart -m n n te an o i io , hi k h lyi g b q z o zo i d T h in its n the andes ites of Elkhorn Peak . e aplite contact agai st the andes ite form s a sharp and eve n s urface and s hows evidence of chill ter ro A t the im m e ate c n c a ne a ing agai nst the lat ck . di o ta t zo of plite an nc in w t is a r c arse ra ne w t uart and e s r i h id h f i ly o g i d , i h q z f ld pa poikilitically i ntergrown and poss ess ing a cons iderable amount of bio s oarse ness is ev identl due to the c ntact sur ace ff r n tite . Thi c v o f o e i g a as sa e for the esca e a ors w se n uence in n uc n a p g p of v p , ho i fl i d i g

coarser crystalli ne structure is well known . Followi ng the narrow band of coars e -grained aplite is a zone s everal feet in width with an indefi nite inner boundary characterized by a radi ate i t c nter r wt uartz and rt c as e the m e ra n po kili i i g o h of q o ho l , ix d g i s in i m e e re is m t t the m n 2 m a et r. uc ess e t an t bei g m . d Th h l bio i h a i me A t n s n en 52 nc es r m n t diate contact. hi ectio tak i h f o the co tac showed certain crys tals of o rthoclas e which becam e poikilitic o nly after half the r r wt was c m lete n cat n a c an e in the s ca con i g o h o p d , i di i g h g phy i l ditions at t at er d w n to w ic the uartz and rt c ase c u h p io , o i g h h q o ho l o ld n n er se arate t emse es r m eac t e r ur n cr sta at n o lo g p h lv f o h o h d i g y lliz io , m n h and we re of si ultaneous growth . A t a dis ta ce of 20 feet t e aplite - shows its normal sugar granular characte r. The importa nce of these contact phenom ena lies in the proof they ni t fur sh hat the aplite is younger than the andes ite . — TIce aplite stock In the aplite stock penetrati ng the sedim ents feet southwest of Elkho rn Peak the aplite s tructu re is nearly r lost and the rock as sumes the appea ance of an acid granite . It has no visible contact with the q uartz -mo nzonite and conseq uently its aplitic affinities are based solely upon its m ineral and chem ical com s t n T he c an e in te ture is no u t due to the ar e ame r po i io . h g x do b l g di te the s t c w c erm tte a t r u eat n the wal s and of o k , hi h p i d ho o gh h i g of l a n e ri f c io n prolo g d pe o d o rys tallizat .

A P L IT E D IK E S N O R T H O F B LA C K B U TT E .

In the system of dikes which have broke n th rough the quartz m n n te l n n rt and wes t ack Butte the n rma ty e o zo i yi g o h of Bl , o l p of a l te is t at i en in ana s s 1 09 on e 540 I i n ac ran te a . t s a p i h g v ly i , p g id g i , t but tt e t t n bu ttl - a t tur wi h li l bio i e a d t li e of the s ugar granul r ex e . It 4 IC R C I A L P R R P H E K R N D S T 5 2 M O S O P C ET O G A Y or L H O I T R IC .

s a es nto two r m ine nt ar et es one w c a r ac es the e h d i p o v i i , of hi h pp o h p g m ti i the t r th r r t t tru r he r r a t c e e c e s ctu e . T m e , o h po phy i i yp of fo ccurs at laces in the center the ntrus ns and one l ca it was o p of i io , o l y noted where the biotite s howed a great exte nsion along one of the r ntal a es rm n t n a es a a m l mete r in w t 1 2 ho izo x , fo i g hi bl d , h lf i li id h by in r ri r n n . T h i n h m to 1 5 mm . le gth e othe va ety s found ear t e a gi s of

l i n n . the ntrus ns an ana sis t e e as No 142 on a e 540. i io , y of b i g giv , p g It is finer ra ne s ws the t ca a te structure and is r c er g i d , ho ypi l pli , i h h r W n th in t te t e atte e st n in sca es and not in c us te rs . t bio i , l xi i g l l i hi e mm e ate c n t the mar ns we er c usters are rm e w c i di vi i i y of gi , ho v , l fo d hi h r m t th artz -m n n ese ble hose of e qu o zo ite .

A P L IT E D IKE S SO U T H W E ST O F BLA C K B U TT E .

T he aplites occurring at i ntervals along the margin in the southern art the str ct S w the n rm a c aracte r e ce t at one oca t p of di i ho o l h , x p l li y

a a m e s ut west ac utte w ere en cr sts a. s a h lf il o h of Bl k B , h ph o y of od n n in m r r n T h r ta s re iki rt clas e a c a ete a e a un a t . e c s a o o ho , i h di , b d y l p liticall nter r wn w t uartz w c is v si e in wa nes to the y i g o i h q , hi h i bl vy li e nd c nst tu s a a u rter h m he cr naked ey a o i te bout q a of t e vol u e of t ys tal . By the excl usion of the biotite and plagioclase m olecules from the area the en cr sts the a t c r un m as s is c rres n n of ph o y , pli i g o d o po di gly ri n n en ched i those com pone ts .

C O N T A CT D IK E B O F A P L IT E I N T U R N L E Y G U LC H .

A few es a te cr ss urn e u c arel w t n the m ts dik of pli o T l y G l h , b y i hi li i

the m a . e are m e um w t and lie etween the uart of p Th y of di id h , . b q z d h tr n r r m n r monzonite an t e andes ite . S i ge s f o them pe et ate the andes ite and p ro ve in the most unequivocal manner the younger age of the d l h - a te an t ere re a s t e uart m n n te . s e uct n es pli , h fo o of q z o zo i Thi d d io giv em r m r d i at tance an r I. t e ence one s ust ate in P X B . h g i po , h ill d L II ,

S E G R E G A T IO N S O F T O U R M A L IN E .

A t a num er aces w t n the a tes es ecial on the r a to b of pl i hi pli , p ly o d rn ea at an e e at n a ut eet and near the r ers Elkho P k , l v io of bo f , bo d the ntrus ns n rt ac utte cr w- t se re at ns a of i io o h of Bl k B , o foo g g io of c m n r re s r in bla k i e al a observed . The e a e us ually abo ut 1 inch diam bu m a b r m to n r e ter t e 4 6 c es ac ss . m et m es t e ar , y f o i h o So i h y e rre u ar en r t c rm s at t er t m es c rcu ar or r m b a in i g l , d d i i fo , o h i i l ho oid l n T he e ut e . s e r at ns are nterm e w t uart and rt c ase o li g g io i ix d i h q z o ho l , and un er the m cr sc e are un to c ns s t a ar t urm a ne d i o op fo d o i of d k o li , n r T h without crystal bou da ies . e tourmaline areas are alway s sur r un e a w te ne r m one- a to 1 nc in w t r m w c o d d by hi zo f o h lf i h id h , f o hi h the ar c m nents a e e ent een e aus te e n w c d k o po h v vid ly b xh d , b yo d hi h the aplite as s um es its norm al appearance and possesses its scales of

544 M IC R O S C O P IC A O L P ET R G R A P H Y or E L KH C R N D IS T R IC T .

1 em e as an a ect e . us one m a s ea a r ssu ar te a ploy d dj iv Th y p k of g o l i , es u an te or a a r d r h a a te r c as t e cas e m be . v vi i , l b o i o k , y T he m etamorphosed sedim ents will be treated in groups acco rding to c m s t n and not c r n ca the s ns e n ta en u o po i io , h o ologi lly , divi io b i g k p in the r er in w c t e u n n o d hi h h y have j st bee am ed .

Q U A R T Z IT E S .

P U R E Q U A R T Z IT E S .

The ure uart tes are ra s -w te r c s ne ranu ar and p q zi g yi h hi o k , fi ly g l n a r s r u n r havi g vit eou luste po f es h fracture . The purest beds withi n the Elkhorn district are found withi n the Quadrant quart z ites and at th to the am r an e p of C b i quartzite. r m t s ure t e w c is relat e rare t ere are e artures F o hi p yp , hi h iv ly , h d p

in se era rect ns . 3ma am unts a n rt c ase and un e v l di io ll o of k oli , o ho l , d com posed rock are commonly present in the origi nal sandsto ne as a c a - e n and u n m etam r s m ese t rn nt t te and l y lik filli g , po o phi th u i o bio i

e ds ars . O n the t er an t ere m a be u e m ate r a s w t n f l p o h h d , h y s ol bl i l i hi the san st ne and t e r rem a and the resence s ca- ear n d o , h i ov l p of ili b i g waters m ay result in a fu rther deposition of silica and ally such m n rtz h replace e t qua ites to t e vein quartzes .

Q U A R T Z IT IC H O R N STO N E S .

Those m etamorphic rocks which approach most nearlv to the quartz ' tes but difler r m t em in ha n r ina ssesse an a m ture i , f o h vi g o ig lly po d d ix c a and un ec m se r c ra ments are dar in c r and re of l y d o po d o k f g , k olo p s nt the e ran f a n m e app a ce o fi ely granular eruptive rock . Exa ples of t s t e are un in the m d e m em er the urn e ser es r hi yp fo d i dl b of T l y i , o ig inall a fine - ra ne a n t n T h r n ns a u y g i d sh ly sa ds o e . e ocks ow co ist of bo t 1 5 per ce nt of olive -green flakes of microscopic biotite and 85 per cent r h d r ua tz . T e c r is ue to the a m ca and the racture is s me of q olo d k i , f o r h r d n w at s u ar m t e eat a un a ce uart e No . h g y , f o g b of q z (fi ld T he greatest coarseness of crystallization in rocks of this com posi tio n is observed in those places where they are adj acent to the granite . n n r h m w un t nsi t O e s uch (field N o . 4) u de t e icroscope as fo d o co s of 70 e r ce nt uart 1 0 er cent ac a oc as e el s ar r a all p q z , p id pl gi l f d p (p ob bly c as e and 20 er cent ee - r wn t te in lates u to a oligo l ) , p d p b o bio i p p h lf m T h r S h n ntat ns a milli ete r in length . e c ystals how t e m utual i de io

due to s m u tane us r wt a eatu re c aracte r st c uart z ites and i l o g o h , f h i i of q h rnst nes and the car onace us matter is scattere rt t r u o o , b o d di ily h o gh T h r r the fe lds pars . e m os t typical rep esentatives of this g oup of

l - l i i ri l in col r bu in th lkh orn r ks is G rosamla rite is a lim e a um na ga rnet; t va es great y o , t e E oc - usua ll ale rown or more rarel reen. Vesuvianite is a as ic lim e alumina silicate carr in y p b , , y, g b , y g o m e iron . m am a ] Q U A R T Z IT E S A N D H O R N S T O N E S . 545 m m r m n are un w t n the M es z c w ere t e eta o phosed sedi e ts fo d i hi o oi , h h y S w as ar r m nent e es w c n the tra ne e st c ul ho h d , p o i l dg hi h o ly i d g ologi o d with certai nty dis ti nguish from er uptive rocks . bed ar c arse ranu ar r c w c s u also be r u e A of d k , o ly g l o k hi h ho ld g o p d un er t ea n was n te in the urn e s a es the west se d his h di g o d T l y h l , lo di i U n er the m cr sc e it mentary horizon of the distr ct (field N o . d i o op is seen to consis t of extrem ely i rregular anhedrons of quartz and feld The e s rs are re m nant r r m 1 t m in am eter. a spa f o o 2 m . di f ld p p do i ly l a n and are v er rre u ar in c m s t n n an une en extinc a k li e y i g l o po i io , givi g v n u n r r s n s U n e the t er uart t c rns tone s the tio de c os ed icol . lik o h q zi i ho , cr stal n ua s are not c ear se arate r m one an t e r but S w v i divid l l ly p d f o o h , ho T h t te cr sta s s w the rm c a acter extensive i ntergrowth . e bio i y l ho fo h r i i rns t nes and are scattere t r u the e s ar s m et m es st c of ho o d h o gh f ld p , o i l T m n a the um e in t er aces exist n in c us ters . he co posi g h lf vol , o h pl i g en tr ut n the t te causes the r c w en seen in la n unev dis ib io of bio i o k, h p i i t in t n sect n to resem e an ar se but s uc an a earance l gh hi io , bl ko , h pp ' ma be an eflect cr sta at n. ere is c ns era e r un m ass y of y lliz io Th o id bl g o d , mostly of a feldspar. The greater freedom of crys tallization which this rock shows m ay to it e n a stratum read erm e r n a r be due s b i g ily p eabl to isi g v po s .

H O R N S T O N E S .

The h rnst nes as the nam e is here use nc u e th s m o o , d , i l d o e eta morphosed rocks that are rich in s ilica and alum ina— rocks which in r or na s ta te e n e to the a s r e thei igi l b lo g d cl y e ies . Th y alway s ca rry w t the a um na a c ns era e am unt a al es m e m a nes a i h l i o id bl o of lk i , li , g i , and r n and u n rec ns t tut n c ns st re m n nt , a y quart i o po o i io o i p do i l of z , t te and e s ar the c aracte r the r c c an n w bio i , f ld p , h of o k h gi g ith th e relat e am unts eac and the c arseness r st iv o of h o of c y allization .

se rnst nes r c in ar il ace us m atter are m uc s ter and Tho ho o i h g l o h of , in the fie m a res em e ar ac u m ld y bl gill eo s li es tones . T he conte nt of m e we e r has a wa s een low and in t r , , y e m etam r ose li ho v l b , h i o ph d te t e ne er effer es e w t sta h y v v c i h acid . They have been origi nally s a es and the two reat c an es w c a e ta en ace a h l , g h g hi h h v k pl re a los s of the fiss e nature and ten a m ar e c an e in c r the il of k d h g olo , latter being due to a clearing up of the groundm as s th rough crystallization into e s ars o r uart the c ect n the r an c m at r i f ld p q z , oll io of o g i te n dots affect in the c r ess t an s sem nate and as t the r g olo l h if di i d , , l ly , colo given by m cr s co c t te a es s et u t r u ut the s e n T h i o pi bio i fl k p h o gho ctio . e U nion ’ s a es s w t es e diflerences we s nce n rt th t h l ho h ll , i o h of e o wn they exis t w t t e r r na ac fiss e c aracter but on the s u i h h i o igi l bl k il h , o thern side of th e C em ete ry R idge s tock they are s oft rocks of a purplis h -red color

- e N o . w t ut m uc c ea a e and w t a fine r ne (fi ld i ho h l v g . i h g ai d splinte ry

2 G E O L P T 2 — 01 — 3 2 , 5 ' ' 6 m ca o m ou . P 54 sco ETR O G R A P H Y or E L KH O R N m s rm or.

t U n er h m cr frac ure . d t e i oscope this hornstone is seen to consist of th e following com ponents

Per cent. Light-brown biotite flakes Q ua rtz grains G ra itic i m ent in O a ue strin and rains ph p g , p q gs g Microgranitic feldspar and quartz groundm ass

T he lowes t m em ber of the Turnley s eries is a som ewhat sim ilar rnst ne a te r ur s red c ns st n 33 er cent bio ho o , of ligh p pli h , o i i g of p of t te a es and er nt n r n l i fl k 67 p ce of fi ely g a u ar q uartz . Its fissile char ter has not n n r W n t ac bee e ti ely destroyed . ithi he M es ozoic series im - s ar e s are u n one e n n te a s ate u r. il b d fo d , b i g o d of l bl e colo

- - L lM E A L U M IN A S IL IC A T E R O C KS .

The lim e -silicate and lim e-alumina—silicate rocks are of great hard ness and in c r ran e r m w te to a e r wn reen or e w , olo g f o hi p l b o , g , y llo .

They indicate thei r large percentage , of lime by showi ng under the r nt th m - e m n m ic oscope esse ially e li e silicat i erals . U sually not more t an t ree suc m nera s c nst tute the u an one r c h h h i l o i b lk of y o k, t u th es ar acc r n to th c m os t n th s ho gh e speci v y o di g e o p i io of e trata . h se c mm nest ccurrence are r ssular- arnet s e wollas T o of o o o g o g , diop id ,

- t n te es u an te and m e s a e s ars . o i , v vi i , li od f ld p Wh erever the strata are of the proper composition they m ay be m etam orphosed bodily for distances of several hund red feet from the ne us c ntacts w t the rm at n certa n t ese m nera s and ig o o , i h fo io of i of h i l he l n t expu s io of the carbonic acid . The Starmount argillaceous limestones are the lowest strata of this m c aracter. ut ac utte and e te n n r t ere to h So h of Bl k B , x di g f o h Elk rn G ulc t e are seen in ccas na utcr s and at t er ho h , h y o io l o op , o h r The e n anes are places are exposed by e osion of the s oil . b ddi g pl st l e ent but the r c s are ar and s m etim es s w c arse cr s il vid , o k h d , o ho o y ta s a e - reen s e at t er t m es a e-br wn arnet in l of p l g diop id , o h i p l o g - - ut banded lens like m asses or in sponge like growths . So h of Elk horn G ulch there is but little m eta morphism within the main m as s of se m ents but w t n the ra m ent s ate etween the uart -d r te di , i hi f g i ol d b q z io i po rphy ry and the diorite -porphy ry laccolith the final degree has been reached . T he im purities of the s trata have been sufficient to com bine with

all the m e c m etel e e n the car n c aci . li , o pl y xp lli g bo i d T he ave rage com pos itio n of these rocks is about as follows

Per cent .

D iopside Wollas tonite L im e-soda feldspar A lkalifeldspa r

'

548 in caoscorIC A L P E T RO G R A P H Y or E L KH O R N D IST R IC T .

t ser tsel w t the sam o we r as s m e t er m nera s no as t i f i h e p do o o h i l , notably garnet . In the eene m esto nes at t e r c ntact w t the em eter R e K li , h i o i h C y idg stoc as e re n te the r ina r c is a s ce us m te w c k , b fo o d , o ig l o k ili o dolo i hi h u n m etam r sm e m es a s e r c t u r ab con po o phi b co diop id o k , ho gh p ob ly na h s w r st n re e t er i ta n n s m e a um . T e e e e e e n i i g o l i diop id h xi i g f , i h ca c te or in w asto n te s ws as c ear e n ate s er a ea s l i oll i , ho l , lo g d , ph oid l b d , W r it r h r 01 m . in am et r. e e m s t ofte n not m o e than . m di e h fo e bulk of the rock it exists as anhedrons rudely fitted together and without r r The ua e the in ua ra ns in suc a sha p corne s . us l siz of divid l g i h ggre m et m t r m m . u s e a e uc a i ut . 5 m c s ates s a 0 t s e r er. g bo , ho gh o i h y h o

O R D E R O F C R Y S T A L L IZ A T IO N O F T H E M IN E R A L S O F T H E L IM E - A L U M IN A S IL IC A T E G R O U P .

When m etamorphosed the rocks belonging to this group indicate t e r c aracter the resence s m e m e-a um n us m nerals suc h i h by p of o li l i o i , h

as arnet es u an te e te and a clase . t u t ese r c s g , v vi i , pido , pl gio Al ho gh h o k , e the rnst nes us ua s w a m utua restra nt am n all the lik ho o , lly ho l i o g m nera s et certa n s ec es te n e b t s m ew at m or c i l , y i p i of xhi i o h idio phi n a efin or er in r s n outli es and i ndicate d ite d c y tallizatio . G arnet is the m ineral which ofte nest has power to assum e its proper

rm and es so in the resence all the t er m inera s . u te fo , do p of o h l A gi ,

w e m e arnet itse m s w as t n te fie No . hil old d by g , lf old oll o i ( ld a c te is ten a sec ndar m neral and in s uc cases fi s m ar t c C l i of o y i , h ll i oli i t es or re aces s m e re us m neral and is natu ra in suc cavi i pl o p vio i , lly h r cas es highly allotriomo phic.

W ere e st n ev identl as an r na m nera fie N os . 9 1 46 h xi i g v o igi l i l ( ld , ) i rm s e t e r a ack r un f or sm a cr sta s t er m nera s s uch t fo i h b g o d ll y l of o h i l , s a s au te or e s ts tsel in u ar m asses as ncl su res a be d of gi , xi i f glob l i o It is never s en to ssum e its cr sta ra c within oth er m inerals . e a y llog phi outlines . ' — T he m n n s ec es is t rm n O ccurre nce q znerals . co bi atio of p i de e i ed to a great extent by the proportionate am ounts of the elem ents in the r c but es es t s t ere are cert n m nerals w c s w a reater o k , b id hi h ai i hi h ho g n i h m t un l tendency to fo rm than do others . G ar et s t e os iversa of these and is very com m on wherever the al um ina and silica exist in proper am ounts . Where the garnets occur of m icroscopic s ize they do not show the ar an es cust m ar in ar er s ecim ens but are a r u sh p gl o y l g p , of o ghly T he u ua s e the ra ns in the t n sect ns is polygonal form . s l iz of g i hi io t n in n r e r as the m . n W re e s c e a a e 0 to . m a d e s r m . 5 10 f o , h xi i g o id bl r - grains are gathered together in m ulber y like cl us ters . i th n r t Its Ves uvianite is com paratively rare n e Elkhor dist ic . m ca c m s t n is near to t at r ss u ar te and the atter che i l o po i io h of g o l i , l . m ann a ] M AR BL ES . un er the c n t ns re a n in th s re n has usual een the d o di io p v ili g i gio , ly b m ineral species with strongest tendencies to form . Where the compositio n of the original rock determines the form a t n e s ars as a metam r c roduct t ese us ua assume the io of f ld p o phi p , h lly i w n n rm anhedrons a ut . 05 mm . in d am eter cr o e an t er fo of bo , o di g o h c se and ent re a tr m r h c rm n a r un mass w se lo ly i ly llo io o p i , fo i g g o d ho n appearance is unlike that of an igneous rock . M icrocli e has been in h r l n te in ar er areas 1 mm . or m re am eter t e c sta s s w o d l g , o di , y ho ‘ in wate re eflects tt n a a nst one an t er rre u ar mar ns g d , fi i g g i o h by i g l gi and incl uding within th em f ragm ents of the other m inerals (field h r tr n resem e the ccurrence m cr ne No . S uc a eas s o gly bl o of i ocli in s m e s en tes and n t n but the e re at ns and a earance o y i , o hi g fi ld l io pp of the hand s pecimen decide as to whether the rock was originally e ign ous or sedim entary . T he mos t general distinction between the rocks of igneous and sedi m entary origin lies in the us ual allotriomorphic character of all the m inerals com pos ing the latter. T he m nera s ec es are usua a s st nct and w ere a e ccur i l p i lly l o di i , h lik o in different pe rce ntages characteristic of the two classes . There are cases we er w ere r c s s uc w e fferent r ns a roac , ho v , h o k of h id ly di o igi pp h r near to t r in m cr e r n ve y each o he i oscopic app a a ce.

M A R E L E S .

T he marbles are composed of a mosaic of anhedral calcite grains of v ar n s e and c nta n sma am unts t er m nera s acc r n to yi g iz , o i ll o of o h i l o di g the nature the m u r t es resent as for e am e the mar e w t of i p i i p , , x pl , bl i h sm all amounts of wollas to nite present which has al ready been m en

tioned. A n t er m ar e ccurr n n rth the to wn h rn o h bl , o i g o of of Elk o , shows in the hand specim e n many slender black needles which under the m cr sc e r e to be s e eta cry sta s trem te the cr sta s i o op p ov k l l l of oli , y l being hollow and havi ng grown through the m arble without the power e c ud n r m h n r of x l i g it f o t e i terio . From the previous petrographical studies it has becom e evident that even u nde r the most i ntense m eta m orphism the carbo nic acid is not expelled f rom the lim es tone except by the presence of silica and the t er m ur t es w c c m an i s t h n n o h i p i i hi h a co p y t . Thi fac as bee ote d else w ere t er ser ers and s r es for the rn strict an h by o h ob v , di p ov Elkho di y e tens e m rat n m a m at c m r a nt th se en x iv ig io of g i ate i l i o e dim ts .