DESCRIPTION OF THE TELLURIDE QUADRANGLE.

INTRODUCTION. along the southern base, and agricultural lands water Jura of other parts of Colorado, and follow­ vents from which the lavas came are unknown, A general statement of the geography, topography, have been found in valley bottoms or on lower ing them comes the Cretaceous section, from the and the lavas themselves have been examined slopes adjacent to the snow-fed streams Economic Dakota to the uppermost coal-bearing member, the only in sufficient degree to show the predominant and geology of the San Juan region of from the mountains. With the devel- imp°rtance- Colorado. Laramie. Below Durango the post-Laramie forma­ presence of andesites, with other types ranging opment of these resources several towns of tion, made up of eruptive rock debris and known in composition from rhyolite to basalt. Pene­ The term San Juan region, or simply " the San importance have been established in sheltered as the "Animas beds," rests upon the Laramie, trating the bedded series are several massive Juan," used with variable meaning by early valleys on all sides. Railroads encircle the group and is in turn overlain by the Puerco and higher bodies of often coarsely granular rocks, such as explorers, and naturally with indefinite and penetrate to some of the mining centers of Eocene deposits. gabbro and diorite, and it now seems probable limitation during the period of settle- sa^juan the the interior. Creede, Silverton, Telluride, Ouray, Structurally, the most striking feature in the that the intrusive bodies of diorite-porphyry and ment, is. now quite. generally >-. appliedT -i to region. and Lake City, all situated in mountain valleys, present attitude of the formations described, from the allied varieties found in the sedimentary beds a large tract of mountainous country in south­ are thus connected with the main lines of traffic. the base of the Devonian upward, is Outline of adjacent to the San Juan Mountains on the west western Colorado, together with an undefined The geological history of the San Juan region is the general southerly or westerly dip structure. are also of later date than many of the surface zone of lower country bordering it on the north, too complex and as yet too imperfectly known to away from a point in the west-central part of the lavas. west, and south. The Continental Divide traverses admit of even an outline statement of satisfactory San Juan Mountains not far east of the Telluride The volcanic eruptions' in the San Juan area this area in a great bow. The principal part of accuracy. The pre-Tertiary surface of the entire quadrangle. As seen in the section of the Animas probably continued at intervals until late in the district is a deeply scored volcanic plateau, region was completely buried by the Valley, all of these formations appear to be con­ Tertiary time, although only the prod- Tertiar more than 3000 square miles in extent, drained volcanic formations which now cover knowledge formable. None of the various unconformities ucts of the earlier outbursts are well hlstory- on the north by tributaries of the Gunnison River the, main. area, and,, while, ., erosion ,has of geology. by overlap represented upon the Hayden map as known. Thus the volcanic period of building-up on the west by those of the Dolores and San again exposed some of the older rocks on all sides occurring in the area between the Animas and was in part synchronous with the third great Miguel rivers, on the south by numerous branches of the volcanic complex and even in some of the San Miguel rivers exists in fact within that terri­ period already referred to - that of sculpturing of the San Juan, and on the east by the Rio interior valleys, the reconnaissance observations tory. But at least one great orographic disturb­ by erosion by which the mountains now exist­ Grande. All but the latter drainage finds its of the Hayden and other early surveys were far ance not indicated in the Animas section is ing have been produced. Within the volcanic way to the Gulf of California through the too meager, and the present resurvey has thus far clearly shown on the northern slopes of the San area little evidence has been discovered by which Colorado River. covered too small an area, to afford solutions to Juan, and probably also on the southern side, not the sequence of events can be correlated with the The San Juan Mountains are now understood many of the problems in the earlier geologic far east of the Animas River. The red Triassic established divisions of Tertiary time. Deposits to embrace the area bounded on the north by the development of this most interesting field. formations and all older sediments are wanting in of Eocene age are known in the zone bordering generally abrupt descent to the sloping mesas In view of this condition, no attempt will be the plateau traversed by the Gunnison and its the volcanic area, but they have not been found extending for 25 miles to the canyon of the Gun­ made at this time to present a thorough review southern tributaries east of the Uncompahgre in direct contact with the lavas. While it may nison, on the west by the great plateau of Colorado of San Juan geology, but in order that the sig­ River, and the granites and gneisses are overlain be safely assumed that the closer study of the San and Utah, on the south by the more gradual nificance of the observations made in the Tellu­ by the probable equivalent of the La Plata sand­ Juan will result in the recognition of different descent to the rolling plateaus of New Mexico, ride quadrangle may be more fully appreciated, stone, of assumed Jurassic age. A similar condi­ epochs of eruptive activity and of orographic dis­ and on the east by the broad and level San Luis an outline sketch of the geologic development of tion exists east of the Animas, in the drainage of turbance, the Tertiary history of this region may Park. From this main area a broad spur leads the region will be given. This outline is particu­ the Los Pinos and Piedra rivers, according to the be summarized as a conflict between volcanic off to the southeast, losing its mountainous char­ larly applicable to the western part of the San Hayden map, but no observations appear to have forces, building up by stupendous emissions of acter near the Colorado-New Mexico line. The Juan, for it is in the valleys of this portion, near been made on either side of the San Juan Moun­ lava, and the agencies of erosion, removing the San Juan Mountains thus have an extent of nearly the mountain front, that the best exposures of the tains to show the actual position or character of igneous material and carving deep canyons to the 80 miles east and west, and from 25 to 40 miles older rocks may be found. the great overlap which must occur at the base very base of the vast lava plateau. The former north and south, and their summits form a great The Animas Valley, between Silverton and the of the La Plata sandstone, or at least below the was most effective in the earliest stages of its group rather than a range. vicinity of Durango, shows apparently a complete Dakota Cretaceous. activity, nearly the entire thickness of 5000 feet In the western part of the San Juan Mountains exposure of all formations of the San Juan, from Other periods of uplift, erosion, or subsidence of volcanic rocks found in the western San Juan the topography is very rugged. There are hun­ the Archean to the Puerco Eocene, inclusive. in Paleozoic or Mesozoic time are indicated by the being of that epoch, while the agents of degrada­ dreds of summits exceeding 13,000 feet Much of this section has now been studied in apparent absence of Cambrian and Silurian sedi­ tion are still actively at work upon the higher Topography. in elevation, and several which reach detail, but definite correlations can not as yet be ments, the insignificant development of the lower mountain masses. more than 14,000 feet above sea level. Here, too, made between the older formations here seen and and middle Carboniferous beds, the local develop­ Quantitatively, the work performed by the the bounding scarps of the group are often very the isolated exposures reported from some other ment of the fossiliferous Trias, and the absence of geologic agencies acting in this region in Cenozoic precipitous, while some of the valleys in the parts of the San Juan Mountains. the marine Jura and of recognized equivalents of time was very great, but the estimation heart of the mountains have been cut down to Ancient granites, gneisses, and schists are the great "Lower" Cretaceous section of Texas. of the post-Cretaceous disturbance, as ofMeasure erosion 9000 feet or less above the sea. To the east the known in the Animas Valley on the south, and The geologic structure and constitution of the wellTT as thei general-i decipheringi T oti» all11 accomplished. configuration is less rugged, and high table-lands, in the Uncompahgre Plateau on the Theoldest San Juan Mountains of to-day are mainly the earlier geological history, has been rendered very north. These rocks have usually been *ormations- of varying extent, represent in a measure old result of the dynamicJ forces which Post-Creta- 4 ... difficult by the mantle of volcanic rocks; and the plateau surfaces. considered as belonging to the Archean, but some were intensely active during three ceous erosion- original extent of this covering is left to specula­ Within the bordering zone of lower country, of them are probably younger than the great great periods of Tertiary time. In the first of tion on account of the more recent erosion. The having a general elevation of from 6000 to 9000 series of quartzites exhibited in the Needle Moun­ these periods the long cycle of upper Cretaceous examination of the Telluride quadrangle has feet, are situated several small groups of high tains and beneath the volcanics in the canyons of sedimentation was terminated by a continental thrown much light on these great problems. Thus peaks bearing special names. The Needle Moun­ the Uncompahgre above Ouray, which have been uplift of unknown extent, but which may have the San Miguel conglomerate becomes of first tains on the south, the La Plata Mountains and referred by Van Hise and Emmons to the pre- been very great. The land thus elevated was importance in their solution, since its base presents the group about Rico to the southwest, and the Canabrian age of sedimentation the Algonkian. greatly eroded, and finally subsided, leading to the best evidence as to the post-Cretaceous erosion San Miguel Mountains on the west, are the most These latter rocks have suffered great metamor- the formation of the early Eocene lakes. Present and its top forms the surface upon which the vol­ important of these outliers. phism and are seen standing on edge or greatly knowledge does not tell us to what extent the San canics rested in the western part of the district. The Needle Mountains are almost continuous disturbed, and the relations of the isolated expo­ Juan Mountain area was covered by the Cretace­ Within the quadrangle the San Miguel conglom­ with the San Juan proper, but the local name is sures to contemporaneous formations elsewhere ous sea, but the sediments of that sea are now erate increases in thickness from 200 feet of bowl­ amply justified by the character of the group, are quite unknown. These quartzites were called exposed, dipping at generally low angles away der beds on the eastern side to nearly 1000 feet which is a result of its geologic structure. The "Metamorphic Paleozoic" upon the Hayden map. from the mountains on the northern, western, and of much finer sediment in Mount Wilson, and La Plata Mountains form an isolated group, and Clearly great continental movement followed by southern sides. In the Telluride and Silverton upon it, in the latter locality, is a remnant of the those about Rico, which may be conveniently enormous erosion preceded the earliest Paleozoic quadrangles is the evidence, to be given much San Juan tuff. Both of these formations are also called the Rico Mountains, are also disconnected deposits thus far discovered in the region. These more fully further on, that the erosion of the present in Dolores Peak, some 6 miles west of from the San Juan in origin. latter formations are now exposed in the Animas period under discussion produced a plain of mod­ Mount Wilson, but there does not seem to be any The eastern summits of the San Miguel Range Valley and near Oura}^, and are referred to the erate relief across the oblique edges of the entire possibility of the preservation of either of them the Mount Wilson group of the Telluride quad­ Devonian, from fossil evidence. Succeeding the series of Mesozoic and Paleozoic formations. This at any point farther west. rangle are a portion of the San Juan, cut off by Devonian comes a great series of calcareous sand­ plain seems to have bordered a higher land mass The amount of Tertiary and recent erosion erosion, as shown by the map and text of this stones and limestones containing abundant Car­ in the heart of the San Juan mountain area, and to which has taken place in the Telluride quadran­ folio. boniferous fossils. have extended a considerable distance how far, gle from the Dakota plateau to the summit of Though the San Juan Mountains are surrounded Above the Carboniferous strata appears an must ever remain a matter of hypothesis to the Mount Wilson is 5250 feet vertically, and a great by an arid plain country the annual snowfall and important series of reddish conglomerates, sand­ north, west, and south. This nearly plane surface thickness of still higher volcanics must still be rainfall upon them is heavy, especially stones,~ marls,' and thin limestones,' in Mesozoic.. of erosion became, in the region where it is now added. on the western portion. The higher «onciapnda= the upper part of which Triassic fossils dep°sits- exposed, the floor of the San Miguel lake, and the The present elevation of this entire region peaks and basins are seldom entirely occur. These beds are the lowest exposed in the sandstones and conglomerates deposited upon it above sea level is to be regarded as the result of free from snow. This abundant moisture sup­ Telluride quadrangle. They occupy a much had already attained a thickness of several hun­ numerous oscillatory movements of Present ports a heavy forest growth in many places upon larger area than the Carboniferous in the zone dred feet when the great period of volcanic activ­ uplift or subsidence which have taken conditions- the western and northern sides. Spruces and adjacent to the mountains, and are conspicuous in ity began, producing the complex of rocks out place since the close of the Cretaceous, affecting aspens cover the higher slopes, yielding to white the Animas, Dolores, San Miguel, and Uncom­ of which the present San Juan Mountains are greater or smaller areas. A slight tilting of the pine, scrub oak, pinon pine, and cedar on the pahgre valleys. sculptured. San Miguel formation in an easterly direction flanks, as the streams sink into canyons cut in the In the Rico Mountains a Permo-Carboniferous In the second period the volcanoes of the San may be connected with the uplift of the extreme lower plains of sedimentary rocks. Timber line fauna has been found in the lower portion of the Juan, assisted, perhaps, by vents in adjacent western San Juan region, leading to the great lies in the zone between 11,500 and 12,000 feet reddish series, and this fossiliferous zone is called regions,<-> ' emitted an enormous amount Volcanic.., . erosion which has caused such an abrupt face to above the sea, and large areas in the interior are the Rico formation. It has also been identified of volcanic material, partly in frag- eruPtions- the mountains in and about the Telluride quad­ thus barren of tree growth, supporting only a low in the Animas drainage, but its presence in the mental form and partly in lava flows, covering an rangle. There are some reasons for thinking that alpine flora on favored surfaces. Uncompahgre Valley has not as yet been demon­ area of certainly not less than 15,000 square miles this part of the San Juan is still rising. Much Valuable deposits of the precious metals have strated. to a depth of many thousand feet in the central more work must be done before the various been found in many parts of the San Juan region. Succeeding the red Triassic beds come other portion. This volcanic area has not been studied movements can be distinguished and given their Coal beds of great extent and fine quality OQCur formations correlated in general with the fresh- in detail except in the Telluride quadrangle. The relative values. In. the resurvey of the San Juan region, now in the plateau above the brink of the San Miguel Three subdivisions are distinguished upon the and Peale. This view found expression on the progress, the Telluride quadrangle was the first Canyon. On the western border is represented map, under the names Dolores, La Plata, and Hayden map by the designation of most of the one taken up. Most of the field work was done the contrast of canyon and plain which is so McElmo formations. These three formations strata of the Gunnison as " Lower " Dakota, or by in 1895, with H. S. Gane as regular assistant and marked over large stretches of country to the make up a complex which, upon the Hayden map the extension of the Dakota to include them. E. C. E. Lord as volunteer aid. Before the next westward. Near the mountains the plain is of Colorado, was also divided into three In describing the Gunnison formation of the Present divi- field season Mr. Gane was unfortunately obliged naturally modified by the numerous dikes, sheets, parts, assigned. -. respectively, to. the,, -T,Irias, . Crested Butte quadrangle, where its thickness to retire from the Survey, on account of impaired and irregular masses of igneous rock which have Jura, an d "Lower" Dakota, for the south- varies from 300 to 450 feet, Eldridge health, his place being taken by A. C. Spencer. prevented the uniform erosion of the Cretaceous western portion of the State. This Survey> states that "At its base is a heavy theFossilsSEik' in In 1896 the ore deposits of the quadrangle were shale, often seen where they are absent. complex is represented on the Hayden map as white quartzite, 50 to 100 feet thick, studied by C. W. Purington, and the results have Isolated mountains. The Mount Wilson group extending up the San Miguel Canyon as far as usually in a single bed. Above it, in some cases been published in the Eighteenth Annual Report of peaks is only partially included within the Big Bear Creek, within the Telluride quadrangle, succeeded by other sandstone layers, is a blue of the United States Geological Survey, Part III, quadrangle. It is the eastern and larg- T eg o( but the individual formations now recognized are limestone containing abundant fresh-water shells pp. 745-850, 1898, with a preliminary edition of est of several groups on a general east- mountalns- not the same in extent as those of the Hayden of the genera Limnea, Valvata, and Cypris. The the geological map. west line which together constitute the San geologists. remainder of the formation consists of gray, drab, Miguel Mountains. These high peaks are due to An accurate correlation of the formations here recognized, pink, and purple clays and marls, through which GEOGRAPHY AND PHYSIOGRAPHY. a great stock of diorite laid bare by erosion of the and included in the Juratrias, with the Hayden Survey sub­ run thin intermittent beds of drab limestone." soft strata penetrated by it. divisions of the same complex (see Hayden map and reports The two parts of the Gunnison thus described An account of the geographic relations of the Tel- by W. H. Holmes and A. C. Peale in annual reports of the J/uride quadrangle to the San Juan mountain To the north of the San Miguel River are three Hayden Survey for 1875 and 1876) is at present impossible, have been separated in the course of the present mountain masses, Gray Head, Whipple, and Hawn because the Hayden classification is based mainly on studies work and named the La Plata sandstone and the area, and of the local physical features. which were most detailed in districts far removed from the mountains, illustrating the laccolithic type of the San Juan front, while no sections seem to have been made in McElmo formation. This subdivision of the Gun­ General relations. The Telluride quadrangle plateau country. They are prominent by reason the quadrangles recently resurveyed. It is known that all nison formation, making it a group term in the is bounded by meridians 10T° 45' and 108° west of their comparative isolation. A corresponding these formations change more or less rapidly in lithologic San Juan region, is required for adequate expres­ character and thickness from the upturned zone about the longitude, and by parallels 37° 45' and 38° north mass is Flat Top, on the southern border, south western San Juan Mountains toward the great plateau and sion of the local geology, and is further justified latitude, and embraces 235.66 square miles. It of the Dolores River. canyon country of Colorado and Utah. An actual tracing by the known development of the individual mem­ lies on the western edge of the great elevated San out of these formations, from the apparent shore line in the New names for mountains. New names have San Juan to the canyon of the Colorado, is necessary to a bers of the complex to the westward, in the pla­ Juan mountain area, wholly to the west of the been given on the map to several prominent sum­ correlation of the various subdivisions made by earlier explor­ teau country. Continental Divide. The principal drainage of mits of the quadrangle for which no Ori inof ers upon observations in localities often hundreds of miles apart. In view of the natural division of the Gunnison into two the quadrangle is by the San Miguel and Dolores local designations were in use at the names- members, Mr. Eldridge acquiesced in a proposition made by the writer to restrict, in future, the name Gunni- rivers, but in the eastern part rise small branches time of this survey. Of these, Ruffner, Whipple, The three divisions here made in the Juratrias son to the upper member, and to distinguish the usaeeCitn°i8th of the Uncompahgre and Animas rivers. By and Hawn mountains, to the north of the San have been established principally on lithologic sandstones at the base as the La Plata sandstone. Annual reference to the index map on the cover of this Miguel, have been named from early scientific grounds, and direct evidence of their age is at This usage appears on the edition of the Tellu- r ' ride geological map published in the Eighteenth Annual folio the general geographic relations of the quad­ explorers of this part of Colorado; Gilpin Peak, present very scanty. The lowest of the three Report of the Survey, Part III, in connection with Mr. Puring- rangle will be seen, and also the course of traffic south of Mount Sneffels, after the pioneer Gov­ divisions, the Dolores formation, contains a few ton's report upon the mining industries of the quadrangle. communication by means of the Rio Grande South­ But it has since been decided that such a procedure is in viola­ ernor of the Territory; Mendota Peak, from the Triassic fossils in its upper portion. Below the tion of a Survey rule. Hence, Gunnison is here again used in ern Railroad with the main lines of the State. noted mine near it; while Gray Head, near Saw- fossiliferous horizons occurs a series of reddish its original comprehensive sense, and the name McElmo is The greater part of the quadrangle is in San pit, Black Face, west of Trout Lake, and Flat grits, sandstones, or conglomerates, in which no applied to the upper portion, designated as the Gunnison for­ mation upon the map in the Eighteenth Annual and in the Miguel County, while smaller areas fall within Top, on the southern border, possess physical fossils have been found. In the Rico quadrangle, report by Purington. Dolores, San Juan, and Ouray counties. features suggesting the names chosen. adjoining the Telluride on the southwest, an Physical features. Situated on the abrupt invertebrate fauna has been found in the lower DOLOEES FOEMATIOE". western border of the San Juan Mountains, the SEDIMENTARY ROCK FORMATIONS. two or three hundred feet of the "Red Beds," Definition. It is desired to apply the name Telluride quadrangle exhibits several which is assigned by Gr. H. Girty to the Permo- Dolores to the Triassic strata of southwestern -I , . i », t _c Mountains, A description of the sedimentary formations rep­ distinct Classes OI topographic lOrmS. canyons, and Carboniferous, in the sense of a transition series. Colorado and of adjacent territory so far as a resented upon the map by special colors or Along its eastern side is the bold front The complex of strata characterized by this fauna direct correlation may prove to be practicable. patterns. of the San Juan Mountains. The western half of will be described as the Rico formation. In the The name has been chosen because of the excel­ the area is chiefly occupied by an undulating plain, ALGONKIAN PERIOD. absence of fossil evidence, the red strata between lent exposures of typical fossil-bearing strata in which represents the Great Plateau of Colorado Quartzites of Canyon Creek. The small body the Rico Permo-Carboniferous and the beds con­ the valley of the Dolores River, at present best and Utah as it abuts against the mountains. In of upturned quartzites, with an intercalated rhyo- taining Triassic remains are grouped with the known in the Rico quadrangle, but probably this plain the San Miguel River has cut a canyon lite sheet, occurring in Canyon Creek north of latter in the Dolores formation. occurring at many places in lower portions of the which reaches a depth of 1700 feet in the north­ Stony Mountain, has been referred to the Algon- The strata between the Dolores and the Dakota valley. The name is now applied to about 2000 western corner of the area. The plateau and kian, or oldest sedimentary group of rocks, because Cretaceous are here referred to the Jurassic por­ feet of generally reddish sandstones, grits, con­ canyon features would doubtless be much more it is known, as stated in the introduction, that an tion of the Juratrias, because of stratigraphic glomerates, and shales, all highly calcareous, distinctly marked were it not for the intrusive immense series of quartzites, schists, slates, etc., position and lithological resemblance to the for­ limited below by beds containing Permo-Carbon­ stocks and laccoliths of igneous rock, which cause does exist beneath the Paleozoic limestones near mations in other parts of Colorado in which verte­ iferous fossils, and above by the La Plata sand­ isolated mountain masses, or small groups, to rise Ouray, projects up into the volcanic series at brate and invertebrate fossils of asserted Jurassic stone of the Gunnison formation, of assumed here and there in the plateau area. Some of these several places 5 miles or less to the eastward, and types occur. No determinable fossils have been Jurassic age. Vertebrate, invertebrate, and plant rises to elevations of over 14,000 feet in the Needle outlying mountains represent a geological type found in these beds in the San Juan The_. question remains have been found in the upper part of the occurring in the Henry, Abajo, La Sal, El Late, Mountains, on the southern border of the San Juan region. They were called "Lower" ofage- formation thus delimited, and upon their evidence and Carriso groups of the western plateau country, region. It is supposed that the quartzites of Can­ Dakota on that part of the Hayden map of south­ the Triassic age of that part of the complex is while the Mount Wilson group, on the western yon Creek represent a sharp pinnacle of the post- western Colorado for which W. H. Holmes and considered as proved. Whether or not all the border of the quadrangle, is an isolated remnant Cretaceous topography which was buried by the A. C. Peale were responsible, while no divisions beds now associated with the fossiliferous series of the volcanic complex, and equals the San Juan great volcanic accumulations of the San Juan. were recognized by F. M. Endlich between the in the Dolores formation are really of Triassic front in the rugged, precipitous character of its The quartzites of this area are rather coarse "Upper" Carboniferous and the Dakota Cretaceous. age remains to be determined by further dis­ slopes. grained, grading into a fine conglomerate. They Beds corresponding to those in question were coveries. The base of the formation as above The San Juan front. The precipitous front of consist almost entirely of pale, smoky or some­ carefully studied by Gr. H. Eldridge in the Elk defined is not exposed in the Telluride quadran­ the San Juan is clearly shown upon the topo­ what clouded quartz grains, with only a small Mountains, some TO miles northeast of Telluride, gle, but appears near Rico, a few miles southwest. graphic map. It begins near the center amount of kaolin in the interstices. The beds are and were grouped as the Gunnison formation and Description of the formation. The Dolores for­ of the northern line, swings around the tneCharacteramoun= of massive banks, the bedding being, however, dis­ assigned to the Juratrias (Anthracite - Crested mation has in general the characteristics of the ° tain front. head of the San Miguel Valley above tinct, with variable strikes. The prevalent one Butte folio, No. 9, Geologic Atlas of the United widely known "Red Beds" of the Rocky Moun­ Telluride, thence runs out to the center of the seems to be about N. 65° W., and the strata are States, 1894). Fresh-water shells were obtained tain region. It consists of an alternating series quadrangle at Ophir Loop, and proceeds in an in places nearly vertical. by Eldridge from a limestone in the lower part of sandstones, grits, conglomerates, and sandy irregular line southerly beyond Grizzly Peak. These quartzites, if not Algonkian in place, must be sup­ of the complex, but no vertebrate remains have shales, the latter often grading into earthy lime­ Nearly all of the indentations in this front have posed to form a block floated up in the gabbro magma of the as yet been collected from this formation on the stones. Individual beds of sandstone or con­ Mount Sneffels-Stony Mountain eruption. But been made by branches of the San Miguel. But since the mass projects on one side into the San Meaning of Pacific slope in Colorado. A personal knowledge glomerate of uniform texture are seldom more this stream has not cut far back at any point, Juan-r tuffs,. . and-i has -ibut j. a narrow arm ofj, j/ithe ofoccurrence quartzite. of the Gunnison strata in the Elk Mountains and than 25 or 30 feet in thickness, although fine­ gabbro-diorite on the other side, it does not and the high divide, followed by the county line, seem possible that a mass of this size can have been floated in parts of the intermediate area leaves no dou'bt grained and thinly bedded sandstones with slight which traverses the area from north to south, into this position. Mr. G. E. Kedzie, of Ouray, states that as to the full equivalence of the formations to be textural variations may exceed 100 feet in thick­ passes eastward into the Silverton quadrangle for there are other much smaller quartzite masses, which seem to described with the series to which the name Gun­ ness. Nearly all beds vary greatly in constitu­ him included in the gabbro of Mount Sneffels, somewhat north only a short distance, around Savage and Ingram of Canyon Creek. The existence of such bodies, however, nison was given by Eldridge. tion and thickness, so that detailed sections made basins. On the east of this divide, in the north­ would not require the same origin for the larger mass of Can­ In stratigraphic position and lithologic charac­ at points not widely separated can seldom be yon Creek. The small inclusions might, indeed, be more easily eastern corner of the quadrangle, is the basin accounted for by the presence of an assumed buried peak of ter the Gunnison formation seems to be, in part closely correlated. head of Canyon Creek, a tributary of the Uncom­ similar rocks. at least, equivalent to the Morrison formation at The reddish color of the series is due partly to pahgre River. In the southeastern quarter rise The rhyolite associated with the quartzites is of a dense, the eastern base of the Front Range, which pink grains of feldspar in the coarser layers, but banded, f elsitic type, much of it carrying smoky quartz pheno- several branches of the Animas River. The crest crysts, with some of orthoclase, while plagioclase is lacking. embraces the well-known Atlantosaurus beds of chiefly to a ferritic pigment in minute of this divide remains above 13,000 feet for This contrasts strongly with the Potosi rhyolite, which carries Oil Creek, Morrison, and other localities. From particles. This color is dark or dull plagioclase phenocrysts and is wanting in those of quartz. nearly its entire length in the Telluride quad­ The rhyolite does not penetrate the San Juan tuffs, and their remarkable dinosaurian fauna these latter red in the lower portion, and a brighter rangle. altogether the character of the rhyolite and its association beds have been referred to the Jura by paleon­ red in the upper part ; but white or pale pinkish Elevated basins whose floors are above timber with the quartzites seem to harmonize with the reference of tologists. sandstones appear here and there, and no color the quartzites to the Algonkian. Dikes of the Potosi rhyolite line, many of them glacial cirques inclosing beau­ in this vicinity are glassy, according to Mr. Kedzie. Since so little new evidence as to the age of the distinction can be used over large areas for the tiful blue lakes, characterize both sides of this JURATRIAS PERIOD. Gunnison formation has been thus far discovered subdivision of the formation. While the greater divide, as clearly expressed by the map. in the San Juan region, the discussion of that part of the red strata of the region belong to the The plateau and canyon area. Under this Subdivision into three formations. question is reserved for a later section of this Dolores formation, it is known that the lower head may be described the country west of the General statement. -All sedimentary formations text; but it may be said, in passing, that on sandstone of the La Plata formation becomes San Juan front, excepting the mountains of igne­ of the Telluride quadrangle below the Dakota Cre­ stratigraphic grounds alone the geologist would brilliantly colored in places, and the Permo-Car­ ous rock which have been mentioned. The map taceous are included in the Juratrias, excepting be forced to class these beds with the Cretaceous, boniferous strata below are also of a more or less brings out very well the undulating character of only the Algonkian quartzites already described. as has been done by Newberry, Endlich, Holmes, distinctly reddish color. A calcareous cement is present in abundanc 1. Coarse sandstones and grits, with occasional sandstone, but it is known that the thin limestone conglomerate layers. Massive as a whole, are seldom pure clay shales, but are commonly throughout the Dolores formation, and this con and forms cliffs; finer grained near top .... 200 of the Telluride area represents a much more both calcareous and sandy. Sandstone layers stituent increases in amount southward from th Total...... 1600 extensive series of shales, etc. in some other regions. appear in the shales, and in following: . . Telluride quadrangle, and forms nodular reddish r». ° Lithologic This section is noteworthy for the comparatively coarse Description. In the Telluride quadrangle the the formation along the walls of the 0? Mceimo011 or mottled limestones or marls. In this abun grain of the sediments in the upper part, for the presence of La Plata formation consists of two massive sand­ San Miguel Canyon a shale stratum formation- dance of carbonate of lime the strata of th large limestone pebbles in so many horizons, stone members separated by a thin, may be found to change, within a short distance, Dolores formation resemble the Carboniferous particularly in the lower half, and for the occur- fhe section* x J ' Lithologic rence of Unio, not elsewhere known in the San dense, bluish or almost black lime- Laap*atffo°r= to an alternation of sandstone and shale. The sandstones and conglomerates, and contrast with Juan region. About 100 feet of fine-grained sandstone is stone. The total thickness in this mation' reverse change often occurs in the case of sand­ the quartzose sandstones of the Gunnison forma wanting at the top, with another limestone conglomerate, both of which are found farther up Mineral Creek. The base quadrangle is seldom more than 100 feet. The stone beds. tion above. of the formation probably occurs in the Silverton quadrangle, distinctive feature of these almost purely quartzose The basal member of the McElmo formation is The strata are chiefly made up of materials on the east, not far from Cataract Creek, but it is covered on sandstones is their unusually fine and uniform sac- a usually highly colored shale resting on the derived from granites, gneisses, and the quartzose slopes where it may be expected to appear, and metamor- phism through a monzonite stock has also prevented its detec­ charoidal texture, causing them to form very mas­ upper La Plata sandstone, and the upper stratum Algonkian rocks of the San Juan con- , , tion. 0 Materials sive banks on canyon walls, although a marked assigned to it is also a marked shale beneath From the character of this section it seems unlikely that the tinental area of Triassic time. The D°oToprellng cross bedding is frequently present. These sand­ a massive sandstone, commonly conglomeratic, finer-grained strata are rich in quartz, strata- strata composing it can be referred to distinctly different epochs. They seem to have been deposited in a littoral zone stones are here very light gray or white; but this which is assumed as the base of the Dakota and the coarser ones contain much feldspar. The during one epoch, and the differences seen at other localities color can not be given as characteristic, since in Cretaceous. The appearance of a similar con­ conglomerates are usually rich in pebbles of the seem due to changes in conditions making themselves mani­ fest only at some distance from the shore. many places, especially to the south and west, a glomerate, in a sandstone bed of the McElmo, dark quartzites and greenish schists of the Algon Fossils of the Dolores formation. In 1880 and in 1882 R. C. distinct or even brilliant coloring in varying about 100 feet below the Dakota, makes the line kian series. By the appearance of limestone Hills announced the discovery of vertebrate, invertebrate, and plant remains in the upper strata of this forma­ shades of red or orange has been observed in the between the formations diflicult to establish in pebbles in many strata, especially of upper tion at San Miguel, 1£ miles below the pi-es- Fossils lower sandstone. But the contrast with the some places. horizons, the existence of Paleozoic beds in the ent site of Telluride (American Journal of R.Uc.dHiiis. bright-red strata of the Dolores formation is very No fossils have been found in the McElmo for- land area adjacent to the Triassic sea is clearly Science, Vol. XIX, 1880, p. 490, and Vol. XXIII, 1882, p. 243). These fossils were submitted by Mr. Hills to striking over large areas. mation of the Telluride or adjacent districts. proved. experts for determination, but were unfortunately lost before The two sandstone members are usually of Subdivision of the formation. As known being fully identified. Mr. Hills regarded the abundant teeth Section of the HoElmo formation The following section of found in one stratum as belonging to a crocodile near Belodon similar character, but in the Telluride quadrangle the McElmo was made by Mr. Spencer on the north side of the between the Animas and San Miguel valleys the prisons, and certain fish remains as representing a ganoid the upper one is sometimes thin bedded and shaly San Miguel, opposite the mouth of Bilk Creek. Dolores formation may be roughly divided into a similar to Catopterus gracilis. A small gasteropod shell and and of much less prominence than the lower. The The section reads downward from the overlying Dakota lower, coarser-grained part, characterized by con­ eleven or twelve apparently determinable species of plants sandstone and conglomerate. were found. The Belodon and Catopterus remains were found base of the lower sandstone is, in certain places glomerate containing granitic and quartzite peb in the limestone conglomerate, occurring here about 50 feet on the San Miguel, a conglomerate of small peb­ Feet. bles, and an upper, finer-grained portion, with below the La Plata sandstone, and the fossil leaves in thin- 17. Shale...... bedded, reddish, micaceous sandstones not far below the con­ bles with much sand. 16. Sandstone, rather fine grained...... 22 limestone conglomerate, often fossiliferous. This glomerate. The limestone between sandstones is usually 15. Shale, sandy, with many thin layers of fine­ distinction can not be applied throughout the In the course of the present survey the upper conglomerate grained sandstone ...... 53 dense, but may be black and thin bedded. It 14. Sandstone, coarse, grading into conglomerate Telluride quadrangle, as the detailed section given has been carefully examined for fossils at many places in the Telluride, Rico, La Plata, and Durango quadran­ varies in thickness from 6 to 16 feet, being thin­ of quartz and chert pebbles at base...... 24 below plainly shows. The difference referred to gles. It contains most abundantly teeth of Fossils nest in the San Miguel Valley and thickest on the 13. Shale, dull red or green, with subordinate may be seen about Rico, but disappears in most Belodon and of a megalosauroid dinosaur, discovered. thin bands of very fine-grained calcareous Palseoctonus, according to the identifications southern border of the quadrangle. A thin, shaly sandstone...... 155 particulars in the valley of Mineral Creek. This of F. A. Lucas. These are Triassic types, species of the latter limestone appears near the base of the formation 12. Sandstone, coarse grained, cross bedded. .... 48 11. Shale ...... 11 speaks for the proximity of the shore line to the from Texas having been described by E. D. Cope, and occur­ in some places, but is probably not thus developed eastward. ring also in the Trias of North Carolina. Further fish remains 10. Sandstone, massive...... 16 have not been found. In the Rico and La Plata quadrangles, within the Telluride quadrangle. The limestone 9. Shale, red, with thin sandstone layers...... 53 Along the San Miguel the fine-grained lime at one locality in each, in the Saurian conglomerate, a gaster­ horizon is important locally, as that of the Sawpit 8. Sandstone, white...... 11 stone conglomerate, containing teeth of dinosaurs opod shell, poorly preserved, has been found, which, accord­ 7. Shale, red, with thin sandstone layers...... 29 ing to T. W. Stanton, belongs to Viviparus or some closely ore deposits. 6. Sandstone, white, cross bedded...... 22 and crocodiles, is a most prominent related genus, and it is stated by him that-the earliest previous On the north side of the San Miguel, opposite 5. Alternating red shale and gray sandstone ... 85 4. Sandstone, massive in lower part, but with horizon. It is usually of pinkish color, !on_ record of this genus is from the Jura. Bilk Creek, Mr. Spencer found the lower sand­ from 10 to 20 feet thick, forms a pro­ While plant stems and indistinct leaves are common at the thin red shale partings above ...... 80 general horizon at which they were found by Mr. Hills, the stone to have a thickness of 64 feet, the limestone 3. Shale, sandy...... 32 jecting ledge, as a rule, and in this quadrangle is only determinable species yet obtained came from the Dolores a thickness of 16 feet, and the upper sandstone a 2. Sandstone ...... 8 Valley, a short distance below the Telluride quadrangle, from 1. Shale, sandy, chocolate colored in upper part, the most persistent and most richly fossiliferous thickness of 26 feet. A similar ratio for the three thin layers of sandstone in upper part. .... 64 stratum of the formation. It has been found con­ coarse grit near the Saurian conglomerate. This has been determined by David White as resembling Paehyphyllum beds way be seen in many exposures. Total...... 724 venient to call it the " Saurian conglomerate," by milnsteri, a Triassic plant. A Unio of specifically undeter­ The upper sandstone layer is succeeded by a which designation it will often be referred to in minable character was found by Mr. Gane in the highest This section is typical of the McElmo formation in the Tel­ exposed conglomerate of the Cataract Creek section, given highly colored shale, taken as the lowest stratum luride region, but other sections show many changes in the this folio. above. This is thought by Mr. Stanton to be in all probabil­ of the McElmo formation. This shale may be relative development of sandstone and shale at On Summit Creek, north of the San Miguel ity one of the forms obtained by Cope on Gallinas Creek, New any given horizon. The conglomerate, number Comment Mexico. sandy or strongly argillaceous. In color it ranges 14 of section, is similar to the Dakota in charac- section!6 River, the Saurian conglomerate occurs about 30 Distribution and correlation. The Dolores formation lias from chocolate through various shades of red, or ter, but is very variable in development. Holmes feet below the La Plata sandstone, but this inter­ been traced continuously from the Telluride quadrangle to it may be greenish. The change from white sand­ noted the frequent presence of a conglomerate near the top of val increases to nearly 100 feet in the southeast­ the eastern side of the Animas River, covering much ground his "Lower" Dakota in the plateau country to the westward, mapped as "Upper" Carboniferous by the Hayden geologists. stone to colored shale is commonly abrupt, but is and sporadic developments of the same in the Morrison beds ern part of the quadrangle and to 500 feet on the Mr. Hills has observed its fossiliferous portion on the Florida, less so on the San Miguel than in most localities. may be seen in various places at the base of the Front Range. Animas River, in the Durango quadrangle. These and it undoubtedly extends eastward until concealed by the The sandstone strata of the McElmo formation as exhibited upper strata are usually bright red in color, of fine­ overlapping La Plata sandstone somewhere east of the Piedra Fossil remains. No fossils have been found in the La Plata in this quadrangle are so similar to those of the La Plata and River. The reddish strata in the Uncompahgre Valley may strata of the Telluride quadrangle, but a few miles south of Dakota formations that it seems highly arbitrary in some grained sandstones and calcareous sandy shales, be confidently assigned to the Dolores, as the Saurian con­ the Twin Sisters some indistinct fish scales and places to attach to the boundary planes that have been chosen without marked conglomerates. They are similar lomerate was noted by Mr. Gane near Ridgway, a few feet vertebrae were noticed. The specimens of these Fossils indi- the great importance that really belongs to them if the forma­ below the La Plata sandstone. The fossiliferous series has remains were unfortunately lost. In the Crested waten fresh tions considered are of the ages that have been assigned to to the strata seen extending for a few hundred not been observed farther north, except on the north side of JButte quadrangle G. H. Eldridge found many them. The justification of the boundaries adopted is found feet below the complex of limestone conglomer­ rand River near Red Dirt Creek, where it was found by Mr. minute shells in the dark limestone, which have been referred in the comparatively constant character of the Dakota and ates as developed south of the Telluride quadran­ Hills (Proceedings Colorado Scientific Society, Vol. Ill, 1890, to Limnea, Valvata, and Cypris, indicating that the limestone the La Plata, above and below, rather than in the McElmo p. 372). Whether this member of the Dolores was once present was laid down hi fresh water. complex itself. gle, and are now thought to represent a part of over the Dolores Plateau and was removed by erosion before the Dolores formation which has been removed ;he deposition of the La Plata sandstone, as I am now inclined FORMATION. CRETACEOUS PERIOD. by erosion in the region north of the Telluride ;o believe, or was never formed in that area, as thought by Mr, Hills, is a question for further investigation. The name. The name here proposed for the The formations succeeding the Juratrias in the quadrangle. The thinning out is apparently due Observations of much value bearing on the distribution of upper division of the Gunnison is derived from Telluride quadrangle belong to the great series to the unconformity at the base of the La Plata all the formations referred to the Juratrias were made by 3.. S. Gane in 1897, during a trip down the San the important branch of the San Juan Eiver which of the upper Cretaceous, which is so sandstone, which has been referred to. .-,,,.., Formations Juan Valley to the canyon of the Colorado. On has its source south of the great bend of the widely distributed through the Kocky S^fpe? to this journey he was able to follow connectedly Mountain country. Strata which can Cretaceous- Detailed section of the Dolores formation. The following all the formations recognized in the Telluride quadrangle, Dolores Eiver and runs thence westward for 50 section of the Dolores was made by H. S. Gane on the western 'rorn the Dolores to the Dakota, inclusive, and, while chang- miles, passing the northern base of the El Late be correlated with the "Lower" Cretaceous forma­ side of Cataract Creek, a tributary of West Mineral Creek, ng greatly in development, the limestone conglomerates of near the eastern border of the quadrangle. The section was the Dolores were found over a large area, bearing the same Mountains, and for most of its length traversing tions of Texas are not known in Colorado, unless made from the stream bed, at an elevation of 10,000 feet, up to elation to the lower La Plata sandstone which he had the arid plain country, the floor of which is the the supposed Jurassic formations, the Morrison the San Miguel conglomerate near the end of the sharp ridge, observed in the Telluride quadrangle. At Clay Hill, Utah, a Dakota sandstone. In the main McElmo Valley at an elevation of about 11,650 feet. srocodile jaw was found in the Dolores limestone conglomer­ and the Gunnison, are referred to that period. The section reads from above downward. At the top is the ate, which has been described by F. A. Lucas as belonging to and in its various side canyons the clays, shales, For reasons given in describing the Gunnison San Miguel conglomerate, resting unconformably on the a new genus, having decided Triassic affinities, to which he and sandstones of the upper Gunnison are excel­ formation, and discussed in greater detail in a Dolores. ;ave the name Heterodontosuchus ganei (American Journal of Science, 4th series, Vol. VI, 1898, p. 399). Poorly preserved lently exposed, and from current reports it appears later section of this text, they are classed in the Feet. shells of Unio, fossil wood, and the usual saurian bones and that vertebrate evidence as to the age of the for­ Juratrias. This implies that the line between 10. Bright-red, fine-grained sandstone; quartzose 82 ;eeth were noted in the conglomerates. These observations mation may there be found in abundance. From 9. Gray limestone conglomerate, with calcare­ ndicate that the Dolores and other lower Mesozoic formations the McElmo and Dakota formations represents a ous sandy matrix; pebbles small, seldom of the San Juan border have a great distribution in the pla- personal observations of Messrs. Gane and Spencer, great stratigraphic break. exceeding 1 inch in diameter. In matrix au region of Colorado and Utah. as well as from the Hayden reports, the equiva­ Only two formations of the " Upper " Cretaceous abundant remains of a specifically undeter­ While the Dolores formation, on the southern border of the minable Unio...... 8 San Juan region, is covered by later sediments for a long lence of the strata here described with those of are now found in the Telluride quadrangle the 8. Alternating sandstone and conglomerate, red­ distance, it seems plainly suggested that the Triassic strata on the McElmo Valley is placed beyond question. Dakota sandstone and the Mancos shale. Above dish, with occasional white layers. Pebbles " -allinas Creek, on the western side of the Sierra Madre, New Description. The McElmo formation as devel­ of granite, quartzite, and schist; few of Mexico, from which E. D. Cope obtained crocodilian and these there once existed in this area the full series limestone. In two conglomerates of upper dinosaurian remains similar to those of the San Juan region, oped in the Telluride quadrangle is a variable of higher Cretaceous strata corresponding to that part pebbles reach 1 foot in diameter. ,re to be referred to the Dolores formation. The same is true complex of shales and sandstones, with the latter still preserved in the Animas Valley near The sandstones are variably fine or coarse >f the strata of the Abiquiu copper mines of New Mexico, grained. No distinct planes for subdivision where J. S. Newberry obtained several Triassic plants while much more prominent than is commonly the case Durango. The upper formations have here been of this complex were noted ...... 910 »n the Macomb exploring expedition. elsewhere. The thickness of the series varies removed by erosion, partly in the period immedi­ 7. Dark-red sandstone with thin mud layers from 650 to 900 feet. The sandstones are fine separating beds; limestone fragments rare. 50 LA PLATA FORMATION. ately succeeding the Cretaceous and partly in 6. Coarse reddish sandstone...... 50 grained, quartzose, of yellowish or gray color, Tertiary and Recent times. 5. Greenish sandy shales characterized by flat The name. It is proposed to name the lower or flake-like limestone pebbles, irregularly and usually soft and crumbling. Some beds are The Cretaceous section of southwestern Col­ distributed, usually small or reaching 6 member of the Gunnison formation after the La locally massive, are cross bedded, and reach 50 orado differs so much in lithologic character inches in diameter. Upper part more sandy, 5lata Mountains, on account of its prominent and feet in thickness, but shale partings of variable reddish, with larger limestone flakes...... 200 from that found in other parts of the State that 4. Thin-bedded sandstone, with conglomerate 3haracteristic exposures in the peaks and on the importance subdivide most of the sandstone mem­ a new scheme for its subdivision into formations at top...... 75 lopes of that mountain group. From the domi­ bers. Small flat flakes of green shale are common is necessary for the appropriate expression of the 3. Sandstone, shaly sandstone, and conglomer­ nant development of white sandstone in the Tel- in many sandstone beds. ate, alternating...... 65 local geology. Only one of these new formations 2. Conglomerate of large, flat limestone pebbles, uride and other quadrangles, it is there quite The shales are either reddish or greenish, or is found in the Telluride quadrangle, namely, the which weather out, leaving cavities...... 10 i appropriate to call this formation the La Plata variegated by a mixture of these two colors. They Mancos shale. Telluride 3. The Cretaceous section of southwestern Colorado. While of the San Miguel River. In this series no per­ Paleozoic formations. The first recorded obser­ feet of the formation often contains much worked-over debris the full discussion of the Cretaceous section and its subdivision sistent lithologic or paleontologic horizon has vations of this conglomerate were made c of the sedimentary formation immediately below. On the will be presented in the La Plata and Durango o Formation eastern slope^of Rolling Mountain a very marked, fine, shaly folios, now in preparation, a concise summary ^tltement been found which could be used for the subdivi­ by R. C. Hills, who referred to it in fs£rs*e°dbby band occurs near the middle of the formation. may be given here. As explained below, the ofnewobser- sion of the formation. The shales are dark gray addresses delivered before the Colorado R' c' Hllls' On Howard Fork the San Miguel conglomerate is not so well Mancos shale includes equivalents of the Benton, exposed as elsewhere, owing to the bleaching and decomposi­ the Mobrara, and a part of the Pierre formation that is, the or lead colored, and are nearly always somewhat Scientific Society in 1888 and in 1890 (Proceed­ tion it has undergone in this highly mineralized area, and also Colorado group and a part of the Montana. In the study of sandy. Thin calcareous layers become almost ings Colorado Scientific Society, Vol. Ill, pp. 174 to the unusually fine grain of the formation as a whole. It the complete section it has been further ascertained that the limestones in places, and are usually rich in fossils. and 407). Mr. Hills noticed this formation in occurs, in almost its customary coarseness and distinctness, as marine fossils of the Pierre extend up into a large shale series a conglomerate at the head of Swamp Creek, and from there above the coal-bearing sandstone formation which forms the Sand locally increases to form sandstone, but no the typical exposures of the San Miguel Valley can be traced, as indicated on the map, on both sides of scarp of the Mesa Verde. In the La Plata folio the latter will limestone or sandstone layer is persistent and near Telluride, and on Canyon Creek above Ouray. Howard Fork until it is cut by the igneous stock below Ophir; be called the Mesaverde formation, and the shale above it the but at many points it is so fine grained and bleached that Lewis shale, from Fort Lewis, on the La Plata River. In the prominent enough to be traced far. The name San Miguel was proposed by the writer unless the unconformity at its base is clearly shown, one might Durango quadrangle a further subdivision is necessary, dis­ About 125 feet above the Dakota is a dark for this formation in a communication presented easily mistake the San Miguel for some of the Dolores grits. tinguishing two heavy sandstone series above the Lewis shale calcareous layer, rich in fossil shells, of which the to the Colorado Scientific Society, September 7, On Bear Creek the lower part of the conglomerate is very and another shale which separates them. Details of this coarse in many places, bowlders more than 2 feet in diameter upper subdivision have not yet been fully determined upon. most abundant is Q-rypJiaea newberryi, and 100 1896 (Proceedings Colorado Scientific Society, being present. While limestone pebbles occur in these lower The Lewis shale is about 2000 feet thick, and is lithologically feet higher another layer is in this quadrangle Vol. V, p. 235), from the locality mentioned, layers, quartzites of white, gray, pink, or bluish colors are comparable with the Mancos shale. The Mesaverde forma­ much more common, with many schists and few pebbles of tion embraces the coal measures most extensively worked in characterized by a fossil oyster, Ostrea congesta. where it was first recognized in exposures dis­ granite or gneiss. that region. On the Hayden map it was referred to the Fox These layers, developed as thin limestones 1 to 3 playing its character and relationships most per­ From the head of the San Miguel Valley above Telluride to Hills. inches thick, may often be found, but the fossils fectly. Ruffner Mountain the formation is generally of finer texture DAKOTA FORMATION. than on Bear Creek, the pebbles seldom exceeding 6 inches in of both are of species belonging to the Benton Description of the formation. The San Miguel diameter, while in many exposures the strata are chiefly sand Description. The Dakota is characterized in shale of the Colorado group, and do not serve to beds vary in thickness from 200 to about 1000 stones and grits, with a few pebbles irregularly scattered through them. Quartzites of various colors are most abun­ this region, as commonly in Colorado, by gray or show the age of the shales above them. At a feet. In the eastern part of the quadrangle, where dant here, and porphyries of several types are unusually brownish quartzose sandstones, often cross bed­ much higher horizon a few fossil shells, which are thinnest, they are uncommonly coarse and massive numerous among the pebbles. These igneous rocks are not ded, with a variable conglomerate at or near the referred to species known in the Pierre division conglomerates, containing round bowlders from 2 of the volcanic series to be described, and their sources are as yet unknown. In Campbell Peak and on Deep Creek the base and several shale horizons at intervals. The of the Montana Cretaceous group, have been found feet down to 1 inch in diameter, embedded in San Miguel formation is much bleached, but its exposures on thickness varies from 100 to 250 feet within the in sandy layers. These sandy layers are very pinkish sand or gravel. The beds are very vari­ Mill Creek and at many points to the east are highly charac­ teristic. quadrangle. local in development. able in texture, however, and change, within short In the mountain northwest of Grizzly Peak the San Miguel As compared with other regions the fine basal Thus it appears that the Cretaceous shale for­ distances, from coarse conglomerate to grits and is about 500 feet in thickness, and increases to 700 feet toward conglomerate, with its small chert pebbles of mation of the Telluride quadrangle probably sandstones with a few pebbles. As the forma­ Sheep Mountain. The lower strata are, as a rule, fine-grained buff and reddish sandstones, with Increase in white, dark-gray, black, or reddish colors, is embraces strata belonging to the Benton, Niobrara, tion increases in thickness westward the average thin,, . layers, of conglomerate., Theremi are many thicknesswestward. especially variable, being absent in some places and Pierre epochs, but that there are no practicable texture becomes finer, and sandy layers even pre­ thin mud layers exhibiting trail markings and apparent shrinkage cracks. Search for fossils in these beds and unusually coarse and thick in others. The horizons for the subdivision of the complex in dominate locally. In Mount Wilson, where the was fruitless. In the conglomerates, which are nearly all of shale members are much more strongly developed areal mapping. This formation has been traced complex is 1000 feet thick, very fine-grained sand­ small pebbles, limestone is very abundant, varying from black, than on the eastern slope of the Rocky Mountains. through the Rico, La Plata, and Durango quad­ stones and sandy shales are largely developed. through shades of gray, to almost pure white. Rapid lateral variation in texture of beds is characteristic of the series here. In two of them there is a large amount of car­ rangles, and adjacent territory, and is found to The pebbles and bowlders of the San Miguel con­ Near the top of the formation in this region is a very light- bonaceous matter, which increases to the extent possess this character as a lithologic unit. It is glomerate represent many rocks granite, gneiss, gray layer 30 to 40 feet in thickness, composed mainly of quartz sand and quartzite pebbles, which is conspicuous from of forming workable coal seams on the Dolores proposed to call it the Mancos shale, from its greenish schists of several varieties, distant points of view. River. In the adjoining plateau country to the characteristic occurrence in the Mancos Valley white, gray, and bluish quartzites, lime- the c"nag?om= On the western slope of Sheep Mountain the San Miguel is , , . erate. west and south coal is commonly found in the and about the town of the same name, between stone, red sandstone, and an igneous nearly 700 feet in thickness, and consists of variable grits and sandstones, with thin conglomerate layers all through the Dakota, but seldom exceeds 4 feet in thickness, the La Plata Mountains and the Mesa Verde. rock of prominent porphyritic structure being series. The light-colored layer near the top is a coarser con­ and is of a quality much inferior to that of the The Mancos shale covers a large area in the common. The matrix is a gravel or sand of glomerate than most of the beds below. The gradual decrease in thickness of the formation from this point along the north higher Cretaceous horizons developed in the La Telluride quadrangle, extending back from the quartz and other hard minerals derived from the slope of the mountain to the west base of Vermilion Peak, Plata and Durango quadrangles. Dakota scarps of the San Miguel and Dolores same rocks that furnished the pebbles. where it is less than 400 feet thick, can be most plainly seen The lower sandstone of the Dakota is especially rivers until overlain by the San Miguel conglom- The composition of the conglomerate varies from points like San Bernardo Mountain. i/ O O In the mountains between Trout Lake and Mount Wilson prominent, not only here but in the plateau region, erate or pierced by igneous rocks. It forms somewhat from place to place, but all through the San Miguel formation is mainly a loose and friable grit or forming the rim rock, or upper scarp, of thousands smooth-sloped ridges or hills, contrasting with the eastern part of the Telluride quadrangle it sandstone series, exhibiting conglomeratic development here and there all through the complex. At Mount Wilson the of miles of canyons like the canyon of the San other formations of the region in this respect. may be noticed that limestone and red sandstone maximum observed thickness of the formation was found on Miguel. Its presence under the Tertiary volcanic series are much more abundant in the lower half of the the south side. The formation is here a succession of fine Plant remains, in the form of carbonized stems has been favorable to enormous landslides. formation, while the ratio of bluish quartzites and conglomerates, grits, sandstones, sandy shales, calcareous shales, sandy limestones, and dark, or even black, shales. The and poorly preserved leaves, are common in the Fossils of the Mancos shale. Fossiliferous strata which dense greenish schists increases upward. Changes layers of homogeneous constitution are seldom more than 10 Dakota, but have not been found in sufficient per­ apparently correspond to the Grryphaea and Ostrea layers above both in texture and in materials occur so rapidly feet thick, and the fine-grained sediments vary rapidly from mentioned liave also been noted at many places one development to another. Thin shale layers may separate fection for identification. in the Rico, La Plata, and Durango quadrangles, localities from place to place as to show that strong shore conglomerate beds. All the finer-grained strata are likely to Detailed section. The following is a section of the Dakota, They undoubtedly represent the horizon at offossils- currents must have played an important part in show curved impressions like fucoidal markings or like trails made by Mr. Spencer, on the north side of the San Miguel, which Newberry found the same forms " at a thousand points " distributing the sand, gravel, and pebbles. or burrows. No fossils of any kind could be found. The con­ on the route of the Macomb expedition, in 1859 (Geological glomerates show many kinds of rocks represented among the opposite the mouth of Bilk Creek, above the section of the In general appearance this formation is light McElmo formation already given. Report of the Macomb Expedition, 1876, p 71). Newberry pebbles, but quartzites predominate. The section reads downward from the dark sandy shales of collected these fossils from many localities in the quadrangles pink or gray in color, and massive, forming dis­ In the vicinity of the Mount Wilson stock the finer-grained the Mancos formation. named, and T. W. Stanton has also obtained them from the strata are often so hardened that recognition of the formation same district, and from near Mancos (The Colorado Forma­ tinct cliffs, and, by contrast with the dark tuff is difficult unless the conglomerate layers are found. Feet. tion: Bulletin U S. Geological Survey No. 106, 1893, p. 32). above, it becomes one of the most striking features Sandstone, hard, with quadrangular jointing. 16 The complete list of fossils known from these two horizons of the precipitous San Juan front. From a point StratigrapJiic relations. The great unconform­ Sandstone, grading into shale in upper part. 22 embraces at present the following forms, as determined by Shale, black ...... 16 7. Mr. Stanton: Gfryphcea newberryi Stanton, Ostrea lugubris like Sunshine Mountain the San Miguel forma­ ity at the base of the San Miguel formation, which 6. Sandstone, massive...... 5 Conrad, 0 congesta Conrad, Inoceramus labiatus Schlotheim, tion can be clearly distinguished beneath the vol­ has been already alluded to, is shown _ . ., _. 5. Sandstone, fine grained, lenticular or wavy I. dimidius White, I. fragilis H. and M., Baculites gracilis J ^ Details of the bedding and thin carbonaceous shale part­ Shumard, Scaphites warreni M. and H , Prionocyclus macombi canic formations, from Ruffner Mountain on the on the geological map, and it may be SSwthesln ings...... f...... Meek, and Plicatula sp. ?. According to Mr. Stanton these north to Grizzly Peak on the south, excepting clearly seen as present on the north Mlguel< 4. Shale, black, with 8 inches of coal at base... forms indicate a horizon well up toward the top of the Benton 27 only where it is obscured by landslide areas or side of the San Miguel River, between Iron Moun­ 8. Sandstone, thin bedded, with shale partings. shale as known in other regions. 5 2. Shale and thin sandstones, carbonaceous.... Fish teeth and scales also occur with these forms, but are cut by the diorite - monzonite stock of Ophir tain and Marshall Creek, by an observer stationed 1. Sandstone, coarse grained, rather massive, not so abundant as farther east, where Newberry found them Needles and Yellow Mountain. The view in fig. at any good point of view on the south of that mainly quartzose, with irregular conglom­ so common that he occasionally referred to these strata as the erate of small quartz and chert pebbles at valley. The general westerly dip of the under­ "fish beds." 1, on .the Illustrations sheets, shows this contrast about 20 feet above bottom...... 81 In loose fragments of sandy strata found on the East Dolores in the cliffs north of the San Miguel River. In lying formations and the low easterly dip of the Total...... 188 River west of Grizzly Peak Mr. Stanton identified Anatina sp. ? the greater part of the quadrangle the cliff-mak­ San Miguel beds bring out this angular uncon­ and Turritella f. The horizon of these shells was not deter­ This section illustrates the general character of the Dakota mined. ing part played by the formation, as illustrated formity most distinctly. In Ruffner Mountain in the San Miguel Canyon, but the several members here dis­ A sandy ledge in the shales at 11,500 feet, on the slope south here, is particularly due to the presence below it nearly 2000 feet in thickness of the dark Mancos tinguished change much laterally. The heavy of Mount Wilson, contains poorly preserved specimens of sandstone at the base always forms a bold cliff, Inoceramus cripsii var. barabini Morton ?, Synocyclonema of the Cretaceous shales; but in nearly all places shales are present below the San Miguel conglom­ but in some places "shaly layers subdivide it. rigida M. and H., and Baculites sp. ? (either B. ovatus or com- it is sufficiently different from the underlying for­ erate. Following southeasterly along the base of The bottom of this sandstone is often a conglomerate and pressus). These forms belong to the Pierre division of the mation, either in texture or in stratigraphic posi­ the San Miguel, it is found that the shales thin generally has a wavy surface of contact with the variegated Montana group, according to Mr. Stanton, and give the only shale of the McElmo, on which it rests. Irregular shale fossil evidence thus far secured in the Telluride quadrangle tion, to form a distinct ledge. out and finally disappear on Butcher Creek, and masses are sometimes included in the basal sandstone. as to the age of the upper Mancos shale. But in the La Plata that in succession the Dakota, McElmo, and La According to the local development and induration of the Details of constitution. The fine exposures of the San quadrangle Mr. Spencer found a number of characteristic Plata formations are cut off, so that between higher sandstones, there may be small ledges or scarps above Pierre shells in the shale, especially near Thompson Park, Miguel conglomerate on Cataract Creek and on Lime Creek the main one; but where shales are plentiful and the sandstone and at several horizons. Among them Mr. Stanton has iden­ near the eastern boundary of the Telluride quadrangle are Marshall and Ingram creeks the San Miguel only 3 miles from the eastern slope of Sultan Mountain, which is soft, the upper part is a broken succession of benches and tified Inoceramus undabundus M. and H. ?, I. cripsii var. comes to rest on the red beds of the Dolores for­ ledges, often obscured near the top by Mancos shales creeping barabini Morton, Baculites asper^/Lorton 1, Leda sp. ?, Mactra lies directly east, in the Silverton quadrangle, where it is now down from above. Where erosion has removed the upper sp. ?, Area sp. ?, Ptychoceras or Hamites sp. ?, and Scaphites known that the formation is only a few feet thick. From Sul­ mation. At no known point directly east of this layers for some distance back from the edge of the canyon it sp. ?. tan Mountain to Mount Wilson is a distance of 18 miles, within locality has erosion cut down sufficiently to reveal becomes difficult to trace the Dakota-Mancos line with which the formation increases from less than 50 feet to about accuracy. EOCENE PERIOD. 1000 feet in thickness. Hence the .variations in character of the San Miguel beds, if present, beneath the vol- Local variations of the Dakota. Eastward from the point Normal sedimentary deposits of apparently the San Miguel within this quadrangle are of much signifi­ canics. In the southeastern portion of the quad­ of the section given above the Dakota increases in thickness cance. to about 250 feet, opposite the Keystone placer, on the north Tertiary age are represented in the Telluride On Cataract Creek, and opposite to it on Lime Creek, the rangle the unconformity is shown in equal degree side of the river. Eastward the sandstone layers become more quadrangle by the San Miguel formation only. San Miguel conglomerate is reddish in color and massive in between Sheep Mountain and the quadrangle massive, and are more indurated, so that the outcrops near bedding, and huge blocks have fallen from the boundary on Lime Creek, though the outcrops Telluride are often very distinct cliffs. This has been assigned to the Eocene, because of overhanging ledge, nearly 200 feet in height, Local varia- On the southern branches of the San Miguel there is no very the great unconformity at its base and because it into the gulch below. The lower 100 feet of the characters. are not continuous, owing to the monzonite stock marked change in the Dakota, but in the southeastern part of beds is rich in limestone pebbles, with an unusual of Grizzly Peak. East of this point, however, the underlies the volcanic complex of the San Juan, number of red sandstone and shale fragments, often not well the quadrangle it is only 100 to 125 feet thick. The basal unconformity is shown for 3 miles farther, to Sul­ conglomerate is here in places rather coarse and prominent, which is thought to be of Eocene age in the por­ rounded. The limestones are of various shades, dark gray and is absent in other places. On the Dolores shale bands are tions here developed. prevailing, with some of white, gray, bluish, and pinkish tan Mountain, near Silverton, where the upper strongly developed, and the coal seams are of some economic color; some are sandy, and some contain black chert frag­ Carboniferous beds appear under the conglomer­ impor Lance. The lower sandstone is massive, and a sandstone SAN MIGTJEL FOEMATION. ments. Some contain Carboniferous and others Devonian above the coal forms the rim of the canyon. fossils. Schists and quartzites predominate in the upper 100 ate, here only 30 to 50 feet thick. It may be that Recognition and naming offormation. Beneath feet of the conglomerate, arid the bowlders are larger than the shore line here indicated marks the eastern MANCOS SHALE. those below. the bedded volcanic series all through the Tellu­ Around the head of Mineral Creek the San Miguel is about limit reached by the formation on the south side Description and name. Above the Dakota for­ ride quadrangle, and in portions of the Silverton 300 feet thick on an average. On the west side of the Twin of the San Juan Mountains, but this generaliza­ mation is a dark shale series, of which a thickness Sisters the lower part is shaly, and contains many fragments tion will not be warranted until the region east quadrangle on the east, occurs a coarse conglom­ of the dark Mancos shale, on which it here rests. This corre­ of nearly 2000 feet is preserved on the north side erate resting unconformably on Mesozoic and sponds to observations in other localities that the lower few of the Needle Mountains has been examined. Whether or not the lower Carboniferous and quadrangle, to the action of water in the San ride quadrangle. They are commonly purplish rhyolite flows, of black or dull-reddish, glassy the Devonian strata are ever found in actual Miguel lake of the period at which the eruptions or dull reddish brown all through the district, appearance, associated with thin tuffs, and all unconformable contact with the San began. The rock fragments are exclusively of the matrix and the larger fragments being often about Marshall Basin and the head of the San Miguel formation, it is plain that they, o tph°euanccoen= andesite, so far as observed, and it is believed of different tones, emphasizing the texture, which Miguel rhyolite flows or tuffs occur below the too, must ihave ibeen anected«. T iby thei formity. that the beds are the product of the first great might otherwise be obscured by the secondary main andesite flow. To the south of Ingram Creek great uplift at the close of the Cretaceous, and explosive eruption in the San Juan area. For coloring. These colors produce very dull and the series changes in character, and is much more that they were exposed in the land mass adjacent this bedded fragmental series, laid down before somber effects in the mountains where the expos­ prominently tuffaceous or breccia-like; both the to the San Miguel lake. Limestone and quartz- the eruptions of rhyolite, the name San Juan for­ ures are most extensive. andesite and the rhyolite flows are less marked as ite conglomerate pebbles derived from Devonian mation was proposed at the same time that the In the areas of greatest mineralization the San such. The base of the series in the central east­ strata occur in the conglomerate of the San Miguel. San Miguel conglomerate was named (Proceed­ Juan tuffs, etc., like other formations, have been ern part of the quadrangle appears to The eculiar The unconformity at the base of the San Miguel ings Colorado Scientific Society, Vol. Y, p. 226). extensively impregnated by minute be a peculiar, unevenly lamellar rhyo- base of th! formation is, therefore, one of the most striking The San Juan tuff, breccia, etc., may be con­ particles of pyrite. The staining of the lite of gray, red, or greenish color, and important ones known in the Rocky Moun­ sidered as a pyroclastic formation which passes tuffs by hydrous oxide of iron, result­ characterized by flat blotches, half an inch or less tain region, and testifies to an uplift of unknown uninterruptedly from regions like the Telluride ing from the decomposition of this pyrite, often across, of dull-red or green color, as a rule. These extent, followed by stupendous erosion in the quadrangle, where the materials were in part at causes brilliant yellow or red colors, but this is are very prominent on the surface of lamination, interval between the close of the Laramie Creta­ least stratified by water, to adjacent land areas, not peculiar to the San Juan formation, being, and cause a streaked appearance at right angles ceous and the subsidence by which the San Miguel where a subaerial bedding may be locally well indeed, most striking in the rhyolite flows of the to it. The blotches are either pressed-out vesicles lake was created. marked, or where the arrangement may be chaotic higher peaks. or mud flakes, and the rock is a devitrified rhyo­ Above the San Miguel beds of the Telluride near the explosive vents. A similar transition is The tuffs, breccias, and agglomerates of this lite (aporhyolite). This blotchy rock was found quadrangle come, with apparent conformity, the undoubtedly represented in the vertical section, series have been somewhat indurated by the cal- at the base of the Intermediate series at many stratified tuffs of the San Juan formation. The the lower beds having been waterlaid, while a cite and other substances resulting from the circu­ places between U. S. Grant Peak and Marshall abrupt change in constitution marks the dividing filling up and forcing back of the water would lation of waters through them, and in consequence Basin. It is only a few feet thick and grades into line very clearly, as will be brought out in describ­ lead to a bedding of the upper portion entirely form very rugged slopes, as shown in the views reddish banded rocks of a dense matrix, holding ing the latter formation. due to subaerial agencies. on the Illustrations sheets and discussed some­ numerous small angular fragments of andesite or The actual contact of the San Miguel formation in uncon- Description. The San Juan formation of the what in detail later on. dull-reddish rhyolite. These pass into apparent formable relation to the beds below may be seen in many Telluride quadrangle is a series of horizontal The thickness of the series varies from 2000 tuffs of rather indistinct character, many of which places. The most striking points are where the white La Plata sandstone disappears and the pink San Miguel comes tuffs, agglomerates, and breccias, composed of feet near Telluride to a few hundred feet at the were found to be flow breccia that is, a rhyolite in contact with the much darker red Dolores sandstones or andesitic debris and reaching a maximum thick­ Lizard Head. There is also a marked flow holding so many fragments of andesite that Thickness. conglomerates. One can put his finger on the spot where this ness of about 2000 feet. It lies unconformably thinning of the formation southward, the fluidal matrix becomes quite inconspicuous contact occurs on the east side of Bear Creek, and it can be located with great precision at almost every other point where on the San Miguel beds, and the line between so that on "West Mineral Creek only about 400 and can not be seen with the naked eye. The it is represented upon the map. The exposures on Lime the formations is sharp only in the abrupt change feet of fragmental beds have been assigned to the true nature of these rocks was not recognized in Creek and on the north side of the San Miguel are especially favorable for examination by any one who wishes to follow from light-colored quartz and feldspar sand to San Juan formation. But the line separating it the field, and, indeed, the base of the series was this unconformity for considerable distances. dark volcanic material. from the Intermediate series above was not very not determined for many localities until the speci­ For 100 or 200 feet above its base the forma­ clearly established at this locality. The variation mens were microscopically studied. IGNEOUS ROCK FORMATIONS. tion is here a fine-grained, well-bedded dark tuff; in thickness is probably due in large degree to Microscopical examination of the rhyolites of the Inter­ A. description of the igneous masses represented that is, a stratified volcanic sand or gravel in erosion in the interval preceding the eruptions of mediate series shows them to have been partly glassy, in most which granitic sand and a few pebbles of Algon- the Intermediate series. cases with strongly developed fluidal structures. Devitrifica­ upon the map, occivi'ring either as surface tion is very common, and many of the rocks are properly lavas, as fragmented material, or as the filling kian quartzites are locally found in quite subor­ From the several textures and the somewhat termed aporhyolite, as expressing their present condition. dinate amount near the base. This lower zone of varying mineral composition of the andesite frag­ The dark rhyolite-vitrophyre of Park Basin has a brown, of fissures or other conduits penetrating the stringy glass, which exhibits the beginning of devitrification sedimentary rocks. dark tuff is prominent in the San Miguel Valley. ments seen in the tuffs, etc., it would appear that in the usual manner, proceeding from perlitic cracks. A Above this dark tuff comes a succession of the explosive eruption which has been assumed dull-red rock results when the ferritic particles have become The igneous rocks of the Telluride quadrangle more or less clearly stratified tuff, agglomerate, hydrated and devitrification has become more pronounced. must have destroyed an earlier volcanic mountain The feldspar phenocrysts are nearly all plagioclase, and when belong in large part to the great series which or breccia beds. The beddingo is A...... bedded built up of lavas represented by these fragments. determinable have been found to be labradorite (Abx An^, form the San Juan Mountains, and these naturally always distinct where cliff faces of ^offKr- If this be true, the San Juan formation seen in as in the other forms of rhyolite occurring in this region. The aporhyolite of the "blotchy type" above referred to fall into two geologic groups of somewhat dif­ some size can be seen, but may not be matloiu this region does not actually mark the beginning exhibits, under the microscope, a peculiar microbreccia struc­ ferent characteristics, namely, the bedded volcanic evident within some of the thicker bands if of the volcanic cycle, but no observations thus far ture, apparent by the sharp interruption of series of surface flows and fragmental material, examined in detail only. Several views presented fluidal structure shown by the strings and films Jltaiiffaphic made indicate the site of the volcano destroyed. of ferritic particles, though the fragments seem and the stocks, or irregular cross-cutting masses, on the Illustrations sheets show the bedding as to have been welded together again. This structure appears which penetrate the former. In the sedimentary present in large exposures. Tuff layers of even INTERMEDIATE SEKIES. to have been produced by shocks while the mass was in a stiff, viscous condition. Perlitic fissures furnish the points of beds beneath the volcanic series are other rocks grain appear at intervals throughout the forma­ The andesitic tuffs, agglomerates, etc., of the attack by the agents of devitrification, as in the vitrophyre. San Juan epoch are overlain in the Telluride The '' blotches " are apparently vesicles pressed out very flat intruded in the form* of laccoliths and sheets. tion, but by the admixture of larger fragments in These are more closely related in general character varying degree, most beds become intermediate quadrangle by a series of rocks of characters indi­ in the flowing of the lava and filled by secondary substances, such as chlorite and calcite, in exceedingly fine particles. to the rocks of similar masses in the La Plata and between tuff and agglomerate a confused min­ cating such a different phase of volcanic activity Above the blotchy aporhyolite occurs more massive flow- other isolated mountain groups of the plateau gling of blocks of various sizes. Beds which may that it is desirable to distinguish them upon the breccia having some evidence of devitrification, generally rather obscure, and containing, as the name indicates, a large country than to the rocks of the San Juan. There properly be called breccia, because made up of map and in discussion. This series is termed number of angular fragments, of both andesite and rhyolite. are, besides the above classes, a large number of angular fragments without a considerable amount the Intermediate series, as expressing both the As the number of these fragments increases, the character of small dikes and a few peculiar igneous bodies of fine-grained matrix, occur here and there, and observed position of the rocks, between the San the rock becomes less and less distinct to the unaided eye. In some cases a very subordinate amount of a felsitic fluidal having obscure relations. The rocks will be probably become more important to the eastward. Juan tuffs and the Potosi rhyolite series above, base is seen in thin sections, but so obscured by linionite and described in the general groups outlined above. In general it appears that the beds are coarser and also the transitional nature of the epoch of other alteration products that the interpretation of these masses as rhyolitic flow-breccia is based on the observed Upon the Hayden map of the San Juan region the igneous and more chaotic in texture near the eastern eruption, from one of exclusively andesitic prod­ gradation to the unquestionable rhyolite with a few fragments rocks are classified as "trachoreite," trachytic breccia, and boundary, but local modifications are often found ucts to one of rh}^olite flows and tuffs. In general rather than on determination of the character of the base. basalt. The former is a term originated by which deviate from this rule. The nearly hori­ the series consists of alternating rhyolite and No andesitic flow-breccias have been observed. P. M. Endlich and defined to include everything "ey namls!"' between rhyolite and basalt. All the volcanic zontal bedding also continues unchanged over the andesite flows, with tuffs of varying constitution POTOSI EHYOLITE SEEIES. series of the Telluride and Silverton quadrangles, fragmental quadrangle. Thus, change neither in texture nor below, between, and above them, grading into a or massive, as well as the granular stock rocks which cut them, The uppermost member of the volcanic series are mapped as "trachoreite," and some of the intrusive masses in attitude of the beds indicates near approach to breccia, mainly of andesite, and in this particular of porphyry in adjacent sedimentary areas are also represented the vicinity of the volcanic vents from which the resembling the San Juan, although commonly is a succession of rhyolite flows and tuffs, the under the same designation. Basalt is known to have exten­ more chaotic in arrangement. The series changes former greatly predominating, with andesitic ash sive development in and adjacent to the San Luis Valley, but materials came. no localities of '' trachytic breccia " have thus far been examined A microscopical study of the fine-grained tuffs in character so rapidly that the description must particles either included in the flows or mingled during the recent survey, and the true character of the rocks and of the larger rock fragments shows that all proceed according to local development. with rhyolite in the tuffs. One thin augite-andes­ so called is unknown. An examination of specimens collected ite flow occurs between much thicker rhyolite by Endlich and preserved in the National Museum shows particles, large or small, belong to rock Details of The lower andesite flow. In the northeastern that rhyolite, a variety of andesites, diorite, monzonite, gab- masses which have been broken up by constltution- part of the quadrangle the series consists largely sheets, in the northeastern part of the quadrangle, bro, and several porphyries were obtained by him in areas some force, presumably an explosive outburst at of lava flows of both augite-andesite but the epoch of these eruptions was plainly one mapped as "trachoreite." one or more volcanic vents. As yet no true and rhyolite. The most prominent in which the andesitic extrusions had become In the San Miguel, La Plata, and other outlying mountains, recognized. the Hayden map classifies the igneous rocks as "porphyritic "bombs," or smaller particles, have been detected member, here, is the,, massive nown ot£ comparatively insignificant. trachyte (hornblendic)." These are mainly areas of intrusive Three divisions of: the series are noticeable. At porphyries, of the diorite or monzonite families. But in which can be considered as having been ejected augite-andesite which is so prominent in Marshall extending this designation to cover the andesitic tuffs and the in a partially fused condition at the time of erup­ Basin at the general level of the Mendota mine. the base is a coarse-grained tuff or flow-breccia; stock diorite of the Mount Wilson group, a more serious error tion. Excepting the granitic and sedimentary This is 600 feet in thickness in some places, but above that comes the principal division, Subdivision of classification was committed. This, like many other dis­ including the important flows which oftheseries- crepancies of the Hayden map, arose from adjusting the obser­ debris found sparingly in the lower beds, the thins out to the northward, and on the south slope vations of two geologists in the border zone between their materials of the San Juan tuffs, etc., seem to be of of Dallas Peak it is reduced to a discontinuous are the cause of much of the rugged topography areas of field work. various types of andesite. Hornblende- and lava sheet 30 feet or less in thickness, and on the of the higher mountains and ridges; the upper­ BEDDED VOLCANIC SERIES. augite-bearing varieties prevail, but of many southeast ridge of Campbell Peak its western­ most member, appearing only in the northeastern section of the quadrangle, is a succession of thin SAN JUAN FOBMATION. different textures and details of structure. most observed outcrop is a ledge of lava 4 feet flows and tuffs, making much gentler slopes and In the fine tuffs the small angular grains clearly belong to high, largely vesicular. Geologic relations. The basal member of the many types of andesite. Some are finely vesicular or fluidal; presenting a marked reddish color, due to the The rock of this flow is augite-andesite, with a small amount bedded volcanic series building up the plateau, a glassy base may be suspected in some cases; but the large andesitic ash in the tuffs. majority are either of microcrystalline groundmasses of very of probable hypersthene, represented by decomposition prod­ of which the present San Juan Moun- Basalmein=, fine grain or cryptocrystalline. The larger fragments are ucts. So far as determined the plagioclase phenocrysts are The greatest observed thickness of the Potosi of the same types as those in the tuffs, and are similar to the labradorite. They are often full of glass inclusions, and exceed series is over 1000 feet, as developed in Gilpin tains are a remnant, is represented 5oidfni?e the pyroxenes in amount. The groundmass is of the typical in the Telluride quadrangle by a com- senes' hornblende- or pyroxene-andesites occurring in flows to be Peak and corresponding summits. In Potosi described. No basalt particles have been seen in the San Juan microlitic form common in andesites. In some specimens plex of more or less clearly stratified beds, which tuffs. The usual state of decomposition in which the rock there is or has been a scanty glass base, but in most cases the Peak, whence the name is derived, situated as mass seems holocrystalline. Fluidal structure is often seen. may be called tuff, breccia, or agglomerate, accord­ particles are found makes any special petrographical descrip­ shown in fig. 'T of the Illustrations sheets, there tion unsatisfactory. Some of the dark tuffs of the lower hori­ ing to the texture presented in various layers. zons contain much more granitic sand than the megascopic Khyolites of the Intermediate series. In the is apparently still more. The beds here lie conformably upon the San appearance suggests. Much of this sand is of microcline ridge between Campbell and Dallas peaks the The lower tuff and flow-breccia. The basal Miguel formation, and while nearly all the mate­ granites. thin andesite sheet seems to be all that can be member of the Potosi series is a gray or light-red­ rials are of volcanic origin, the bedding of the Such andesitic beds are usually dark bluish referred to the Intermediate series, but on the dish massive band, seeming in many places like a lower part at least seems due, throughout this gray, but this color is seldom found in the Tellu- ridge north of Park Basin there appear some thin coarse gravel bed. It is variable in thickness, Telluride 5. ranging from a few feet to more than 100 feet, and of magnetite, and spherulitic crystallization is commonly Mignel Peak is made up more prevailingly of In the Mount Wilson stock the prevailing rock is clearly distinguishable from the darker flow- coarser, if developed at all. This corresponds to observations in many well-known acid lavas of Institution, a single rock type than any other of the corre­ is diorite, tending toward monzonite through the breccia or tuff-agglomerate of the Intermediate much fresher condition, such as those of the sponding masses. This type is pink- General presence of considerable orthoclase, but quartz- series below and from, the lamellar flows above. Yellowstone Park. ish gray, rather fine grained, with descriPtion- bearing monzonite and granite occur locally. The chemical composition of these rhyolites is assumed to The gravel-like beds consist predominantly be represented in a general way by the analysis of the vitro- orthoclase, plagioclase, and quartz as the most In the mass of Yellow Mountain and Ophir of angular fragments of gray porphyritic rhyo­ phyre, given below. None of the gray rhyolites here under abundant constituents, and with several ferromag­ Needles a quartz-monzonite is very conspicuous, lite, with variable amounts of reddish felsitic discussion was fresh enough to warrant quantitative analysis. nesian minerals biotite, augite, hypersthene, and probably exceeds the dioritic facies rhyolite and darker andesites. The matrix is finer The upper division. Above the heavy-banded and hornblende also variably prominent. Each in amount. But the most basic modifi- these rocks. dust of rhyolite, and when this is present in large flows just described occurs a complex of thin flows of the light-colored constituents is equal to the cation. ot<» this1 1 eruption/ is loundr> i in thei amount the tuff appears like a massive rock, for and reddish tuff layers, of which about 400 feet total of the dark minerals in much of the rock. Ophir Needles, especially in the upper part of the there is not much bedding distinguishable within is preserved in Grilpin Peak. The original thick­ The plagioclase is chiefly referable to labradorite mountain. Here gabbro rich in the dark sili­ the band. ness of this series is not known, and it is not pre­ (Ab.,^ An 1 ), and is nearly equal in importance to cates occurs, either in the fine-grained dike-like The fragmental character is most evident near served at all south of Telluride Peak. orthoclase. The latter has a pinkish color, while bodies or in more irregular areas grading into the bottom of the band, and in several sections a In general this succession of thin flows resem­ the plagioclase is white. Quartz is not very other facies less abruptly. massive flow with many inclusions bles the earliest rhyolites of the Intermediate noticeable to the unaided eye, but is shown by The structure of these rocks varies, but by far a flow-breccia follows without any into«ow=es series. There are reddish and pinkish the microscope to be an important element. Bio­ the greater part is moderately fine and even . breccia. clearly defined line of separation; in lamellar varieties, and some black glassy fiowsnandng tite and augite are more persistently developed grained. Coarsely granular development is local. fact, some specimens collected to represent sup­ flows with numerous white feldspar throughout the mass than the other dark constit­ But while the stock parts are generally granular, posed gravelly tuff were found on microscopical crystals, a typical rhyolite-vitrophyre, exactly like uents. the sheet-like connection and the Black Face examination to be flow-breccia; so that it is not that on the ridge north of Park Basin, at the base A rather peculiar mottling is seen in much of intrusion are more or less distinctly porphyritic. known how much of the lower member of the of the Intermediate series. A chemical analysis this monzonite, caused by a tendency of the white The groundmass in these portions is much less in Potosi series is actually fragmental rock. of this rock is given below. plagioclase and the darker minerals to segregate amount than the phenocrysts, but occasionally a This band of gravelly tuff and flow-breccia The upper flows are reddish rhyolite-porphyries, in spots, leaving the quartz and orthoclase to form few large orthoclase crystals develop and make forms massive outcrops of rather rounded outlines, and contain rather more feldspar and biotite than a more continuous matrix of a pink color. Occa­ the structure pronounced. Such a growth of and often marks the floor of a bench above which the lower rocks. Dull, felsitic, streaked, brown sionally a part of the orthoclase develops in large orthoclase may also occur locally, even in the rise the banded flows. It is well shown in the or gray bands, containing few crystals and show­ grains or rude crystals, making a porphyritic large stock of the Mount Wilson group. region from Campbell Peak to the head of Bridal ing flow structure, are numerous. Andesitic inclu­ phase of the rock. The transitions observed between the many Veil Basin. The tuffs appear to be best devel­ sions occur to some extent in all flows, and in the structural and mineralogical varieties occurring Under the microscope it is found that augite, hornblende, oped in Mill Creek and in the region westward, thin, variable tuff layers between flows there is and biotite are often intergrown as primary contemporaneous in the rock masses here under consideration are, while flow-breccia is very prominent in Marshall sometimes a considerable admixture of andesite. constituents. Both augite and hornblende are as a rule, so gradual that the several facies can of very pale-green color, and the biotite is light detaHsraphiC Basin and southward. In the mountain south of The series as a whole has more or less reddish brown. Hypersthene is not so abundant as the not be regarded as of distinctly different periods Howard Fork a thinner but still very distinct color, making it conspicuous at long distances. other constituents. There is very little magnetite, apatite, or of eruption, the later ones cutting the older in layer of this material appears beneath the banded other accessory minerals. dike form. It seems more in accordance with These rocks exhibit inicrostructures and stages of devitrifi­ Allanite occurs sporadically in the usual prisms. Ordinarily flows. cation, when examined microscopically, similar to those of quartz and orthoclase are in separate grains, but occasionally observation to assume that the magma injected lower flows. Some bands are rudely spherulitic, others are they appear in micrographic intergrowth. The flow-breccia contains plagioclase and alkali feldspar into these stocks was not homogeneous in compo­ partly glassy. One black vitrophyre from the ridge east of A fresh specimen of the prevalent facies of the stock, from phenocrysts, usually broken and much altered, a very few the pass between Marshall and Virginius basins was so nearly near the lake northwest of San Miguel Peak, was subjected sition, and that much of the existing variation in biotite leaves, and a groundmass generally much fresh that a quantitative chemical analysis of it was made by to quantitative analysis by H. N. Stokes, with the following the rocks is due to original variation in the obscured by decomposition. In the freshest tetaiis.COplc H. N. Stokes, with the following result: results: rocks examined microscopically fluidal structures magma. There is no apparent regular relation of are very distinct through curling bands of ferritic globulites. Analysis of black vitrophyre from ridge east of pass between changes in composition to form of the masses, A very rude spherulitic structure appears in some cases, but Marshall and Virginius basins. Analysis of monzonite from near lake northwest of San is so obscured by limonite flakes and other decomposition Miguel Peak. suggesting a differentiation within the magma products that its character is not plain. Evidently there was 64.72 MgO. 0.50 after eruption. An exception to this statement 0.43 K2 O . 1.82 ...... 65.70 K2 O 4.62 a considerable amount of glassy base in most of the rocks may be presented in the Black Face mass, where studied, but devitrification, producing a very fine cryptocrys- 14.18 3.88 ...... 0.72 3.62 talline mass, doubtless of quartz and alkali feldspar, has often Fe2 O» 1.58 0.08 A12 0 3 ...... 15.31 P 2 0 5 ...... 0.33 orthoclase and quartz are frequently found in 0.40 Fe.O,...... 2.54 so...... 0.12 entirely replaced the vitreous base. Proofs of the former FeO.. HS 0 below 110° C..... 2.68 vein-like or irregular areas, often intergrown as glass are found in the perlitic fissures traversing the crypto- CaO . 2.62 H 3O above 110° C..... 6.82 FeO.. 1.62 Cl...... 0.03 crystalline mass, and in occasional remnants of glass. BaO . 0.28 CaO . 2.56 H 2 O below 110° 0.17 graphic granite, but yet showing zones of transi­ The included rhyolite fragments are similar to the varieties SrO.. 0.21 Total...... 100.20 BaO . 0.12 HoO above 110° 0.42 tion into diorite-monzonite of subordinate por­ Traces of MnO, CO S, S, SO 3, and Cl. SrO.. 0.03 observed in the Intermediate series, and the andesitic particles phyritic structure. These appear to be due to are like those of the San Juan tuffs. MgO. 1.62 Total...... 99.53 This analysis is vitiated to some extent by minute inclusions Sp. gr. 2.720 at 34° C. Traces of Li2 O and MnO. of andesitic ash, but serves to show that this rock is not a segregations of magma rich in potash and silica, The heavy-banded flows. The chief member of typical rhyolite, and explains the phenocrysts of The observed ratio of constituents and the above analysis and are not limited to any particular portions of lime-soda feldspar. As these phenocrysts char- Systematic the Potosi series consists of two or three flows , . ,, , , . j. I-, x . i -, position of show that this rock is intermediate between granite and the exposed rock. of a banded rhyolite of characteristic features. acterize the whole series of Potosi rocks, and as the rock. quartz-diorite, and it therefore comes within the the black vitrophyre of the lower division is group to which BrOgger has given the name {^anaVite *0 A full discussion of the character of the numerous rock facies These flows, generally from 100 to 200 feet in quite indistinguishable from the one analyzed, the composi­ monzonite, a term used in the folios of the Sur­ occurring in the stocks of Mount Wilson and Ophir Needles thickness, are separated by thin tuff, or, as in one tion given above may be assumed as representative of the vey as coordinate with diorite, granite, etc. This particular will be possible only after much more extensive field study banded rhyolites as well. The rocks are rather intermediate case, by a flow of andesite. Vertical columnar type belongs with the quartz-bearing monzonites, not far from than was permissible in connection with the present work. between rhyolite and dacite, and may, perhaps, be considered the modification to which the name banatite has been given. Other stocks of this and adjacent quadrangles should also be structure is marked within each sheet, and the as highly siliceous lavas in the series of which some forms Variations from this type observable in this stock consist, included in the discussion. But some details may here be steep cliffs or jagged ridges caused by this joint­ richer in the ferromagnesian silicates have been termed latite on the one hand, in an increase in the amount of quartz given as an aid to the understanding of these remarkable rock by F. L. Ransome. It is not thought wise to introduce at this and orthoclase, producing as an extreme form a granitic facies, masses. ing are most prominent features of the high divide time a new name for the rock. which is also somewhat porphyritic through the develop­ The minerals entering into the constitution of these rocks at the head of the San Miguel drainage. Benches ment of orthoclase phenocrysts. This facies is very local, are the same as those found in the monzonite. Plagioclase is INTRUSIVE SERIES. or horizontal lines, visible for long distances, mark and the monzonite-granite intermediate rocks are also rare. commonly labradorite, of composition Ab x An1; as determined STOCK EOCKS. On the other hand, dioritic facies, either rich or poor in quartz, by the Michel Levy method, which is particularly applicable the zones between the flows. These characteristics have been noticed, but all tend toward monzonite by the here because of the common association of the Carlsbad and are to be seen in several figures of the Illustrations The great cross-cutting masses of the Telluride presence of considerable orthoclase. Aplitic bands traverse albitic twinning in the same crystals. Varieties richer in the the monzonite in some places. anorthite molecule occur in some of the gabbro facies, and sheets. quadrangle are petrographically complex, and it At several places near the edges of the monzonite stock a andesine appears in some of the monzonites. The rhyolite of these flows is usually a light- is impossible either to represent the known com­ white porphyry was found by Messrs. Gane and Lord as an The alkali feldspar is apparently always orthoclase, doubt­ gray rock, very clearly laminated in process of apparent dike in the monzonite. This rock, less with some soda in it. Augite is the most important of the plexity upon the map or to express it in legend which was not mapped, may be referred to here Compiemen= ferromagnesian silicates, but hypersthene is -. -, -, a j_i ^ x TI tary dikes flowing, and containing numerous crystals of both names for the rocks of the larger bodies. While as probably one ot the complementary dike associated usually present in all but the highly feldspathic Petro- -, f ,-, T+ s * ^ i graphic de= plagioclase and alkali feldspars, with a little bio­ some of the existing complication is apparently forms associated with the monzonite magma, monzo'nlte. members of the series. It is of moderate pie- tails. The porphyry has a largely predominant, very ochroism, and is often altered in the common tite in thin leaves. No quartz crystals have been due to several eruptions of somewhat different fine-grained, light-gray groundmass, in which are embedded manner. Intergrowths of hypersthene, augite, and biotite noticed. Usually there is a contrast between magmas within certain stocks, a still larger part small feldspar phenocrysts of less than 2 millimeters diameter, are common. Hornblende appears as a primary element in dull-gray bands and those of white or earthy-gray with thin biotite leaves. Seen with a hand lens the ground- the strongly dioritic facies only. As uralite it replaces augite is due to gradual transitions in the composition mass appears distinctly granular in some specimens and or hypersthene in altered phases. shade, each less than a centimeter in thickness. of the rock from place to place. The designa­ felsitic in others. The porphyritic structure occasionally seen never becomes Where this banding is most conspicuous the feld­ tions of the map merely express the predominant The microscope shows the groundmass to consist principally comparable to that so characteristic of laccolithic intrusions. of dusty orthoclase grains, with some quartz and a consider­ The contrast between groundmass and phenocrysts is never spar crystals are less distinct. But in some rock characters in the larger masses, as explained able amount of green hornblende. There is a very little mag­ so sharp as in the latter masses, and transitions in size between places, especially in contact zones, the ground- in detail below. netite, titanite, and zircon. The feldspar phenocrysts are the groundmass grains and the larger crystals are usual. mass of the rhyolite is more homogeneous in oligoclase and, probably, alkali feldspar, both very much A megapoikilitic structure is often noticeable in the diorite- General characters. The rocks of these stocks obscured in the sections examined. This rock may be called monzonite facies, especially in zones between monzonite and appearance, and then the large crystals become are mostly granular, with a local tendency to por­ a granite-porphyry or a quartz-syenite-porphyry. gabbro, as in the Ophir Needles. very prominent. A fluidal structure is evident in phyritic structure in the large masses and in dikes The rock in a measure complementary to the above in com­ The transition in chemical composition which would be position is an augite-mica-syenite a narrow dike in monzo­ expressed by a series of chemical analyses is so obvious from the wavy curves of the banding, and especially if or arms extending out into the surrounding rocks. nite. It consists of orthoclase, largely in grains as much as the mineral constitution that no such series has been consid­ the rock contains many angular inclusions of rhyo­ They are usually rather fine grained, but coarse 1 centimeter in diameter, filled with pale-green augite prisms, ered necessary. But one of the dark gabbroic facies of the lite and andesite, like those of the flow-breccia, as red-brown biotite leaves, and magnetite grains. Apatite Ophir Needles is so common both here and in the Stony facies occur in most of the larger stocks. needles are numerous, and a little quartz is present. The Mountain-Mount Sneffels stock, that a very fresh specimen is often the case. This rock splits easily into thin The variation in composition, partly expressed cleavage planes of the large orthoclase grains, interrupted by was analyzed by H. N. Stokes, yielding the following result : plates parallel to the banding. in the names on the map, is gradual in many the many minute inclusions, illustrate the poikilitic structure beautifully. Analysis of diorite-monzonite from Ophir Needles stock. Under the microscope a most delicate and striking fluidal places,r ' as in the Ophirr Needles and at Variationv ... in. structure is common in the rock of these flows. It is expressed Mount Wilson; but rather abrupt constitution. Diorite-monzonite. This compound name has ...... 56.93 MgO. 3.30 by the curling and twisting of strings or films of dark ferritic changes at other places seem to indicate dikes. been applied upon the map to the rocks of the ...... 1.03 K3 O . 2.58 globulites. The thin wavy bands between these dark lines A12 O 8 ...... 17.03 3.19 are either glassy, with some irregular trichites, or rudely The rocks as a group are made up of the minerals Mount Wilson and Ophir Needles stocks, of the Fes O 3 ...... 3.67 0.44 spherulitic, in the freshest specimens. More commonly, how­ plagioclase, orthoclase, and quartz on the one connecting arm between them, and of the irregular FeO...... 4.54 H 2 O below 110° C..... 0.13 ever, obscure devitrification has set in, from perlitic cracks MnO...... 0.10 H 2 O above 110° C..... 0.45 in some cases, and much of the rock is to be designated hand, and the dark silicates augite, hypersthene, Black Face mass. The name expresses the fact CaO ...... 6.51 aporhyolite-porphyry. hornblende, and biotite on the other. According that the greater part of the rock in question is BaO ...... 0.08 Total...... 100.04 The feldspar phenocrysts of these rhyolites are seldom fresh, to the ratio of these minerals to one another, the intermediate between diorite and mouzonite, in SrO...... 0.06 but it is clear that, as in all other varieties of the bedded Sp. gr. 2.860 at 33° C. series, a plagioclase probably labradorite (Ab ± An!) and rock assumes from place to place the characters that soda-lime feldspar plagioclase predomi­ several forms of alkali feldspar are present. Biotite has been of the various types to which distinct names are nates, while potash feldspar orthoclase is also The rock of this analysis is dark gray, fine and even grained, commonly resorbed, with usual products. and megascopically almost aphanitic. Labradorite is its The banded or streaked appearance is due to alternation of given. The principal groups will now be prominent. There is also great variation in the principal constituent, with considerable amounts of hyper­ layers, drawn out by motion of the lava, in which slightly dif­ described. amounts of quartz and of the dark silicates and sthene, augite, and biotite, in order of importance. These ferent structures or degrees of crystallization may be noted. Monzonite. The largest stock of the district magnetite, so that the observed extremes are are all somewhat irregular in form, the prismatic hypersthene In the dull-gray bands there is commonly a coarser develop­ being more nearly automorphic than the others All three ment of the ferritic constituent, in distinct grains or trichites that of Grizzly Peak, Rolling Mountain, and San granite and gabbro. are intergrown in some grains. Orthoclase and quartz are present in very subordinate amounts, but serve to connect clase and chlorite. The latter is derived chiefly from biotite, tion between gabbro and diorite lies here in the quantitative Head and the one shown on the ridge south of this rock with the dominant types of the region. of which small fresh leaves are found included in quartz, and relation of the ferromagnesian silicates to the feldspathic ele­ Mount Wilson. The analysis is to be compared with analyses of similar partly from augite. The distribution of the former dark ment. The character of the soda-lime feldspar is ordinarily rocks from Stony Mountain and Mount Sneffels, given mineral in thin layers causes the banded appearance. found to vary in harmony with this ratio, but certainly does A few inconspicuous dikes in Campbell Peak below. (/) Gfneissic rocks, with broad or narrow alternating bands not do so in these rocks. and Ruffner Mountain are hornblendic. of dark-green color, rich in augite, and others nearly free from The labradorite, hypersthene, and diallage of the summit Inclusions in the diorite-porpliyry at Ophir any dark silicate and rich in feldspar and quartz. Veins of rock of Stony Mountain possess the characters common in Plagioclase-basalt. The dike at the mouth of Loop. Both railroad cuttings at the Ophir Loop feldspar and quartz cut the dark bands. All the banded rocks gabbros ; but the diallage parting is not commonly found in Big Bear Creek is a very simple normal basalt, show the diorite-monzonite at that locality to be exhibit contorted folding in some bowlders, but no shearing even more basic forms than that. When the plagioclase is having abundant fresh olivine crystals and a holo- or lamellar schistose structure is shown. richer in lime than Ab± A^ it usually has some peculiar habit characterized by numerous inclusions of many (g) Hornblende - diorite, seeming coarse grained at first and is ordinarily very full of glass and globular augite inclu­ crystalline groundmass consisting of augite, oli­ varieties of rocks. The inclusions are most abun­ glance, composed chiefly of hornblende and feldspar sharply sions, as in the gabbro-porphyry. Magnetite seldom becomes vine, labradorite, magnetite, and a little biotite. separated into aggregates of small grains. These aggregates strongly developed in these rocks, and, aside from apatite and dant at the level of the lower railroad cutting, are more or less connected, but seem to be in distinct areas on titanite, accessory constituents are very rare. Augite-minette. In the ridge running southeast where for nearly half a mile the diorite-porphyry many fracture faces. Now one, now the other, is the more The gabbro of the summit of Stony Mountain has the fol­ from Gladstone Peak, in the Mount Wilson group, is so crowded with them that they make up more continuous, and there is much variation in the relative lowing composition, as determined by analysis made several are two narrow dikes of a dense, dark, almost amounts of hornblende and feldspar in different inclusions. years ago by L. Gr. Eakins : than half the mass. This line of the railroad is In some fragments a tendency of the aggregates to draw out black rock, cutting Cretaceous strata. These con­ within about 50 feet of the lower contact of the into wavy lenticular form is visible. Occasionally totally Analysis of gabbro from Stony Mountain. tain abundant augite and brown biotite, with a irregular variation in composition appears, each of the ele­ igneous mass, which is, however, not exposed. SiO3 ...... 52.05 K3 O 1.61 few decomposed olivine crystals, in a smoky-brown ments forming massive rock either grading into the other or A12O3 ...... 17.96 Na3 2.99 Apparently this is about the point at which the being bounded by a rather sharp line. Fig. 14 shows several Fe2 0 3...... 4.09 H S O 0.97 glassy base with cryptocrystalline spots. It is sheet-like arm extending westward toward Mount of the structures and phases of variation exhibited in these FeO...... 6.33 P.O« 0.31 believed that this glassy base must be rich in singular rocks. MnO...... 0.43 Wilson leaves the main stock of the Ophir Needles. The microscope shows that magnetite and biotite are com­ CaO...... 8.64 Total...... 100.41 potash and that the rock belongs with the minettes No reason is apparent for the concentration here mon associates of the hornblende, which is of the common MgO...... 5.03 on account of the prevailing character of similar green variety, and that orthoclase is variably mingled with Sp. gr., 2. 891 at 13. 5° C. of such an enormous number of inclusions. Occa­ the predominant labradorite in the feldspathic portions. No dikes in adjacent districts. sional inclusions are found in Yellow Mountain, evidence appears that these structures were of mechanical The fresh gabbro-porphyry of the pass south of Mount Yogesite. On the ridge running east from Sneffels was analyzed by H. N. Stokes, with the following and in the arm forming the cliff shown in fig. 12 origin after the consolidation of the mass. Gladstone Peak and in Bilk Basin are narrow (h) Diorite rich in magnetite. A massive, rather coarse­ result: of the Illustrations sheets. dikes of an ash-gray rock with black semivitreous grained rock, consisting mainly of hornblende in stout prisms Analysis of gabbro-porphyry from pass south of Mount Fig. 13 shows a small section of the cliff on the several millimeters in diameter, labradorite in compound contact zones. The center of the larger dikes grains or stout prisms, and magnetite in large amount included Sneffels. lower railroad cutting and gives a fair idea of contains much augite and dark-brown hornblende, in both silicates, almost obscuring the labradorite megascop- ...... 47.32 5.69 the number and relations of the inclu­ ically, and making the mass notably heavier than the pure ...... 1.50 2.02 with some olivine, in a colorless base, which con­ sions. In size these included masses ofefhe'occur- amphibolites. A little biotite and chlorite are seen in thin A18 6,...'...... 16.71 2.70 sists largely of delicate interlocking, branching sections. Fes O3 ...... 6.92 0.96 vary from the common diameter of 1 or (i) Sanded augite, hornblende, or Motite rocks. Under this crystals of orthoclase. The contact zones are FeO...... 5.94 SO...... 0.19 2 feet to 15 feet. They are angular or subangu- head are included a whole series of rocks of rude schistose or MnO...... 0.08 H 3 O below 110° C.. 0.24 dark glass with augite crystals alone. gneissic structure, in which the three dark silicates predomi­ lar, and a very rude horizontal arrangement is CaO ...... 8.51 ELO above 110° C.. 1.04 Augite-camptonite. The small lenticular plug nate, either alone or in association, with a lesser amount of BaO ...... 0.07 visible in large exposures. The inclosing rock feldspar, a large part of which is labradorite with some SrO...... 0.06 Total...... 99.95 to which Black Face owes its name is composed is a fine-grained, hornblendic diorite-porphyry, apparent orthoclase. Much of the feldspar is unstriated, and Sp. gr., 2.949 at 26.5° C. of a dense black rock in which the naked eye can generally occurs in small round grains. Titanite and magne­ showing distinct fluidal structure winding about tite vary greatly in amount. The banding comes entirely detect only a few minute particles of feldspar and LACCOLITHIO BOOKS. the inclusions. Fig. 14 shows one of the blocks from the alternating abundance of dark silicates and feldspar. dark grains of augite. The microscope shows Nearly all are granular. The biotite-rich rocks seem like bio- blasted from the cliff, the fine-grained porphyry, tite-schists, but have- much augite, hornblende, and feldspar Among the igneous rocks of the Telluride much augite, brown hornblende, plagioclase in holding the inclusions, contrasting with the darker associated. One schistose-looking biotite rock has strongly quadrangle are certain varieties which have been microlites, orthoclase in irregular grains, and a or coarser-grained inclusions. pleochroic hypersthene in narrow irregular zones about mag­ intruded into the sedimentary complex in the dust of magnetite. The chemical composition of netite grains and intergrown with the biotite. The inclusions of the Ophir Loop exposures (j) Hornblende rocks. There is a great variety of massive form of laccoliths, lifting up the strata above this rock is very nearly the same as that above present great variety in composition and structure, and schistose rocks made up almost entirely of hornblende, them. These rocks here differ sufficiently from given for the gabbro of the Ophir Needles. but in both of these respects there is , with small amounts of augite, feldspar, and magnetite. Some the stock rocks in structure to be distinguished t Large series have small, long prisms of hornblende, and in these a decided MISCELLANEOUS IGNEOUS ROCKS. so complete gradation represented, even fourndkass schistose structure is developed. Others consist of large very readily. Two varieties have been indicated in the 1 comparativelyi.* 1 smalln portioni.' or£ inclusions. hornblende prisms, sometimes an inch long and half an inch upon the map. Rhyolite. Two occurrences of rhyolite shown thick, irregularly grouped, with small grains of other minerals the mass here visible, that the question of their crowded into the angular interstitial spaces. Many of these Diorite-porphyry. The rocks forming the large upon the map can not be assigned to the Potosi origin becomes of great importance. The striking inclusions are similar to the hornblendic inclusions often found bodies of Gray Head, Whipple, and Hawn moun­ series. One of these is represented by two small fact is that many of the inclusions are practically in massive diorite. tains, Flat Top, and numerous thin sheets adjacent, plugs near the railroad line southwest of Trout coarse-grained diorite, and that gradation seems G-abbro-diorite. This term is applied upon the have been designated diorite-porphyry. They Lake. This rock is a dark-gray, dense felsite, to exist toward masses which are nearly pure map to the stock rock of Stony Mountain and contain phenocrysts or distinct crystals of a soda- with a few small decomposed feldspar crystals segregations of each of the constituent minerals Mount Sneffels and of a number of small cross- lime feldspar (andesine or labradorite), horn­ and reddish-brown mica leaves. The mass of the of the diorite. Further, nearly every mineral cutting bodies. The compound name indicates blende, biotite, and occasionally augite, in a gray rock is finely granular and cryptocrystalline, and aggregation is found, both in granular and in primarily that much of the rock is intermediate groundmass of orthoclase, plagioclase, and quartz. is possibly a devitrification product, though no banded forms. The study of the great number between gabbro and diorite, and also that there The crystals are never very large. In the lacco- definite proofs of this origin have been observed. of transition forms present makes it seem highly is a transition from gabbro to diorite. lithic masses the groundmass is so In the plug nearest Trout Lake a breccia occupies probable that all these rocks are most intimately The principal rock of Stony Mountain, as seen coarse and the crystals are so small structure110 most of the space, but is cut by arms of the same connected in origin, and when the variations at its summit and down the eastern slope, is a that the porphyritic structure is rather rock in massive form. within the mass of the Ophir Needles stock are dark, coarse- or medium-grained gabbro, General subordinate, but appears distinctly when thin Interbedded with the Algonkian quartzites of borne in mind, the association of the inclusions composed chiefly of a plagioclase rich in descr|p«on. sections of the rocks are studied under the micro­ the small exposure in Canyon Creek occurs a with the igneous rock inclosing them has an lime, with abundant hypersthene and diallage- scope. In the thin adjacent sheets the ground- banded rhyolite, > ' veryJ different in its Algonkian., apparent significance which is of great theoretical augite. Even in this rock there is some ortho­ mass is very fine grained, and hence contrasts with habit from the flows of the Potosi series. rhy°lite- importance in connection with the general problem clase and quartz, while in some phases of the the phenocrysts embedded in it. This rock is felsitic, gray or pinkish, delicately of the origin of igneous rock varieties. The infer­ rocks here included there is a considerable amount The mineralogic composition of these rocks banded and then nearly free from phenocrysts, or ence seems most natural that this diorite-monzo­ of these two minerals, and a transition develops does not vary greatly. Quartz is not very abun­ it may exhibit many sanidine and smoky quartz nite magma and these inclusions have a common toward the rock called monzonite. dant in any of them and is nearly lacking in crystals and be less distinctly banded. The dense source, and that the great variations in chemical In Mount Sneffels, so far as the mass is now the rock of Flat Top. Orthoclase is always so portion exhibits under the microscope much quartz and mineralogical composition found in each have known, gabbroitic phases are predominant, partly subordinate that no decided approach to monzon- and a cryptocrystalline granular part, which is resulted from the processes of "magmatic differ­ coarse grained, but with large amounts of the fine­ ite-porphyry can be found. probably mainly orthoclase. No biotite or other entiation," the nature of which is, unfortunately, grained, almost aphanitic type seen in the Ophir Granite-porphyry. Above Ophir occurs a por­ dark constituent now remains. as yet a matter of hypothesis. Words can hardly Needles, of which an analysis has been already phyry body, cut in two by Howard Fork, which Andesite. Capping Diamond Hill, east of Big give an adequate conception of the strong evidence given. A dark gabbro-porphyry occurring in is in some respects analogous to a laccolith, Bear Creek, is a thin sheet of a much decomposed in favor of this conclusion afforded by the phe­ dikes is very prominent on the southwest side of although somewhat irregular in its relations to the rock, in certain portions of which are many small nomena visible at Ophir Loop, and an extended the mountain and at the pass on the south. This inclosing sedimentaries. This rock is called a cavities containing perfectly clear crystals of discussion is out of place in this folio text, but porphyry is characterized by thin plates of plagio­ granite-porphyry. It contains pink orthoclase quartz, whence the local name of the hill. The short descriptions of the main rock varieties clase in more or less marked parallel arrangement. crystals, some of which are half an inch long, and rock is so highly impregnated with calcite and occurring among those inclusions will be given They appear dark themselves, from the great num­ many smaller ones of white oligoclase, quartz, chlorite, and all its constituents are so much below. ber of dusty inclusions they carry and because green hornblende, and brown mica, which lie in a altered, that its original character is uncertain, they lie in a dark aphanitic groundmass of the groundmass consisting chiefly of orthoclase and although it is designated andesite upon the map. The rock inclusions of Ophir Loop, so far as represented by the specimens collected, may be grouped as in the following other minerals common in these rocks. quartz. The large orthoclase crystals especially It may be an intrusive sheet of porphyry, but, if descriptions. It is to be kept in mind, however, The lighter-colored fine-grained rocks of the characterize this rock. so, it has undergone alteration in a manner not that many more varieties are undoubtedly pres- L'st of rocks connecting arm between Stony Mountain and elsewhere observed among the intrusive sheets. ent,, makingi the, i complexi one off almosti x perfectf j. observedinclusions. as DIKE BOOKS. gradation from all extremes toward the average Mount Sneffels, and the rocks of the smaller stocks rock, which is of the general composition of a diorite. and dikes mapped as gabbro-diorite, are less rich Under the general name " Basic dikes " a num­ DESCRIPTIVE GEOLOGY. (a) Labradorite rock. A white granular mass of labradorite, ber of dark dike rocks have been represented on with a little orthoclase, no magnetite, no quartz, no horn­ in the dark silicates than the gabbro proper. Their A description of the quadrangle, showing the dis­ blende, or other primary f erromagnesian silicate. Small specks plagioclase is like that of the diorite-monzonites, the map. These dikes are younger than The oun est tribution and local characteristics of the rock of epidote and chlorite may be decomposition or infiltration any other igneous masses of the region, 's"60"8 rocks- and they have some orthoclase and quartz. Usually -formations and their influence in producing products. and several varieties occur which will be specially (&) Aplitic granite-porphyry. Variable large pink ortho­ these rocks are fine grained in texture and gray in the existing physical features under the clase phenocrysts and some smaller ones of quartz lie in a fine­ color, and hornblende is sometimes the most dis­ described. agencies of erosion. grained groundmass of orthoclase and quartz. A few minute tinct constituent. They have the general habit Pyroxene-andesite. By far the majority of all augite grains and specks of magnetite are present. Plagio- OUTLINE SKETCH. clase not identified. of diorite. the observed dikes are of a pyroxene-andesite (c) Q-ranite. Of rude gneissoid structure, consisting chiefly very similar in composition to the rock of the The Telluride quadrangle affords a striking of pink orthoclase and quartz, with a very little chlorite and In the more basic rocks of this series (as in the gabbro-por­ epidote. The rock is unevenly granular and of rather coarse phyry of Mount Sneffels) plagioclase as rich in lime as bytown- flows in the Intermediate series, but dense in text­ instance of the dependence of physiography upon grain. A little plagioclase is present, and one crystal of allan- ite may be found, but the most common variety ure and usually quite aphanitic. These dikes geology. The strongly contrasting physical fea­ ite was seen in the thin section. is labradorite of Ab x Anl5 the same that occurs Sj°if.raphlc prevail in the eastern portion of the quadrangle, tures of the area, which have already been men­ (d) Gneiss. A banded rock in which certain bands 2 or 3 in many of the monzonites. This feldspar forms inches wide consist almost wholly of pink orthoclase, with a a large part certainly of the plagioclase in the gabbro of the all of those represented on the map in that section tioned, are most intimately related to contrasting little quartz; others have some white plagioclase, and still summit of Stony Mountain. This persistence of one variety belonging to this type. The small dikes in the geologic features. The mountains and valleys others are variably dark by the admixture of green augite. of plagioclase through such a series of rocks makes impossible (e) Mica-granite-gneiss. A thinly banded but evenly granu­ a satisfactory subdivision of the series on the basis of that vicinity of Sawpit, not shown on the map, belong are the result of long-continued erosion of a part lar rock, with much quartz and orthoclase, and some plagio­ component, and it appears that the criterion for the distinc­ in this group, as do the dikes about the Lizard of the San Juan volcanic plateau and of the Telluride 7. ^

older formations beneath it; and it is possible northwest. In the foreground is the rolling pla­ westerly dip of the Mesozoic formations, combined of a thin andesite flow. This line can be seen for ble to recognize the relation between many of the teau of Mancos shales, covered here and there by with a gentler eastern dip of the San Miguel miles, as appears from the views given in figs. 2 physical features and the character and mode of detrital material swept t down from the Description_ conglomerate, brings the latter down to rest on and 5. The columnar structure in these sheets is occurrence of the different rock masses which mountains. The light band of cliffs next of fig> * the La Plata sandstone at a level only a few hun­ due partly to shrinkage cracks and partly to fis­ have been described. These facts lead to a above the shales is of the pale-reddish San Miguel dred feet higher than the valley lake bed above sure systems, which traverse all the formations. natural subdivision of the quadrangle for descrip­ conglomerate, here about 300 feet thick. Succeed­ Telluride. Within this distance Butcher and The rounded summits of the divide are caused by tive purposes. ing that comes the San Juan tuff-conglomerate, Cornet creeks and Owl and Royal gulches descend the more easily disintegrating, thin, glassy flows The eastern half of the quadrangle belongs to fully 1500 feet thick, stratified as perfectly as the rapidly from the mountain crest of Potosi rhyolite and purplish tuffs of rhyolite belonging to the the San Juan mountain complex of to-day. The San Miguel in its lower portion, but of contrast­ across the Intermediate series, the San Juan tuffs highest member of the volcanic series thus far

bold mountain front exhibits most The moun= ing dark color, as is well shown in the figure. and agglomerates, and the San Miguel conglom­ examined. Where this upper series has been clearly the bedded volcanic series tain area. The San Juan reaches to the horizontal ledge erate, each stream bed in turn showing the latter removed the narrow serrated crest becomes impass­ which makes up a large part of the mountains to near the top of the dark point on the right, and in unconformity upon various older strata. A able. Its character appears above the Virginius the eastward. Penetrating these volcanics are to the little scarp below the main cliffs of Camp­ waterfall or cascade occurs in each as it crosses mine and in St. Sophia Ridge, north of Mendota great stocks and smaller bodies of granular igne­ bell Peak, near the center of the picture. This the San Miguel ledge. Peak. A larger thickness of the upper tuffs and ous rocks, known also in other parts of the San little scarp is caused by a 4-foot sheet of augite- From the extreme eastern limit of the lake beds flows appears in Gilpin Peak and other summits Juan region. The mountain front is imposing, andesite, all that here represents the Intermediate above Telluride the somber cliffs of the San Juan above the 13,200-foot level. but the deep dissection to which the interior of series. The more rugged upper part of Campbell formation rise with startling abruptness on the At the horizon of the Virginius mine, in the the San Juan Plateau has been subjected is well Peak and the higher peak on the right are due to north, east, and south. From the mouth of Bridal center of the view, appears a line between the illustrated by the eastward drainage of Canyon, the massive rhyolite flows of the Potosi series. Veil Creek to the end of the ridge between basal tuff of the Potosi series and the darker tuffs Mineral, and Lime creeks, and in the valleys of The view in fig. 1 shows that, rugged as are the Ingram and Savage basins, known as Telluride of the Intermediate series, which are locally these streams the relations of the volcanic rocks mountains to-day, they were much more precipi­ Peak, there is a rise of 3000 feet within one mile. exposed in several outcrops, though commonly to the underlying sedimentary formations are well tous at a time not very remote. The great talus Ingram Creek descends in a cascade for more than concealed by talus. The San Juan tuffs are sel­ exposed. slope which extends from, near the summit of 1000 feet, and Bridal Veil fall, over the San Miguel dom well exposed in the upper parts of Canyon The western half of the quadrangle is an undu­ Campbell Peak down to the Cretaceous shales has ledge, is over 300 feet in height. The character Creek, owing to the great amount of debris from lating plain, interrupted at several places by iso­ almost covered the scarp of the lower San Juan of the country may be seen in fig. 5 BridaIVeil the higher zones. The smooth slopes of the mid­ lated mountains or groups of peaks. Theplain tuff and the San Miguel formation. The glacial (Illustrations sheets). This view is Basin> dle ground, seen in fig. 6, are yearly swept by Viewed in its broader relationships area- cirque in front of the peak, at the head of Eder from a point well up in Bridal Veil Basin, looking terrific snowslides, some of which have been this plain is the eastern limit of a great plateau Creek, has now no solid rock exposures anywhere directly north. In the center are the wonderful attended by loss of life in the immediate vicinity country covering thousands of square miles to the near its floor, and all the high basins are in a buttressed cliffs between Marshall and Ingram of the Virginius and Terrible mines. westward, the floor of which is the Dakota sand­ similar condition. But the mountains north of creeks, formed by the San Juan tuffs and agglom­ Fig. 7 is a view from a small bench south of stone with local remnants of the Mancos shales the San Miguel are particularly characterized by erates. From the lowest point visible to a level Stony Mountain looking down to the forks of upon it ; and the San Miguel Canyon is a type these debris slopes, on account of the decomposed several hundred feet above timber line all belongs Canyon Creek below the Trust Ruby Potosi Peak. of the gorges cut by many streams of the Colorado state of the rocks, causing them to split into small to that formation. This view illustrates the dis­ mine and showing the almost unbroken Plateau into the sandstone and shale series beneath angular pieces under the influence of frost. In tinct bedding throughout the formation as well as slope of Potosi Peak, 3000 feet in height. In the the Dakota. In the walls of this canyon one may many places the altered rocks are bleached or the massive effect of the whole in cliff exposures. left foreground is a projecting spur of the Stony study the details of sculpturing produced by the heavily iron stained and the color contrasts are In the left-hand portion of the view is seen the Mountain stock. The volcanic series of Potosi erosion of horizontal rock beds of varying consti­ often striking. zigzag trail to Marshall Basin, from about the Peak has not been examined in detail, but it is tution. Fig. 2 shows the sculpturing of the same moun­ point where it crosses to the western side of the known that glassy rhyolite flows have here a Viewing the plain area from a more local stand­ tains as seen from the south side of the San stream up to a level beyond the buildings at greater development than at any point in the point, it presents the record as to the extent of Miguel, about 2 miles below Telluride, Mountain the mouth of the Bullion tunnel. A little higher Telluride quadrangle. erosion below the floor of the San Juan volcanics, looking somewhat east of north. The shownTn a section of the wagon road in the basin may be The massive forms produced by the stock of and the isolated mountains become of prime inter­ ledges of Dakota sandstone in the fore- flgs' 2 and 3' seen, but the mines on the Smuggler vein are Stony Mountain contrast markedly with those est, since in the Mount Wilson group and in the ground are on the north bank of the river, the hidden. The Intermediate series of flows and resulting0 fromp the erosion of the bed- Stonye Moun­ summits east of it is the proof that the San Juan valley bottom not coming into the view. Back of tuffs occupies a zone running through the amphi­ ded volcanics. The stock forms the tain- volcanic complex and the underlying San Miguel the rolling shale slopes appear the cliffs of the theaters and shoulders, separating them. The angle between the forks of Canyon Creek, and formation once extended for an unknown distance San Miguel conglomerate, cut by Eder Creek. On lowest cliff, facing Marshall Basin and nearly sur­ presents rough cliffs of massive, irregularly jointed beyond the Telluride quadrangle to the west. the left is the long talus slope seen in fig. 1, lead­ rounded by talus, is formed by the augite-andesite rock on all sides except the west, where a narrow The mountains of the Wilson group exhibit the ing up to the andesite ledge of Campbell Peak. flow near the base of the series, here some 600 feet divide but little lower than the summit connects rugged forms caused by a great stock from which The higher mountains of the ridge between Camp­ in thickness. This sheet thins out southward, it with the main ridge. The gabbro mass extends the surrounding beds have been largely removed. bell and Dallas peaks exhibit the general bedding toward the point of view, and the flows become across the north branch of the creek, penetrating In Gray Head, Whipple and Hawn mountains, of the Potosi rhyolite series, and, so far as it is much subordinate to tuffs and agglomerates. The the San Juan tuffs and so metamorphosing them and jFlat Top are shown excellent illustrations exposed, of the San Juan tuff. In several places columnar jointing of the rhyolites of the Potosi that the line between the rocks can be traced only of the mountain forms resulting where large igne­ may be seen the rounded pinnacles of the San series and the general rugged character of the with great difficulty, even in the bare glacial sur­ ous masses of the intrusive laccolithic type have Juan, nearly buried under the talus, and in fig. 3 crest of the divide between Marshall Basin and faces or ledges of the creek bottom. Arms of the been laid bare by the removal of the inclosing is represented a characteristic bit of detail of this Canyon Creek are clearly shown. gabbro-diorite run into the tuffs east of the sediments and have themselves been sculptured kind. The view is from timber line on Campbell In and about Marshall Basin the character of Yankee Boy mine and as narrow dikes reach far in bold relief. Peak, looking across Eder Creek at a group of the Intermediate series can be studied to good up the slope of the ridge from Potosi Peak. Much The area thus exhibits, on one hand, the struc­ these turreted forms of cliff erosion, and beyond advantage. On the western ridge, opposite the of the detail on this slope is too complex for rep­ ture of the San Juan Mountains and of isolated them to the massive cliffs of the Potosi series, just Smuggler workings, the flows of dark glassy rhyo­ resentation upon the map. outliers once connected with them, and on the west of Dallas Peak. A small ledge midway in lite are very distinct, and the principal augite- The small mass of Algonkian quartzites and other the sedimentary plateau and the laccolithic the cliff represents the horizon of a thin andesite andesite flow has its maximum development here, rhyolite at the north base of Stony Mountain is type of mountain which in several areas modifies sheet separating the heavier rhyolite flows. The causing the projecting shoulders which separate of much interest,7 especiallyA «/ if it is,' as Old- . buried the monotony of the great plateau country. rhyolite debris fallen upon this ledge often con­ the minor basins between Marshall and Savage represented in the map and sections, a SSS^Man ceals the andesite and causes a lighter band. One basins. Directly west of the Mendota, on the peak of the old mountains covered by quartzltes- THE SAN JUAN MOUNTAIN FRONT. of the narrow and somewhat irregular andesite trail to the head of Cornet Creek, the base of the the San Juan tuffs. The outcrops are not very As represented on the geological map, the entire dikes common in this vicinity is shown cutting Potosi series is very well exposed, and below it, favorable to a study of the mass, for all about western front of the mountains, from Ruffner the tuffs on the left. in a hollow of the andesite sheet, occurs a consider­ them the tuffs are highly metamorphosed, arms Mountain to Grizzly Peak, is formed by In Iron Mountain the tuffs about the summit able thickness of tuff of the Intermediate series. of the gabbro-diorite partly inclose them, and all the bedded volcanic series of andesitic are heavily iron stained, but on the lower slopes Head of Canyon Greek. In the northeastern rocks have here been mineralized, then bleached and rhyolitic rocks which have been they are bleached and much hardened near the corner of the Telluride quadrangle is the head of out, and are in addition so crushed and fractured described, excepting only the salient gabbro-diorite dike shown on the map. On the Canyon Creek, which enters the Uncompahgre at that in and near the creek bed they break up into points at Ophir Needles and Grizzly Peak, where east branch of Deep Creek the same general devel­ Ouray. It is separated from the drainage of the minute angular pieces on exposed surfaces, so that two large and massive stocks appear, causing a opment of the volcanic series is found, with very San Miguel by the high divide formed of the Potosi recognition of the original rock character by the rugged topography very different from that found brilliant yellow and reddish coloring in places. rhyolite series, seen in fig. 5. On the northeast naked eye is sometimes impossible. The lami­ elsewhere along the front. Except for the small Ruffner Mountain, the summit of which is is the corresponding crest, extending from Potosi nated rhyolite is distinct in the creek bed, and so stock at Stony Mountain and a few lesser intru­ beyond the quadrangle line, is especially rugged, Peak to Mount Sneffels. In the center of the is the narrow quartzite ledge on the east of it, sions, the bedded volcanics extend eastward by reason of the number of andesite basin rises Stony Mountain, due to a small gabbro while parts of the larger quartzite area west of beyond the limits of the quadrangle, producing dikes penetrating the San Juan tuffs, RuffnerMountain, stock connected with that of Mount Sneffels. The the rhyolite are beautifully exposed on a smooth the same general features of mountain and valley and particularly through the indura­ geology of this basin will be described with refer­ glaciated surface. observable on the actual front. Beyond Ruffner tion of both sedimentary and igneous beds adja­ ence to three views given on the Illustrations Between the quartzite area and Mount Sneffels Mountain the bedded series extends but a few cent to the broad gabbro-diorite dike on its south­ sheets. a widening band of rather fine-grained gabbro- miles northwest, to Mears Peak, and then swings western slope. The San Miguel conglomerate and Fig. 6 is from a photograph taken from Stony diorite can be traced to the cliffs of the Mount rapidly eastward, while for 20 miles east of the Mancos shales below it are very well exposed Mountain looking south at the ridge separating the peak, though many details of its con- Sneffels- Grizzly Peak there are no remnants of the vol­ on the western side, the latter in almost continu­ Virginius branch of Canyon Creek from tact are concealed by talus. The extreme west­ of canic series south of the parallel forming the ous outcrops for 1500 feet, down nearly to Deep Marshall Basin. At the foot of the mid- ern head of Canyon Creek, under Mount Sneffels, southern boundary of the Telluride quadrangle. Creek. On the southwest ridge of the mountain die cliffs are the Virginius mine build­ and the mountain itself, are shown in fig. 8, a It is literally the southwestern front of the San is a small irregular stock of gabbro-diorite cutting ings, and below them are those of the Terrible mine. reproduction of a photograph taken from Stony Juan Mountains which is here presented. horizontal shales, which are greatly metamor­ The trail to Marshall Basin passes through the Mountain. This view exhibits the character of North side of the San Miguel River. The gen­ phosed near it. This mass is like a number of little windy gap above the Virginius. This view the mountains, due to the massive stocks of granu­ eral character of the mountains facing the San others found in the volcanic area. Fig. 4 shows is well adapted to show the nature of the details lar rock penetrating the bedded volcanics of the Miguel Valley between Ruffner Mountain and this stock as seen from the south side of east Deep commonly found in the horizon of the Potosi San Juan Mountains. Unfortunately it has been Telluride can best be described with reference to Creek. The lower limit of the outcrop is shown series, which extends from the Virginius buildings impossible to examine this mass except on the views reproduced on the Illustrations sheets. Fig. and on either side appear ledges of hardened shale. to the crest of the ridge. The main cliff-making ridge leading to the pass on the left, and it is not 1 represents the mountain face as seen from the Marshall Basin and the head of the 8an flows of the series are distinctly shown, separated known how extensive the stock may be on the ridge between Mill and Butcher creeks, looking Miguel. Between Mill and Marshall creeks the by a narrow debris -covered bench, at the horizon north. It is possible, moreover, that indurated tuffs take some part in tlie very summit or on the The separation of the Potosi series into several The Dolores formation is best exposed in Water­ directly under the summit by a blunt wedge of the right-hand point of the mass. The view shows massive bands with crumbling material between fall and Swamp creeks, and in the ravine west of stock rock, turning up the higher strata at a steep the great amount of talus in the basin south of them causes a marked development of benches the Badger mine, on the north side of the river. angle, while the lower ones are cut off sharply Mount Sneifels. and ledges in the basins under the higher points. But the hardened, bleached, and fissured rocks by the nearly vertical contact, which can be fol­ To the west of the saddle between Gilpin Peak This is very noticeable from Lookout Peak north­ seen in the locality last named and in the lower lowed from that point down the ridge. Below and Mount Sneffels rises a fork of Dallas Creek, east. On these benches are numerous lakelets of part of Waterfall Creek can scarcely be recognized the San Miguel the hardened Mancos shales appear, in a basin called the "Great Amphi­ clear blue water, which add much to the beauty of as belonging to the Dolores, except by their rela­ and at the base of the slope is seen the white theater" on the Hayden map. Only Amphithe= the landscape. At the head of Bridal Veil Basin tion to the white band of La Plata on the slope of Dakota sandstone, both of these steeply upturned the upper slopes come within the Tel- several of these ponds occur very near the base of Yellow Mountain. The red color of the Dolores by a fold older than the San Miguel, whose strata luride quadrangle, and they exhibit the bedded the Potosi series. strata appears, however, in the upper parts of are not affected by it. This contact is typical of volcanic series which has been described as appear­ From Lookout Peak westward to Silver Moun­ Waterfall and Swamp creeks. those ordinarily found about the great stocks of ing on the southern side of the crest between tain the mountain crest is very narrow and jagged, The San Miguel conglomerate is scarcely recog­ the quadrangle, though lateral arms of varying Gilpin and Campbell peaks. This is one of the presenting forms similar to those found in the nizable along the north side of Howard Fork, form and extent are sometimes seen, as particu­ elevated glacial basins the origin of which was mountains north of Telluride. A view of Silver being greatly altered in appearance by larly illustrated in this stock. ascribed by F. M. Endlich to a sinking of the Mountain and the spur toward Gold Hill, as seen the decomposition of the limestone and sedimentary* The diorite-monzonite mass forms the north­ .. ill 11 ill i formations. floor for more than 2000 feet (Annual Report of from the west, is given in fig. 10. It will be granitic pebbles and the general bleach­ western end of Yellow Mountain and sends off the Hayden Survey for 1874, p. 206). No evi­ noticed that the San Juan slopes are partially ing out which all formations have here undergone. a large irregular dike, in which are some of the dence of such subsidence was cited by Endlich, talus covered, while the higher levels occupied But in the southern tributaries it is again well workings of the Montezuma and adjacent mines. and it seems, in fact, that this amphitheater was by the Potosi series are more rugged, with a developed. In Swamp Canyon the limestone There are doubtless other offshoots of this mass, formed by the same agents of erosion that pro­ serrate crest above them, as in other regions pebbles have frequently been partially replaced not represented upon the map, for the recognition duced all the other basins of the region. Several already described. by iron oxide in the outer shell, while the interior of small arms in the metamorphosed sediments is small blue lakes are found in the various branches In this ridge the rocks are either bleached or may have been dissolved, leaving a cavity. often difficult and requires more detailed exam­ of this basin. That it was once filled by snow highly colored, and are so much fissured that, The San Juan and Intermediate series are here ination than could be given to any portion of the and neve fields feeding a glacier below is directly particularly on the southern face, great talus developed much as in Bridal Veil Basin, but the area. Mr. Purington reports that a broad dike suggested by an actual remnant of neve ice lying slopes descend almost unbroken from the summits latter varies much in thickness. In the of dioritic rock was cut in the Badger (?) tunnel. at an elevation of about 12,500 feet, directly under to the valley bottom of Howard Fork. Ophir Pass divide and in the Lookout ofethe°vouent If this dike is exposed on the surface it escaped the sharp ridge east of Dallas Peak. Howard Fork and tlie mountains about it. Peaks on either side, this series has its detection, but it is presumably an arm from the Area between Telluride and Howard Fork. Howard Fork of Lake Fork of the San Miguel greatest known thickness and contains several Ophir Needles. The greater part of the mountain area lying River has excavated a deep, broad basin in the augite- and hornblende-bearing andesite flows of The diorite-monzonite extends westward across between the main or Telluride branch of the front of the mountains, from which it flows limited lateral extent. Westward the Intermedi­ Lake Fork of the San Miguel, penetrating the San Miguel and Howard Fork is made up of the through a narrow v-shaped gorge cut in the dio- ate series thins out rapidly, owing partly to an Cretaceous in three directions. The San Juan tuff breccia or agglomerate, penetrated rite-monzonite stock. A general view of this increase in the thickness of the San Juan tuffs, etc., two arms at the eastern and northern Westernshootefrnom off= by a few narrow dikes of pyroxene-andesite and valley is given in fig. 9, a reproduction Viewfroln in Yellow Mountain, and partly to removal by ibases oic banr\ -r»x>ernardo -i Mountain-**- i are the stock. seamed by mineral veins, some of which can be of a photograph taken from the ridge fEfJiStf °f erosion in the interval preceding the eruption of found with crumpled and hardened shale about seen miles away. The basins at the heads of the north of South Lookout Peak, looking the Potosi series. The blotchy aporhyolite at the them, few contacts being clearly shown, on main streams and their tributaries are either high nearly west. On the right hand is seen a portion base of the Intermediate series was found both account of the wooded slopes or the soft shale up in the San Juan series or in the overlying rocks. of the valley bottom, with the busy little mining east and west of U. S. Grant Peak, but was not debris from above. The representation is, there­ As shown by the topographic sheet, the fall of the town of Ophir appearing just over the monu­ identified in the ridge between Yellow Mountain fore, somewhat diagrammatic on the map. The streams from these basins is often very rapid, mental forms of the foreground. On both sides and Pilot Knob. Certain remnants on the former northern arm, however, is seen in the cliff expo­ while between them occur many broken precipices of the town are steep debris slopes, rising almost seem to be of andesite flows, but the basal tuff of sures north of Wilson Creek for nearly 2 miles as a thousand feet or more in height. These great to the top of Silver Mountain on the north, while the Potosi series rests on an irregular surface of an apparently regular sheet immediately below faces of dull red, purple, or brownish tuff, etc., are the positions of Waterfall Gulch and Swamp San Juan tuff on the ridge at the head of Roger the Dakota sandstone. The cliff formed by this monotonous in detail and need no special descrip­ Canyon are indicated beyond or in front of the Gulch. mass is shown in fig. 12, a view from near Ophir tion. In Mr. Purington's paper in the Eighteenth smooth ridge characterized by areas of fallen The Potosi rhyolite series is present in a thin station, on the railroad. The almost vertical cliff Annual Report, Part III (1898), are several views timber, seeming like piles of jackstraws at this remnant from U. S. Grant Peak to Pilot Knob, is 800 feet high in places. The Dakota sandstone illustrating the character of this rugged district distance. causing the usual impassable sawtooth crest is at the top of the cliff, and McElmo beds are as seen from different points of view on the north. Below Ophir the v-shaped gorge is outlined where the lower massive flow remains, and rounded exposed in the small railroad cutting seen near Bear Creek has cut deep into the red Dolores sand­ against the dark spruce-covered eastern base of forms in the horizon of the basal tuff or flow-brec­ the center of the view. The actual base of the stones, and the unconformable relations of the San Sunshine Mountain, lying beyond Lake Fork. cia. The decreased prominence of the Potosi eruptive rock is concealed by talus. Miguel conglomerate are here very plain. From this point the line of Yellow Mountain series is chiefly due to a gradual rise southward At a point nearly opposite Ames the mass cuts From the base of the Intermediate series to may be traced in its projection against the sum­ of the floor upon which it rests, amounting to rather abruptly across the Dakota and runs west­ the summits of the mountains there is much more mits beyond Lake Fork, which culminate in the about 400 feet between North Lookout Peak and ward to the north slope of Sunshine Mountain, varietyJ to the formations. The Inter- Development_ , lofty peaks of the Mount Wilson group in the U. S. Grant Peak. where it unites with a branch of the Mount Wil­ mediate series increases in thickness metd\lteter= middle background. The granite-porphyry areas in the valley above son stock, the minimum thickness of the sheet-like and changes in character from Ingram senes' Howard Fork has cut deep into the sedimen­ Ophir doubtless belong to one irregularly intruded body at this point being about 100 feet. The Basin southward. The andesite flows and the tary formations below the volcanic series, but, mass having a general lenticular cross regularity of the intrusive body in the cliff of vitreous rhyolites are much less distinct here, while the unconformity below the San section. The representation of these fig. 12 is thus very local. The rock is not present while flow breccias and tuffs composed of both Miguel conglomerate is well exposed at Indurocikpes areas is somewhat generalized, as the on the eastern side of the river, and on Wilson rocks become more prominent. Some of the some points. , in fewampa Canyonn and.J in coloration. contacts are seldom clearly exposed. No other Creek it again cuts up across the Dakota into the tipper agglomerate bands are locally coarse and Waterfall Gulch, the geology of the lower slopes occurrence of this rock has been noted within the Mancos shales. chaotic in arrangement. In the mountains west is greatly obscured by the great talus slopes, the quadrangle. The structure of the porphyry is At the base of the lobe of this mass, between of Bridal Veil Basin the tuff-agglomerates of this metamorphism adjacent to the diorite-monzonite that ordinarily characterizing laccolithic bodies. Howard and Lake forks, on the lower railroad series are very similar to the San Juan, and the stock, and the general decomposition of some large At the head of Swamp Canyon, below Ophir grade of the loop, is the place, referred to in the absence of prominent rhyolite or andesite flows areas. Especially on the north side of the valley Pass, and high up on the north side of the valley description of the diorite-monzonite, at which it in some places makes the field recognition of hori­ have the rocks been highly mineralized, not only east of Staatsburg Gulch are several dikes or is filled with rounded inclusions of many kinds, zons uncertain. But the "blotchy aporhyolite" in the vicinity of the numerous ore veins, but small stocks of gabbro-diorite, which form pro­ as illustrated by figs. 13 and 14. which has been described can be traced through generally, by a thorough impregnation of the rocks jecting outcrops similar to the one on Ruffner Directly opposite this point a thin, sheet-like all this section as an unfailing criterion for deter­ by iron pyrites in minute particles. The leaching Mountain, illustrated in fig. 4. Those near Staats­ arm lies on top of the Dakota sandstone for a dis­ mining the base of the Intermediate series. out of this pyrite and of the original iron-bearing burg Gulch are very prominent as seen from the tance of about one mile. The base of the Potosi series is well marked constituents of the andesites has left the rocks valley. These masses are doubtless arms con­ Southwestern promontory of the San Juan. through the mountains east of Bridal Veil Basin either bleached or stained yellow and red, often nected at some depth with the larger stock of From Pilot Knob to a point a little south of Griz­ zly Peak the formations of the volcanic byJ the contrast between< the dark andes- _,.The nPotosi , , in most vivid hues. Other processes of decompo­ Ophir Needles. itic beds and the lighter-colored rhyo- series- sition have likewise gone on in the permeation of The great cross-cutting body of variable con­ series extend connectedly as a high and lite-tuff. But on the west side and at the head the rocks by solvent waters, and many types are stitution through which Howard Fork has cut rugged promontory of the San Juan of the basin the bleaching or discoloration to now recognizable with great difficulty or not at all. a deep and narrow gorge is one of the Mountains. Along the irregular and extremely which the rocks have been subjected makes the The iron extracted from the rocks has been most remarkable igneous masses of the monzonYte" rugged crest of this promontory rise Pilot Knob, stock* line much less plain. The lower member of the again deposited on the lower slopes, either as a quadrangle and deserves far closer Golden Horn, Vermilion, Fuller, and Beattie Potosi series here is often a fine-grained flow-brec­ cement for the mantle of debris, making study than could be given to it. The petro- peaks, all capped by the rhyolite flows of the cia, distinguishable from the entirely fragruental a hard resistant shell of dark breccia graphical complexity here found has been already Potosi series, giving them the character already bed of other localities only on microscopical materal, or, where springs have long issued, in alluded to. The irregularities of its contacts are described and illustrated for other points to the examination. Above this bed, which is of vari­ considerable masses of brown iron ore, or bog ore. in small measure shown by the map. The north­ north. Beyond Beattie Peak the divide passes able thickness, though always considerable, come At a point about one mile above Ophir is situated ern portion, causing the craggy ridge known as into the great monzonite stock within which are two heavy laminated rhyolite flows like those of one of the strongest of these iron springs (see the Ophir Needles, is shown in figs. 9 and 10. In the summits of Rolling Mountain, San Miguel the northeastern part. They are separated by a map), which has built up a cone of iron sinter the latter the bedding of the San Miguel forma­ Peak, and Grizzly Peak. All of these but Beattie thin band of dark-red or almost purplish tuffs holding a beautiful pool of limpid blue water. tion should be visible between the jagged crest Peak exceed 13,500 feet in elevation, and the and by thin flows of rhyolite. These tuffs may be This cone has the characteristics of those formed and the strip of snow-covered talus. The contact lowest pass in this range, traversed by the trail at the horizon occupied by the thin andesite flow about the hot springs of the Yellowstone Park and is nearly vertical, with many minor irregularities, between Lake Fork and Mineral Creek, is above in the mountains above Marshall Basin, or may other districts. The mass of ore deposited at this and the stratified rocks are so hardened near it as 12,000 feet. correspond to the upper purple bands of Grilpin cone and on the slope below it is of such purity to form unusually bold cliffs. A bit of detail The abrupt western face of Pilot Knob is well Peak. In the latter case one of the lower flows and extent that it is now being used in large along the contact line between the stock and the shown in fig. 15, which is a view from the rhyolite of the northeastern district has disappeared and amounts as a flux in the smelter at Durango. The sedimentaries, from the San Miguel down, is knob at the head of the landslide be- Description_ a higher one has come in. This development of iron cap resulting from the cementing of debris shown in fig. 11, a view from near Ophir station tween Ground Hog and Leslie gulches. of figt I5> the Potosi series is very plain in the crest from is to be noted at many points on Howard Fork looking northeast. The light-colored San Miguel On the left is a building at the mouth of a tunnel Waterloo Peak, at the head of Grays Basin, to and also on the east of Ophir Pass, in the drainage beds, which lie almost horizontal on the left-hand on the Sulphuret No. 2 claim. San Juan tuff and Lookout Peak. of Mineral Creek. face of the mountain, are seen to be split apart agglomerate, of more nearly homogeneous charac- Telluride 9. ter than usual, forms the entire cliff to a level just able that the base of the Dolores formation is but sary to assume that violent earthquakes have Landslide east of Trout Lake. From Trout Lake below the smooth saddle. The cliff at the top of little below the quadrangle line. The structure of occurred in that region, only a few miles distant almost to the crest of the Yellow Mountain ridge, Pilot Knob is due to a rhyolite flow, and below it the sedimentary formations here will be described from the large slide areas of the Telluride quad­ about 3000 feet above the lake, and extending for is a series of thinner flows and rhyolitic tuffs, with in a later section. The prevalent southwesterly rangle. The conditions which now exist or which nearly 3 miles along the slope, is a landslide area a thin band of the Intermediate series, which was dip is indicated by the outcrop of the La Plata may reasonably be supposed to have existed in exhibiting in great clearness the various evidences not very clearly distinguishable from the San sandstone. this region are considered competent to explain of its origin. It is probable that the lake owes Juan tuff in this neighborhood. Nearly all of On the trail leading to Ice Lake Basin and west the areas under discussion as due to landslide its origin to the dam of slide material at its these summits in the Potosi rhyolite series are of the stream from Clear Lake is a small but note­ action. lower end, now almost entirely eroded away. The brilliantly colored, as is indicated by the names worthy landslide block, in which are represented ^Evidence of sliding observed. The internal evi­ outline of this area upon the map is accurate for Vermilion Peak and Golden Horn. the Dolores, San Miguel, and San Juan formations, dence offered by the areas themselves as to their only the upper part, at its contact with the cliff On the east of this serrate range lies Ice Lake now dipping at about 20° northward. This block origin may be briefly discussed :^ 1. The CIassification of San Juan tuff and agglomerate, seen in fig. 15. Basin, holding a dozen beautiful blue glacial has a diameter of about 300 feet and is much rock formations at the surface in these of evidences- The lower line is obscured on the surface by both lakelets, most of which are near the 12,500-foot broken, though the relations of the formations are areas occur in the mountains near by at much grass and forest growth and by the numerous level and lie upon the tuffs, etc., of the Interme­ plain. The point from which this block came is higher levels and must have come to their present small blocks of slide material which have become diate series, if the distinction here drawn upon evident in the cliff above. position by structural faulting or surface slipping. detached from the original slide mass and have the basis of an augite-andesite flow is correct. The head of Lime Creek affords excellent oppor­ This dislocation is more than 2000 feet in some disintegrated into a confused talus-like mingling On the east and west of the monzonite stock tunities to study the San Miguel formation as to places. 2. The areas are so circumscribed that of huge bowlders. are three isolated areas of the bedded volcanic its constitution and its unconformable relations to structural faults can not explain them unless the Fig. 16 is from a photograph showing the upper­ complex. The one on the west, from Variations the Mesozoic series. The Saurian conglomerate of dislocation was the sinking of a block without most part of this slide mass resting against the San Miguel Peak to Sheep Mountain, the Dolores is also very well exposed in a persist­ disturbance of adjacent country a phenomenon cliff of San Juan tuff and agglomerate. _ ma o° Description is most beautifully exposed in the cliffs ent ledge around the head of the gulch and in the not easily accounted for. 3. At the Currency The point of view is the little saddle of «g-'6- on all sides. The chief interest here attaches to ridge just south of the quadrangle line. mine, within one of the landslide areas, Mancos between the heads of Leslie and Ground Hog the San Miguel formation, which visibly thickens shale was found under 190 feet of San Juan and gulches, looking north toward Yellow Mountain, LANDSLIDE AREAS ON THE FACE OF THE San Miguel material, showing that this area is not whose summits are seen in the center of the from about 400 feet on Lake Fork to 700 feet MOUNTAINS. at the west side of Sheep Mountain, indicating a a faulted block. 4. In many places the forma­ view. On the right hand is the cliff of bedded change in character as distance from the shore General occurrence. At the western base of tions of the supposed slide areas dip at 30° or tuff, etc., continuous with that seen in fig. 15. On line increases, and preparing one to recognize the the San Juan front, in the Telluride quadrangle, more toward the adjacent cliff of the mountain the southwest or left-hand face of Yellow Moun­ thin-bedded fine-grained strata of Mount Wilson are several areas within which the rock front, the recognized normal attitude of slide tain is seen a similar cliff, of the same formation. .. -IT « -i Landslides of as the formation which on Lime Creek and else­ formations normally occurring tar above pleistocene blocks to the mass from which they have been But this cliff is interrupted in the center of the where is a coarse conglomerate. Above the San on the mountain are found in very detached. 5. The rocks within the separate areas view by a shoulder of very different character. Miguel are the monotonous dark-gray slopes of irregular and confused relations to one another. are greatly shattered and present abnormal and In its upper part this shoulder is largely grassed the San Juan tuffs, etc., with possible remnants From the size of these areas and the fault-like confused relations at many points. 6. The topog­ over, with huge debris blocks scattered about and of the Intermediate and Potosi series in the high­ boundaries seen in certain places, especially along raphy of the larger landslide areas is extremely several outcrops of light-gray rock forming knoll- est points. The brilliant coloring of the sharp the upper borders, they would naturally be peculiar, with many knolls and rounded hills like projections. Below appears a distinct ledge knobs west of San Miguel Peak suggests the explained as due to ordinary structural faults of among which the drainage channels wind in or cliff, with straggling timber-line spruces strug­ presence of the rhyolite, but these points were some remote geologic epoch. But from internal irregular course, and there are numerous sinks gling for a foothold. All of this shoulder seen not visited. Between Sheep Mountain and Grizzly evidence of various kinds, and from the limited and pools without surface outlet. 7. At the in the view is made up of the Potosi series, and Peak the San Miguel is well exposed, and the extent and the form of the areas, they are consid­ Currency mine and at a few other points there is in all the distinct outcrops the lamination dips San Juan tuff caps the unnamed summit north­ ered as having been caused by gigantic landslides evidence that sliding is still in progress within toward the east or northeast at variable angles, west of the latter peak. in Pleistocene time. The unusual scale of the the area. 8. Small slide blocks adjacent to the often exceeding 30°. The apex of this rhyolite Rolling Mountain is cut in two by the nearly phenomena exhibited gives these landslide areas larger areas are of unquestionable origin, and mass is very near the crest of the main ridge on vertical contact between monzonite and the bed­ an exceptional interest and makes them worthy of exhibit many features in common with the larger the right, just under a little knob, visible on the ded volcanics. Here there is great decomposition rather extended discussion. ones. sky-line of fig. 15, which is made up of the tuff- of all rocks, and huge talus slopes obscure the Since the survey of the Telluride quadrangle The smaller slide blocks are often seen in pro­ like fiow-breccia at the very base of the Potosi contacts at many places. A branch of Mineral was completed landslides have been noted in cess of disintegration, in the course of which they series. This apparently rests on the San Juan Creek between Rolling Mountain and the Twin other districts, and it is probable that must eventually form talus heaps indis­ tuff, the Intermediate series being absent at this this.-,.-, phenomenon 11has been 01n common elsewhereLandslides in tinguishable from those of slower point, so that there is now no lamellar rhyolite, Sisters has cut below the level of the San Miguel . the San Juan. conglomerate, thus completing the isolation of the occurrence in the San Juan Mountains. accumulation. Certain talus masses such as that of the shoulder described, on the area of bedded volcanics extending eastward on It is stated by Endlich that "Numerous places are surely of this origin. By the existing grada­ ridge above it. The base of the rhyolite in the south side of Mineral Creek as far as Sultan may be found in the volcanic section where large tion in size down to masses less than 100 feet in Ground Hog Gulch is 1000 feet below the apex Mountain near Silverton. masses of rock have fallen down, at times for length a transition to the ordinary talus accumu­ of the mass, and under it comes San Juan tuff, etc., The monzonite stock of this promontory has several thousand feet, and are now lying immedi­ lation is evident. Where a detached block, in as in the normal section. On the ridge north of been deeply scored at the head waters of three ately below the perpendicular cliff that their fall­ sliding down the slope, maintains in some degree the gulch the rhyolite comes several hundred feet ing produced " (Annual Report of Hayden Survey the normal attitude of the bedded formations its distinct streams,7 and at the head of Cas- The_. monzo- lower, but rises again to 11,700 feet on the divide cade Creek there is a wide amphithe- nite stock- for 1874, p. 194). While some of the instances origin can be recognized. But a similar mass between Roger and Minnie gulches. These vari­ ater whose floor is nearly 1000 feet below the here included by Endlich may be basins of ero­ may break into several smaller fragments by the ations are in general accompanied by changes in normal level of the San Miguel conglomerate. sion, like the " Great Amphitheater" near Mount shock of the fall, and appear to be the result of dip and strike of the rhyolite bands, but not The contacts of this great mass are, as shown by Sneffels, it is altogether probable that others numerous separate falls. always. Fracture planes marking lines of dislo­ the map, very steeply cross cutting, except on the observed by him are directly comparable with the Age of the slides. The age of the larger land­ cation traverse the mass in several directions. north of Grizzly Peak, where a wedge of the strat­ landslides of the Telluride quadrangle. slides of the Telluride quadrangle is not definitely One of these is indicated upon the map. ified rocks is upturned by the monzonite and Conditions favor able to landslides. It is evident, known in relation to other phenomena On the ridge north of Roger Gulch a massive given an irregular northwesterly dip. These from the present rugged topography of the San of Pleistocene time, but while they flow of pyroxene-andesite appears beneath the amphitheaters are no doubt glacial cirques, but Juan front,' as illustrated in this folio,' PresentD con= seem to be the oldest of the slides now rhyolite and is taken to mean that the Interme­ polished rock faces are now rare, owing to the and from the character of the rock for- ditions- recognizable the landslide period must be consid­ diate series is present in a thin remnant, as disintegration of the rock in outcrops, or the enor­ mations present, that natural conditions very ered as extending down to the present time. That expressed on the map. Below the line of rhyo­ mous talus and slide accumulations which conceal favorable to landslides still exist in the Telluride similar phenomena played an important r61e dur­ lite and Intermediate andesite is a broken, wooded them. quadrangle. The great complex of volcanic rocks, ing the whole period of erosion since the shales country, with numerous knobs or huge blocks of The eastern drainage areas. From Three mainly fragmental and highly permeable to water, were first exposed below cliffs of the San Miguel San Juan tuff, etc., and occasional ones of rhyolite. Needles to Grizzly Peak the southeastern slope from rain or melting snows, is underlain by the formation and the volcanic series can hardly be The structure in these exposures is very variable, of the high mountain divide is drained by tribu­ equally porous San Miguel conglomerate, and the doubted. The landslides in question occurred in no two being quite alike, and the conclusion is taries of the Animas River, but the streams to the whole rests upon the sandy Mancos shale. Numer­ after the mountain front had reached its present that they are not outcrops of rock in normal rela­ north of the South Fork of Mineral Creek have ous fissures also penetrate the rocks, affording position, but while the upper members of the tion to the rhyolite above. not cut to the base of the San Juan tuffs within channels for percolating waters to reach the shale volcanic complex were much more prominently The only line of outcrops affording a good idea the Telluride quadrangle. In Paradise Basin the horizon. It seems inevitable, then, that each spring developed in the peaks and ridges above that of the composition of this area is in Ground Hog andesitic tuffs, etc., are greatly decomposed and the shales below the San Miguel conglomerate front. That fact bears on the recent erosion of Gulch. Here, at about 10,800 feet, the heavily iron stained, so that horizons can be traced must be softened and rendered somewhat plastic the region, and indicates that the physiography San Miguel conglomerate is found rest- Formationsexposed in with difficulty. A small gabbro-diorite stock on by the waters which have found their way down of the San Juan Mountains was much bolder and ing on ^rMancos shales,11 iboth ,1 exhibitingi "i , the slide. the divide south of this basin is prominent by through the porous rocks above. That under that the summits were higher at the time of these an eastward dip of nearly 45°. The outcrops are reason of its projecting crags. these conditions landslides of greater or lesser landslides than they are at the present day. very distinct and the San Miguel is almost con­ In the south branch of Mineral Creek the rela­ magnitude should yearly occur at many project­ It is difficult to determine whether the glacier tinuously exposed in a thickness of between 200 tions of the San Miguel to the Mesozoic formations ing points of the San Juan front seems quite which left the moraine still visible on the east side and 300 feet. The shales below it are crushed are especially clear on account of vari­ natural. That huge masses like some of those to of Lake Fork was older than the large landslide and are not exposed except very near the San able dips of the latter, in which the 3>Mineraik be described should have been detached en masse east of it. The map shows the slide in contact Miguel. Below and on either side the wooded Creek. San Miguel does not take part. Above in this way seems to require the assumption that with the moraine and crossing its southern end, slopes were carefully searched in vain for further Bandora a thin remnant of the Mancos shales somewhat exceptional conditions prevailed at the as if more recent. This is believed to be the exposures on this general horizon. The San Juan appears under the conglomerate, and one going time of their fall. The fact appears to be that the actual relationship of the two phenomena, but it tuffs, etc., are exposed in several places between down this stream crosses the Dakota, McElmo, La topography of early Pleistocene time Pastcondi= is possible that the part crossing the end of the the San Miguel and the rhyolite above, but there Plata, and Dolores formations in order. The was bolder even than that of to-day, tlons- moraine is a more recent slide than the part above is manifestly not room for the entire section of formations are much hardened adjacent to the and it is quite natural to assume that this region the moraine to the north. The moraine has been these beds present in the cliff face of Yellow monzonite stock, but farther down the slopes the may have experienced earthquake shocks of con­ so nearly destroyed and the secondary slides are Mountain, or in that seen in fig. 16. strata have their normal constitution to about the siderable severity in times past, one effect of so numerous that the primary relations of the two From Trout Lake up to the rhyolite or andesite boundary of the quadrangle. Here they have which must have been to cause landslides of more phenomena can not be clearly demonstrated at flow the area traversed by Roger and Minnie been greatly affected by the monzonite stock of than ordinary magnitude. From the superficial this day. creeks is very irregular in its topog- Landslide Bear Peak, in the Silverfcon quadrangle, and are also slipping combined with extensive shattering of The several landslide areas will now be raphy, with many hillocks and short toP°graPhy- much decomposed by thermal waters. It is prob­ the rocks, as observed about Rico, it seems neces­ described in some detail. ridges covered by aspens, with sink holes on the upper side of many of them and drainage ravines 21-easterly.-...... ,...-«/.. This... seems... to have. . been a recent Material representing the San Miguel conglom­ mountain northwest of Grizzly Peak. Here are winding intricately around among them. In the slide of the lower few hundred feet of the San erate, much broken up, occupied some 15 feet several pinnacled projections in the zone of the frontispiece to Mr. Purington's paper in the Eigh­ Juan formation. The trail from the Gold King above the shale in the Currency shaft. In some San Miguel formation, already detached by cre­ teenth Annual Report this topography is illus­ road to Ophir Loop passes at the base of this levels and tunnels a greater thickness was found, vasses from the mountain and slightly dislocated, trated, as seen from San Bernardo Mountain. slide block, over a bench on which there is a small but in both this and the San Juan tuffs, etc., the which must ere long fall or slide en masse down Below the 10,400-foot level large blocks of San lake. At about this level to the north is a much normal bedded structure could seldom be made the steep shale slope beneath. Miguel conglomerate are numerous, and surfaces more extensive bench with two ponds, which are out, and the innumerable fracture planes running It is quite probable that small slides, more or of considerable extent show only debris of this shown upon the map. These benches are typical in all directions plainly showed the cause of the less broken up, exist on the north slope of Sheep formation, but perhaps still lower may be found of many in this slide area, having a steep outer existing conditions. It seems probable that the Mountain and to the north of the San Miguel an area equally characterized by the San Juan slope in which the crushed San Juan tuff does not Currency mine is in a slide block which may have River, on the rounded slope below the cliffs seen tuff. The larger knolls may have ledge-like out­ form a distinct ledge outcrop, though its presence originally been much more solid than at present, in fig. 1. crops, showing that either the San Juan or the is plain. and that secondary sliding has increased the dis­ THE MOUNT WILSON GROUP AND ADJACENT San Miguel formation is present in mass, but the From the south branch of Turkey Creek around location upon the fractures produced in the fall, SUMMITS. dips vary, and in the strike of a ledge of one of Bald Mountain to Prospect Creek the slide line is and perhaps created new ones. It has been stated above that the San Miguel these are knolls exhibiting the other rocks. No not sharply indicated, except that above it are Below the Currency mine the confused min­ formation and some of the overlying volcanic outcrops of shale could be found down to the seen nearly continuous outcrops of the San Juan gling of San Juan, San Miguel, and Mancos shale rocks have been found in the Mount Wilson group level of Trout Lake. tuff in normal position and below it the confused extends down to Lake Fork. At one point the and adjacent summits. The geological map On the ridge running west from Yellow Moun­ landslide topography begins. On the west slope wagon road close by the stream passes over the expresses the observed distribution of tain a small block of the San Miguel has dropped of Gold Hill, however, lateral ridges, with a trench loose black shale 500 feet below the Dakota formations, demonstrating at once that a distance somewhat less than the thick- Adjacent... back of them, are found in several places near the ledge, over which it has been pushed by the slid­ the Mount Wilson group is geologically ness of the formation. It is much fis- slideblocks- cliffs of the San Juan. In some of these ridges ing mass of the other formations. This shale and an outlier of the San Juan Mountains, isolated by sured and will surely be precipitated down the the outcrops show the San Juan tuff dipping at that of other exposures higher on the slope is the deep erosion of the San Miguel and Dolores steep slope at no distant day. A few hundred various decided angles toward Gold Hill. One of thought to belong to the great furrow of this soft rivers. A glance at the map will show the almost feet below that a similar block of the San Miguel these is below the point where the normal cliff of material which must have been thrown up by the complete correspondence in constitution between rests on the Mancos shales, marking an earlier the San Miguel formation reappears. plowing force of the original slide. the mountains under discussion and the south­ slide, and in the valley of Lake Fork is an area, Rhyolitic debris is scattered over the upper There is much landslide material in disconnected western promontory of the San Juan ending in shown by the map, within which is a jumble of part of the slide area, but was not found in mass, masses on the slope west of Ophir Needles, and Grizzly Peak. In each area a great stock of gran­ large and small fragments of the San Miguel con­ as east of Trout Lake. San Juan tuff and agglom­ the southern border for the landslide mass, as ular rock is partly surrounded by remnants of the glomerate, thought to represent a slide block in erate forms most of the knolls and benches down shown on the map, is thus a necessary general­ San Miguel formation and of the overlying bed­ an advanced state of disintegration. to a level somewhat below 10,500 feet. In several ization. ded volcanics. In the one case connection with The lower border of the large slide area east of outcrops below the Gold King road dips of as From the extent and observed structure of this, the main mass of the San Juan still exists ; in the Trout Lake is indicated on the map by a line much as 60°, generally somewhat north of east, the largest landslide area of the Telluride quad­ other the isolation is complete. were observed. showing the lower limit of a confused mass of rangle,o ' it is supposedJ. J. to be the i.product A. compound Summits west of Trout Lake. To the west of San Juan and San Miguel materials which con­ The most regular element in the composition of of several slides from the mountain face, landslide- Trout Lake is a long, curving ridge with an abrupt tinuously conceal the Mancos shales. From the this area seems to be the presence of the San which were perhaps contemporaneous or nearly so, face on the southern and southeastern sides and nature of the case this line can be drawn with Miguel formation in a broad band extending and that ever since the primary slide there has with great talus streams extending far out over only approximate exactness. Spots of debris from Prospect Creek to Turkey Creek. The lower been continual slipping of minor masses within the lower shale slopes. This ridge is due to a occur in places below it, and the grassy or wooded line of the slide and the line between the San the area. The disintegration of the whole mass great irregular injection of gabbro-diorite in Man- slopes above it may in some places be underlain Miguel and San Juan formations can be pretty is going on, under the active operation of a geo­ cos shale. Just under its summit on the south by shale. It is supposed that the shales beneath clearly made out within this space. Mr. Spencer logical agent of no mean importance in such a side a dense, dark igneous rock allied to campton- the San Miguel in Ground Hog Gulch belong to noted several outcrops of the San Miguel having region. ite penetrates the gabbro-diorite in an irregular the landslide mass, on account of their dip. strikes between N. 15° W. and N. 20° W., with Landslide at west base of Sheep Mountain. small stock or plug. Its vertical cliffs contrast The southern boundary of this mass as repre­ dips of 45° to 60° easterly. The map shows a small landslide area just below strongly with the gray mass of the Black Pace. sented on the map is possibly incorrect. The line South of the south branch of Turkey Creek no the cliff of the San Miguel formation ks mountain and form so prominent a fea­ may be in Leslie Gulch instead of in Poverty regular relation between San Juan and San Miguel on the west slope of Sheep Mountain. exp°sed- ture as seen from the south that the hitherto Gulch, as drawn. Observations of sufficient outcrops can be made out. Above the Currency In this mass Potosi rhyolite flows and tuffs, the nameless elevation has been called Black Face. detail to determine this point were not made. mine is a knoll of San Miguel conglomerate with San Juan tuff, etc., and some coarser agglomerate This mass of gabbro-diorite is plainly to be con­ Reviewing the evidence of this area, it seems irregular eastward dip, the reddish western face referred to the Intermediate series, have been dis­ sidered as an arm of the Yellow Mountain stock, that a great slide has taken place, and either at being visible miles away. This is the oval tinguished and represented upon the map in their but the contacts are seldom visible. At the north the time it occurred or subsequently the block mass represented upon the map. All around it is proper colors. The main part of this mass is end, above San Bernardo, the shales are seen in a has been much broken up, so that only the gen­ debris of the San Juan tuff. Potosi rhyolite, of two lamellar flows, with the ravine, and are much contorted and baked near eral relations of the various parts can now be The Currency mine (No. 30 on Economic sheet), usual flow-breccia below them. Under this rhyo­ the intrusive mass. The shales may also be seen made out. The base of the rhyolite series in a little below this exposure, has a shaft 200 feet lite comes some very much crushed San Juan tuff above the mass, dipping northwesterly from the

Ground Hog Gulch is 1000 feet vertically below deepA sunk through° San Juan and San The_. Currency_ in a thin band less than 200 feet thick at its summit of Black Face and easterly from the hill its normal place on the ridge above, but the base Miguel formations to the Mancos shales. mine< maximum. Bowlders and pebbles of quartzites south of the Lizard Head. On the southeast side of the San Miguel is only 200 or 300 feet lower By an unfortunate error on the Economic sheet and granite found at its base in a few spots indi­ of the latter hill the lower contact is also seen, than the level it occupies to the north or south of the Currency shaft has been placed north of the cate a disintegrated layer of the San Miguel con­ and on the northwest a thin branch in sheet form the landslide block. This discrepancy is partly boundary of the landslide. It belongs within glomerate. Across the southern end of the rhyo­ is present in the shales. At the end toward the accounted for by the increasing dip of the lower the landslide area, immediately below the knoll of lite flows runs a cross fracture, beyond which a Lizard Head the mass comes in contact with the part of the dislocated complex, but a larger factor the San Miguel conglomerate. This shaft was chaotic agglomerate of pyroxene-andesite appears, San Miguel conglomerate and sends off a narrow in this case is probably the grinding up and dis­ sunk on an ore-bearing vein which was traced seeming to belong to the Intermediate member of dike into it. At several places in this dark-gray appearance from the section of much of the San down to the Mancos shale, though much fractured the volcanic complex. The lower boundary of gabbro-diorite are coarse-grained veins or small Juan tuff, a result of the complex fracturing and dislocated. Drifts from the shaft and various this mass is obscure, in a heavy growth of spruce segregations rich in orthoclase and quartz, often which the slide block as a whole has undergone. surface workings have shown the broken-up char­ forest, but shale exposures at several points and intergrown in the form of graphic granite. Landslide area northwest of Silver Mountain. acter of the San Juan formation about this point the smooth character of the slopes below indicate The first trace of the San Miguel conglomerate The largest landslide area of the Telluride quad­ and have furnished evidence of slipping still in that the map is here nearly correct. west of Lake Fork is in San Bernardo Mountain, rangle, embracing about 10 square miles and progress. Thus tunnels running nearly parallel The laminated rhyolite flows dip toward Sheep which is capped by a remnant of that shown on the map, extends from Lake Fork to the general slope of the country in the vicinity Mountain, but not regularly, since cross fractures formation some 400 feet in thickness, and sunshine . , . -, mountains. near Ames nearly to the Telluride branch of the exhibit a crushing of the timbers, especially on plainly show the mass to be much broken up. On its base being about 500 leet higher San Miguel. Its upper limit is along the slopes the upper or mountain side. The Currency shaft the west face of Sheep Mountain above this slide than on the west slope of Yellow Mountain. of Silver Mountain, Bald Mountain, and Gold could not be kept vertical, the bottom moving is a depression drained by two shallow ravines, West of San Bernardo Mountain Wilson Creek has Hill, and its lower border is in general about down the slope and with a twist indicating some shown on the map, and one can not avoid the cut down 1200 feet into the soft Mancos shales. 1500 feet below, except at the southern end, undulation in the shale surface upon which the conclusion that the depression marks the place On its western side rises a high ridge, at the where the slide material comes quite down to the sliding mass rests. A short distance from the from which the slide block came. If the original northern end of which is Sunshine Mountain and stream of Lake Fork, more than 2500 feet below Currency shaft a prospect tunnel in broken-up mass included much of the San Juan, that mate­ at the southern end the singular monolith called the upper line as it crosses Turkey Creek. San Juan material was run into an older tunnel rial was so ground up by friction that little of it the Lizard Head. In this ridge the San Miguel The topography within this area is that most whose timbers were all crushed together, the now remains. formation has a variable development. Under the naturally characteristic of a surface made up of entrance to this old working having been entirely This block shows that at the time of its fall summit of Sunshine Mountain the formation has landslide blocks. In fig. 10 is illustrated the con­ obliterated by recent sliding. Mancos shale was the Potosi series extended out along the crest a thickness of nearly 1000 feet, while at the figuration of the southern part of the landslide also found here, much nearer the surface than in from San Miguel Peak to Sheep Mountain. It is southern base of the Lizard Head the San Juan mass as seen from the western side of Lake Fork, the Currency shaft. not now present at the extremity of Sheep Moun­ tuffs come down to less than 200 feet of the Man- looking toward Silver Mountain. There are a The attempts to find a continuous ore body in tain, and only small remnants may be still found cos shales. This seems explainable only as due great number of knolls, longitudinal ridges, or the Currency mine resulted in the discovery of in the points above 13,500 feet near San Migue*! to erosion, and indicates that the San Juan tuffs were not deposited at this point in such perfect benches, the majority of which have steep outer sufficient ore in small stringerso and dis- The_. Currency Peak. Much less erosion has been necessary to slopes, with trenches, or depressions, often contain­ connected masses to induce the erection ore body* remove this rock from Sheep Mountain than must continuity with the San Miguel beds as they ing a stagnant pool, back of them, on the moun­ of a steam hoisting plant and other mine machin­ have taken place on the Yellow Mountain-Pilot appear to have been elsewhere. tain side, and the drainage is extremely irregular. ery, but the total irregularity in dislocation and Knob ridge. A thin remnant of rhyolitic flow-breccia and While this area is large it exhibits relatively the cumulative evidence that the whole mass was Other landslide masses. Other landslides mixed tuffs of the Intermediate series occurs on few exposures in which the attitude of the bedded slide material caused the abandonment of further besides those above described have undoubtedly the ridge about the Lizard Head. The The Lizard latter is a column with nearly vertical Head> formations there J-present can be clearlyJ Observations.. exploration. There are some indications of taken place at many localities along the San Juan seen. On the upper limit, north of inthisarea- secondary deposition of ore on the shale contact, front, but they are either so small or have become walls on all sides, rising nearly 300 feet above Ophir Needles, a small slide block of San Juan but the thoroughly crushed condition of vein and so disintegrated in course of time that they can its platform. Its summit is inaccessible, and the tuff, etc., interrupts the San Miguel ledge between rock matter and the likelihood of some ore being not be represented on the map. One point where reason for its preservation is not evident. At the the levels 10,500 and 11,000 feet. In this block dragged into the zones of movement make posi­ small masses have been detached and have broken base it is a bedded mass of andesitic breccia, the tuffs strike somewhat west of north and dip tive statements on this point unwarrantable. into smaller blocks in their fall is on the unnamed which might belong to the Intermediate series, Telluride 11. and a Horizontal banding is visible far up on its lower contact in many places, especially on the THE CRETACEOUS PLATEAU AND ITS VALLEYS. within the Telluride quadrangle. Of these the walls, although a vertical fissuring renders this Barlow Creek side. San Miguel River is locally the more important, obscure in many places. It is possible that there Gray Head. In the northwestern part of the delations to the mountains. From the western and the deeper parts of its canyon for 10 miles is here a rounded or oval neck of massive rock, quadrangle are three intrusive masses of the same base of the San Juan Mountains in the Telluride above Sawpit are in many respects typical of like some of the basic dikes of the vicinity, which variety of diorite-porphyry that occurs in Flat quadrangle a gently undulating plateau extends thousands of miles of canyon valleys traversing has indurated the surrounding tuffs, so that the Top. One of these is the resistant rock of the due west for 120 miles to the brink of the Colo­ the arid plateau region to the westward. core is concealed by a shell of this character. mountain called Gray Head, the summit of wThich rado Canyon. This plateau is immediately under­ At Sawpit the San Miguel Canyon is 1/TOO feet The Mount Wilson group. On the northern is 3500 feet above the river at Sawpit. The lain by the Dakota sandstone over large areas, deep, from the plateau level to the stream bed, and but toward the mountains variable thicknesses of and southern sides of the Mount Wilson group general plane of intrusion of this mass is also the it is more than 1000 feet in depth-L at A. typical. . , the San Miguel formation occurs in its upper surface of the Dakota sandstone, although the Mancos shales locally remain upon the sand­ the mouth of Bilk Creek. The Dakota canyon- maximum ^known thicknessJ_T i 01j» abouti j_ beddedRemnants for- of on the southern side some shales come between stone. The western portion of the plateau, the sandstone is at once the rim rock of the canyon . mations. 1000 leet, with much thinner remnants the porphyry and the sandstone, as indicated Great Sage Plain, is interrupted in this east-and- and the floor of the plateau. It frequently pre­ of the San Juan tuffs above. The character of upon the map. Mancos shales rise to the summit west line only by the small laccolithic group of sents an almost vertical cliff for its entire thick­ the San Miguel may be especially well studied on of the mountain on the east, with a general east­ the Abajo or Blue Mountains, in Utah. The ness, varying from 100 to 250 feet, but the shaly the ridge south of the summit of Mount Wilson. ward dip of about 30°. They are soft and friable eastern portion, the Dolores Plateau, layers more commonly cause slight benches. The base is commonly very distinct in contrast to except in the immediate contact zone, a few feet would certainly have developed to abut piateauores Below the Dakota comes the series of alternating the Mancos shales, even where the latter are much in thickness, where they are hardened and some­ abruptlyi, xi against j. 4.-Uthe aban JuanT -eiront j. modw!ed- sandstones and shales of the McElmo formation, indurated. The San Juan tuffs are not preserved what bleached. The Gray Head mass extended were it not for the igneous intrusions of the San forming a corresponding succession of rock ledges in characteristic condition, being greatly decom­ but a short distance beyond the quad- A dissected Miguel Mountains. Of these, the western sum­ and debris-covered slopes. Many sandstone ledges posed and iron stained, but the microscope leaves rangle line on the north, being now cut laccolith- mits, Lone Cone, Dolores Peak, and some lower overhang the soft, friable shales beneath them. no doubt as to the nature of the tuffs. off by a steep curving gulch, which enters the elevations, are certainly in large part of lacco­ The La Plata sandstone is sure to form a nota­ The high peaks of the Mount Wilson group are San Miguel at Sawpit. The approximately dome- lithic origin, as described by W. H. Holmes, ble horizon along the canyon walls by reason of all within the irregular diorite-monzonite stock, shaped form which this laccolith must have pos­ analogous to the smaller masses, Flat Top, Gray its more massive sandstones and its usually dis­ which has been deeply scored by the head waters sessed is shown by the fact that it did not extend Head, and others of the Telluride quadrangle. tinct white color, contrasting strongly with the of several streams. The various peaks and far enough to the southwest, northwest, or north The Mount Wilson group, geographically belong­ red Dolores strata below. But the lower portion narrow, serrate connecting divides present the to reach the opposite side of the canyons, the rim ing to the San Miguel Mountains, has been shown of the La Plata is itself sometimes highly colored extremely precipitous and rugged forms charac­ rock of which is the Dakota sandstone, with the to be geologically an isolated outlier of the San in shades of orange or red; ordinarily, however, teristic of the dissected stocks. These summits Mancos shale resting on it. The maximum verti­ Juan volcanic area. All these elevations are different from those of the Dolores beds. The are the highest remaining in the Telluride quad­ cal thickness of the porphyry mass was more than locally of much prominence, but they are dwarfed thin bituminous limestone separating the massive rangle ; several exceed 14,000 feet in height above 2000 feet, for the present summit, upon which the by comparison with the broad expanse of the sandstones is prominent in many exposures about sea level, the point known as Mount Wilson hav­ shales have an easterly dip, rises to that elevation plateau from which they rise. Sawpit. Within the Dolores formation the red ing the altitude of 14,250 feet. above the horizontal Dakota sandstone of the can­ The level borders of the San Miguel and sandstones and conglomerates cause very rugged As these mountains stand isolated they present yon wall. The apex of the dome was somewhat Dolores canyons in the Telluride quadrangle are topography, the latter bands being commonly a particularly striking appearance in contrast to to the west of the summit. The rock presents distinctly the eastern limits of the Dolores Pla­ most prominent. In this canyon the principal the gentle slopes of Mancos shales an extremely rugged face, with many cliffs and teau. Between 1500 and 2000 feet above this ledge of Saurian conglomerate is less than 100 feet about them. Fig. 17 is intended to acutlr ol thT* gray crags on all sides where it is exposed. The plateau level is the old plain of post-Cretaceous below the La Plata sandstone and is almost con­ give some idea of the rugged grandeur Mancos shales are much crushed and contorted erosion, which upon subsidence became in this tinuously distinguishable for the entire length of of this alpine group as seen after one of the light about the blunt ending of the porphyry on the region the floor of the San Miguel lake. From the canyon. snowfalls of early autumn from the valley of the south, and are mainly concealed under large talus Ruffner Mountain to Sheep Mountain the slopes The narrow dikes of basalt or other dark igne­ East Dolores, at the eastern base of Flat Top. On slopes on the north. of the zone between these plains are mainly occu­ ous rocks which cut the canyon walls in the the left hand is a ridge of San Miguel and San Whipple and Hawn mountains. East of Gray pied by the Mancos shales, and the lower part vicinity of Sawpit and at the mouth of Dike features. Juan beds, the stratification being apparent Head occurs a larger body of the same type of of the mountain front thus presents the rounded Big Bear Creek are striking features. through the snow. The view also shows the porphyry. There are two distinct summits of this forms normal to the degraded slopes of such The one running from Newmire up to the plateau extremely jagged forms of Mount Wilson and of mass, named respectively Whipple and Hawn material. level is characterized by a regular vertical and Gladstone and Wilson peaks. mountains, after early explorers in this region, Fig. 18 presents a view of the Dolores Plateau horizontal jointing, parallel and at right angles to Another view of these summits is given in fig. while a large basin has been excavated between in its relation to mountain and canyon, as devel­ the walls. The magma of this dike has also 18, from the point of Dakota sandstone directly them, at the head of Willow Creek. Mancos oped in the San Miguel drainage area of the Tel­ hardened the shales and sandstones in contact north of San Miguel. In this view Wilson Peak shales are found on the summit of luride quadrangle. The view is looking southwest with it. The combined effect of these two con­ becomes most prominent. To the left of it are Whipple Mountain in almost horizontal in Mancos from the top of the Dakota sandstone ledge north ditions is to produce a cleft in the cliff face, due Magpie and Bilk basins. In the distance, on the position, andt extendi down thei northi shales. of San Miguel, at an elevation of about 10,200 to the removal of the joint blocks of the dike, the right hand, are the peaks of the Dolores group; face with a northerly dip, connecting with the feet, here less than 300 feet below the base of indurated contact zones of sedimentary rock on the left hand, Sunshine Mountain and the shales surrounding the mass, thus limiting, at a the San Miguel conglomerate. The scene is, standing out as walls, in places 30 or 40 feet high Lizard Head. Even at this distance of 10 miles short distance beyond the quadrangle line, the then, nearly equivalent to that which would fall and only 10 to 15 feet thick, themselves resem­ or more one can clearly distinguish the bedded two porphyry lobes represented on the map. under the eye of an observer looking down upon bling the more common outcrop of dikes in such San Miguel formation from the massive stock While it is plain that the contacts of the por­ the Dolores Plateau from the level of the post- strata. This dike is of plagioclase-basalt. The which cuts it, but much of this contrast has been phyry about the present mountain must be with Cretaceous plain of erosion. narrow dikes near Sawpit are visible for long dis­ lost in the reproduction. Mancos shales, the exact nature of the lower line The Dakota sandstone descends rapidly from tances as projecting ribs cutting the sandstones. was not found at any point, owing to the great the point of view to the general level of the table­ The more important southern tributaries of LACCOLITHIC MOUNTAINS. talus heaps at the base of the precipitous slopes, land,' as illustrated byj the map.L In the Description_ the San Miguel Big Bear and Bilk creeks and The remaining mountain masses of the Tellu­ and this talus extends so far out over the shale foreground is the alluvial floor of the S£liJJuthea" Lake Fork flow in their lower courses in can­ ride quadrangle show by their structure and the that the structure of the latter is not evident near San Miguel Valley below Telluride, and relations' yons repeating in miniature the features of the character of the igneous rocks in them that they the mass. This body of diorite-porphyry is along the edge of the plateau beyond it runs the larger gorge. About the forks of Big" Bear are not geologically connected with the San Juan regarded as a laccolith intruded into the soft Dakota, covered in most places here by glacial or Creek are several faults crossing the stream and Mountains in origin, but represent instead the lac- Mancos shales, its principal mass, at least, lying slide material, but exposed in a distinct ledge near the adjacent plateau. These are very noticeable colithic type of the plateau country to the west­ at some distance above the Dakota sandstone. Lawson's ranch, on the left hand. at many points on the walls where the Dakota ward. The shales on the summit of Whipple Mountain The level middleground, with its grazing land ledge is dislocated, but they are not sufficient in Flat Top. Near the southwestern corner of are more than 2000 feet above the base of the or cultivated fields, belongs to the table-land displacement to change materially the relation of the quadrangle is the mountain mass called Flat massive porphyry cliffs on the southern ridge. between Bilk Creek and Lake Fork or to the canyon to plateau. Top, caused by a thick body of gray Other intrusive masses. Between the two large area east of the latter, above Vance Junction, a Lake Fork of the San Miguel flows in a canyon Structure. diorite-porphyry. The porphyry rests porphyry laccoliths above described occurs a veritable bit of the Dolores Plateau, continuous decreasing in depth upstream as far as the junc­ on the Dakota sandstone, as may be seen at many smaller one of the same rock, exposed by the from this point down the San Miguel Valley. tion with Howard Fork. The western Lake Pork. points, and has a small remnant of baked and erosion of Summit Creek. This body lies on the The canyons of the streams cutting it are but side of this shallow canyon has the somewhat bleached Mancos shales on its summit. Dakota, with a wedge-like arm in shales at the east­ indistinctly outlined in the view by strips of usual rim of Dakota sandstone until the intrusive As shown by the map, the Flat Top porphyry is ern end of the outcrop. There can be no doubt aspens, or by the white lines of the Dakota ledge arm of diorite-monzonite shown in fig. 12 adds 1200 feet thick under the summit, but its thick­ as to the connection of these three laccoliths in on their farther rims. The undulating wooded nearly 1000 feet to the height of the wall at this ness decreases so rapidly to the northeast that an some manner, but the soft and crumpled shales ridges which rise on the left hand to Bald Moun­ point. On "the eastern side the glacial gravels apparent sheet, 100 feet or less thick, represents above them do not preserve the structure due to tain and Gold Hill, and in the distance to Sun­ extend from Turkey Creek southward until the it on the opposite side of the narrow East Dolores the intrusions with suificient clearness to indicate shine Mountain or Wilson Peak, are characteristic arm of the landslide mass below the Currency Valley, while to the west and northwest no trace where the connection takes place. It is assumed of the intermediate country lying between the base mine is encountered, which sweeps quite down to of it appears on the edge of the plateau. It is in section AB of the Structure Section sheet that of the San Miguel formation and the plateau level. the stream bed, replacing the normal canyon wall easy to understand how the overlying shales must it is north of the line of that section. Numerous In fig. 17 the southeastern slopes of Mount Wil­ by the irregular, uneven slope shown in part in have been domed up over this mass, and the body small tongues of dense porphyry occur Relatlon of son, of the same character, are well shown. fig. 10. seems from the map to be almost a typical lacco­ in the shales all about these masses, laccoliths- Between Bilk and Big Bear creeks is a promi­ The bed of Lake Fork rises rapidly across the lith. But the porphyry extends from Flat Top assuming sheet, dike, or irregular shapes, and a nent hill capped by a very much decomposed diorite-monzonite, and on its southern side, at San for 3 miles southeastward to Hermosa Peak, in few of them could be mapped. Near the Last igneous rock whose original character is much Bernardo, is in the Mancos shales. It is proba­ the Engineer Mountain quadrangle, occupying, Dollar claim, on the ridge northeast of Gray obscured. It has numerous small rounded cav­ ble that the Dakota is very near the surface at San so far as now known, this same horizon at the Head, and also farther north, are some dikes and ities containing very clear quartz crystals having Bernardo, and it can not be very deeply buried at base of the Mancos shales. The mass is, there­ sheets of diorite-porphyry in the Mancos shales, polished faces and sharp angles. Some of these Trout Lake or on the flat divide of hay lands fore, not symmetrical, but differs in no other which exhibit the porphyritic structure to per­ crystals have weathered out and can be picked about Lizard Head station. The true dips of the respect from the laccolith as originally defined fection by the strong contrast existing between up. The name Diamond Hill, locally in use for crumbling shale can not be definitely made out by Gilbert. the uniform gray groundmass and the plagioclase this hill, was doubtless suggested by these clear in this vicinity, but the position of the Dakota Several thin sheets of dense gray porphyry and hornblende crystals embedded in it. Several crystals. below can be inferred from the lenticular expo­ appear in the Mancos shales to the northeast of small sheets and dikes on Deep Creek are sup­ The San Miguel Valley. The Dolores Plateau sure of its upper strata about 2 miles southeast of Flat Top, which are considered as offshoots from posed to be offshoots from the mass of Hawn is traversed by several important streams, two of Trout Lake and from the level of its appearance the laccolith. Enormous talus slopes obscure the Mountain. which have their extreme eastern head waters on the Dolores drainage to the southwest. The Dolores Valley. The East Dolores River, a great overlap unconformity, demon trating ele­ must be pointed out that, while the San Miguel assume that the material of these beds was derived from Barlow Creek upward, flows in a valley of vation, erosion, and subsidence, separates the La formation is doubtless limited in extent, the Hay- from the San Juan Mountains, but they contain a . . . . den map represents the entire volcanic complex fossil flora similar to that of the Denver beds, and the same ogeneral character as that of the San Plata sandstone from the Dolores forma- An important Miguel. The Dakota forms the bounding ledge tion on the northern and southern slopes of the San Juan as resting upon a surface which if they are really to be correlated with the San between the plateau and canyon, and the Flat Top of the San Juan. Little evidence is in a broad way the continuation of that beneath Juan tuffs of the Telluride quadrangle, the San porphyry mass adds locally to the depth of the bearing upon this problem has been observed the San Miguel in the Telluride quadrangle. The Miguel formation must be older and would then canyon, as does the Gray Head mass at Sawpit to within the Telluride quadrangle. The varying quartzite and granite peaks of the Needle Moun­ correspond with the Arapahoe formation. The the San Miguel Canyon. thickness of Dolores strata occurring between the tains rise above this level, and on their north­ Animas beds now appear in seeming conformity Below Barlow Creek the influence of the dome La Plata sandstone and the uppermost Saurian western side is a known shore line for the San with the Laramie in the only places where they uplift of the Rico Mountains is felt, and the conglomerate, increasing from 30 feet in the San Miguel formation, but it is quite possible that have been seen in contact, and no possible equiva­ stream cuts rapidly into the Dolores formation. Miguel Valley to several hundred feet in the La these beds reappear beneath the andesitic tuffs lent of the San Miguel formation has been found Plata Mountains, may represent a gradual trans­ southeast of the Needle Mountains. In any case south of the outcrop shown in the Telluride quad­ GENERAL GEOLOGY. gression of the La Plata outward from a zone of it is demonstrated that the period of uplift and rangle. A. discussion of the general problems respecting more marked unconformity near the San Juan erosion before the deposition of the San Miguel The only hope for a solution of the problems the geological history of the region presented continent of Dolores time. But this transgression conglomerate was one of great importance. The involved lies in determining the age of the San by the facts observed in the Telluride quad­ is so gradual that the thickening might easily extent of its influence is at present unknown, Miguel or the San Juan formation, or in finding rangle. be due to original conditions of deposition of the though it must be assumed to have been far proof of the relation between the Animas beds Dolores sandstone. reaching. and the volcanic series of the San Juan Mountains, The Introduction to this text gave some idea of No satisfactory fossil evidence has been found That a general subsidence led to the formation which are petrographically so similar. The only the scope of the geologic problems presented in to prove the age of the Gunnison formation. But of the San Miguel lake is a most natural supposi­ region in which this can be reasonably looked for the San Juan region and of the scanty basis for it is clear from the lithologic character of the tion, but little evidence indicating the amount of lies to the east of the Animas River, near the their discussion afforded by our present knowl­ sandstones of both the La Plata and the McElmo that movement is as yet available. point where the San Juan River issues from the edge. The survey of the Telluride quadrangle division that the conditions of sedimentation dur­ mountains. has brought to light many lines of evidence ing those epochs were very similar to those pre­ PROBLEMS OF THE SAN MIGUEL FORMATION. The land mass adjacent to the San Miguel which, when followed out through adjoining dis­ vailing while the Dakota sandstones were being The San Miguel beds occupy a position of lake. The San Miguel conglomerate is of much great importance in relation both to the events tricts, will lead to a much better understanding laid down. Especially noteworthy in this con­ importance± for the evidence it bears as Rocks . of... the ' of the complicated geologic history of the San nection seems the presence of a peculiar fine­ which preceded and those which followed their to the character of the San Juan land land mass> Juan. Naturally the correct interpretation of grained conglomerate both in the McElmo and in deposition. Could the age of the San Miguel be mass during the epoch of its deposition. The much of this evidence must depend on future the Dakota formation. The small pebbles are closely determined a valuable contribution to our nature of the pebbles in its strata testifies to the investigation, but a brief discussion of the largely of chert or dense siliceous rocks, white, knowledge of the orographic history of the Rocky proximity of a land area made up of quartzites observed facts will be given, in order that the dark gray, pink, or red in color, and many of Mountains would be made, and at the same time and schists, granite, and gneiss, like the Needle reader may obtain a better idea of the geologic them appear to represent silicified limestones. the epoch in which the volcanic eruptions of the Mountains of to-day, and probably these moun­ development of this particular area and also a The origin of these materials is unknown, but San Juan began would be fixed. But no fossils tains are a remnant of that land mass. Strata of conception of the broader significance of the local they speak for similar land conditions in the two have been discovered in the San Miguel beds, Paleozoic and lower Mesozoic age, upturned geology, as it is at present understood. epochs of sedimentation. Yet, if the McElmo be although careful search was made in the fine­ around the mass of older rocks, must have fur­ of Jurassic age, the long period' included in the grained strata at many points. Only some car­ nished the red sandstones, the limestones, and THE PRE-DEVONIAN CONTINENT. "Lower" Cretaceous must have separated them. bonized plant stems and trail-like markings in some of the quartzites occurring in the San The fundamental element in the geologic struc­ There is no known reason to suppose that the full mud layers were seen. Miguel. The decreasing amount of these mate­ ture of the San Juan Mountains concerning which " Upper " Cretaceous section was not deposited in Age of the formation. The only direct evidence rials in the upper beds is testimony of the wear­ we have any considerable knowledge at the pres­ the Telluride and adjacent areas in a similar devel­ as to the age of the San Miguel formation now ing away of these formations. There must have ent time is the continental mass, composed largely opment to that now seen on the Animas River known comes from its position upon the plain of been some large porphyry body exposed in the of Algonkian quartzites, with granites, gneisses, near Durango. In that section the total thickness erosion above discussed and beneath the volcanic land area, as there are many pebbles scattered and schists, some of which are probably Archean, of the " Upper " Cretaceous, from the Dakota to complex of the San Juan. According to R. C. throughout the conglomerate layers. This rock against and around which sedimentary formations the Laramie, inclusive, is about 6200 feet. Hills, the later eruptions of the main San Juan is unlike any of the dioritic or monzonitic por­ were deposited during many periods of Paleozoic The San Juan shore line during the " Upper " series were " involved in the disturbances which phyries forming the intrusive sheets or laccoliths and Mesozoic times. This land mass has doubt­ Cretaceous may have been far east of that which produced the San Luis Park depression and the in the sedimentary series of this region, and it is less varied in size in different periods, but facts existed during the Dolores period. Indeed, the final upheaval of the Sangre de Cristo Range." probably of much greater age. have not yet been discovered to prove that it has sediments of the former may have entirely covered This uplift is called by Hills the " post-Bridger, ever been entirely submerged beneath the sea the area of the present San Juan Mountains, so far for the reason that it terminated the period of THE VOLCANIC SERIES. level. Yet it is also true that the shore lines of as any definite evidence now known indicates. Eocene sedimentation in the Huerfano Basin, The bedded volcanic rocks of the Telluride the various surrounding formations have not been where the uppermost beds are paleontologically quadrangle bear witness to the character and POST-CRETACEOUS UPLIFT AND EROSION. determined with accuracy. referable to this age " (Orographic and structural magnitude of the San Juan volcanic outbursts The only evidence afforded by the Telluride Nature and extent of uplift. The evidence features of Rocky Mountain geology : Proc. Colo. during a considerable portion, at least, of the quadrangle concerning this old continent mass is which has been found in the Telluride quadran­ Sci. Soc., Vol. Ill, pp. 407, 408). In the address period of activity. Until much more extended gle as to the great uplift which brought examinations have been made even the outline of that of the isolated exposure of quartz- « just cited (p.VJr 407)/ Hills remarks _Post=Laramie , , ite in Canyon Creek north of Stony to an end the long period of continuous porfanw "oi that the San Miguel conglomerate, orEocene?- this volcanic history can not be sketched, but it is Mountain. This proves that the Algon­ Cretaceous sedimentation, and the enor­ which he had observed near Telluride, " belongs, of interest to consider the evidence presented by kian quartzites are present in this vicinity, and mous erosion which followed that uplift, is so presumably, to post-Laramie or early Eocene the formations that have been described. they are doubtless continuous, beneath the plainly shown by the geological map and has been times ; " and in the original description of the for­ The San Juan formation. The first evidence younger formations, with the quartzites of the so frequently referred to in the details of local mation the present writer called attention to the of the San Juan volcano thus far discovered is lower part of Canyon Creek, near Ouray, where geology that little more need be said in this place. analogy in composition and general relations contained in the tuffs, agglomerates, and breccias they are overlain by Devonian limestone. Since The upturning of the Mesozoic formations is seen between the San Miguel and the Arapahoe for­ of the San Juan formation. From their character­

the oldest strata found resting upon the Algon­ about Telluride and Ophir, in the South Fork of mation of the Denver Basin. istics,' which ^ have alreadyJ been given,o ' A. f ragmental'

kian quartzites of the San Juan are probably of Mineral Creek,' and in Lime Creek. In the adia-V Correlation. To suggest a correlation of the these materials are thought to be toexp'fosl've Devonian age, it must be inferred that a land mass cent Silverton quadrangle the Carboniferous and San Miguel and Arapahoe formations is to suggest the product of explosive eruptions at eruptlons- Devonian strata are also upturned. While several one or more centers situated some little distance existed here during Silurian and Cambrian times. further that the San Juan tuffs of the Relation of earlier periods of orographic movement are sup­ Telluride quadrangle correspond in age, a east of the Telluride area. It is supposed that MESOZOIC HISTORY. posed to have taken place in this region, and there as they do in composition, with the formatlon- these materials received their stratified arrange­ No Paleozoic formations are exposed within have certainly been later uplifts, it seems clear Denver beds, and thus is raised the whole broad ment in part in the waters of the San Miguel the Telluride quadrangle, but it must be assumed that the present attitude of all the Paleozoic and question as to the relations between the physical lake, but that, as they overlapped the shores of that they are present beneath the Dolores forma­ Mesozoic formations in the western part of the history of the Rocky Mountain region and the that body of water, a similar structure was pro­ tion, since they appear within a few miles to the San Juan Mountains is mainly due to the great changes that took place in animal and plant life duced by subaerial agencies alone. Two thou­ southwest about Rico, to the southeast in Lime post-Cretaceous uplift. during the time interval between the period of sand feet of this stratified formation is preserved and Mineral creeks, and to the northeast in Can­ The broad structure resulting from this uplift the uppermost conformable Cretaceous formation, in the Telluride quadrangle, and this amount was yon Creek. was a general dip north, west, and south, away the Laramie, and the lowest recognized Eocene doubtless exceeded in proximity to the vents. The Dolores epoch. The Dolores formation of from the western center of the San Juan moun­ formation, the Puerco. The Arapahoe and Den­ Mr. Hills speaks of this agglomerate, etc., as an this quadrangle is coarser grained than in the tain area. This is apparent from the structure of ver beds contain a rich fossil flora and a fauna " outflow of breccia," implying a molten matrix or Rico, La Plata, or Durango quadrangles. Its char­ the Telluride quadrangle, but it also appears that "of wonderful vertebrate forms, the evidence of some unusual form of eruption from the depths. acteristic limestone conglomerates prove there were subordinate folds or wrinkles in this which has inclined many paleontologists to group No evidence of a molten base holding the frag­ that in the adjacent land areas older Sarthi"6 area, the extent of which can not now be fully these formations with the Laramie Cretaceous in ments has been observed in the beds here called limestones were exposed, and the greater made out. spite of the great orographic movement which the San Juan tuffs, etc., though in the Intermediate number of conglomerate layers and the large size Erosion following uplift. The measure of the preceded them. For a full presentation of this series a rhyolitic magma has locally caught up of the pebbles in the easternmost section examined, erosion following the great uplift is found in the question the reader is referred to the discus­ great numbers of andesitic and rhyolitic frag­ that of Cataract Creek, seems to indicate approach unconformity at the base of the San Miguel con­ sion, by the writer, of the age of the Arapahoe ments. to the shore line in that direction. The conglom­ glomerate, for the region occupied by that forma­ and Denver formations, in the Survey mono­ The San Juan formation contains andesite of erates frequently contain a large amount of quartz- tion. On the supposition, scarcely questionable, graph upon the geology of the Denver Basin several varieties and of many textural phases, and ite and greenish schists derived from the Algon­ that the entire "Upper" Cretaceous section was (Monographs U. S. Geological Survey, Vol. XXVII, the most natural supposition as to its kian complex, together with granite and gneiss. present in this area at the time of uplift, about Geology of the Denver Basin in Colorado, 1896, origin appears to be that it is the result Snier Irup- All these facts lead to the conclusion that during 7000 feet of strata were removed above the spot pp. 206-252). oi£ explosive1 outbursts^ x oi£ greatx violence, 1 tions' the Dolores period a land mass existed not far where Ingram and Bridal Veil creeks now unite. The necessity for considering this correlation by which a volcanic cone, or perhaps several cones, east of the Telluride quadrangle, composed of A glance at the map shows, moreover, that the in spite of the lack of known fossils in the San formed in the earliest stages of the eruptions in essentially the same formations now found in the surface upon which the San Miguel PIainon Miguel or San Juan formations arises from the the San Juan region, were largely destroyed and Needle Mountains. was deposited was a gently undulating presence of the andesitic tuffs known as the lavas in fragments of various sizes were showered delations between the Dolores and La Plata plain, and although elaborate discussion Animas formation between the Laramie and upon the surrounding country for many miles formations. it has already been explained that of the subject at this time would be premature, it the Puerco, near Durango. It is but natural to around. This occurrence would be of the same Telluride 13. order as the explosions which in recent years have the stock eruptions is based upon the relation of types of eruption "through the investigations of tion of the strata. In the central portion of the been witnessed at Krakatoa, in the Straits of sheets and stocks of similar rocks in the Rico and other portions of the San Juan. But the form quadrangle a northeasterly dip of a very few Sunda, and at the volcano of Bandaisan, in Japan. La Plata mountains. In both of these districts which the problem takes as presented in this degrees prevails, with many local irregularities There is great similarity in composition and in diorite- and rnonzonite-porphyries are cut by dio- quadrangle will be briefly stated. due to the large igneous intrusions. In the south­ structure between the bedded series under dis­ rite stocks. In the Telluride quadrangle no evi­ The question may be expressed in general terms eastern corner of the quadrangle the base of the cussion and certain members of the volcanic com­ dence bearing upon this point was observed. as follows: Were the magmas of the intrusive San Miguel is 2000 feet higher than at Ingram plex of the Yellowstone National Park, but in the laccoliths, the stocks, and the effusive flows of Creek, and the strike is nearly east and west. latter case evidence of slow accumulation is found STOCK ERUPTIONS. the Telluride quadrangle derived from the same Local undulations, such as those in the Twin Sis­ in the succession of fossil forests destroyed and The large stocks of the Telluride district pre­ subterranean source and impelled to or toward ters and in the ridge south of Rolling Mountain, buried by the andesitic tuffs and breccias. sent a number of interesting problems. The form the surface by volcanic forces of the same origin, are found here and there. The Intermediate series. The alternating series of the masses and their clearly exposed or were they derived from different levels and This general northeasterly dip of the San e '*° of andesitic and rhyolitic lavas included in this relations to the sedimentary formations otehtr'erup impelled by forces differing somewhat in kind? Miguel and overlying volcanic formations has, no group serves to show that at a certain time a and the bedded volcanic series show The laccoliths are similar to thousands of por­ doubt, had an important and perhaps predominant change took place in the character of the volcanic that the stocks now seen represent the filling of phyry masses scattered through Colorado and the influence in determining the broad features of the products. Lavas of rhyolitic composition were immense conduits which penetrated to the highest adjacent plateau country of Utah, Arizona, and dissection of the San Juan volcanic plateau by erupted for the first time, but alternated with levels now existing in this part of the San Juan, New Mexico, in regions where no evidence now erosion, at least in its western portion. andesitic outpourings in a manner to suggest that and which, it must be assumed, ascended to the remaining suggests that surface volcanic eruptions Faults andfissure systems. The faults observed different vents must have been emitting different surface of the time of eruption. If they did ever took place. The stocks of this region are to cut the volcanic series are of small dislocation, materials at the same time. extend to the surface, lavas must have issued from conduits, so near the old surface that they seem trend in various directions,' and display-t J Many frac> The Intermediate series has a much more varia­ them, and they are thus volcanic channels, but no to represent channels through which lavas were no system. Some of them are ore-bear- JutreSSliz- ble development than the San Juan, as now seen. evidence has been found indicating that they are quietly poured out. But exactly similar stocks ing veins, and possibly some of the large able system* It is thickest near Ophir Pass, and extends as far the throats of explosive volcanoes, such as those are present in the Kico, La Plata, and Elk moun­ veins upon which there is no notable displacement west as the Lizard Head. from which the great fragmental masses of the tains, and probably in other mountain groups of where exposed may belong to the same period of The Potosi rhyolite series. The uppermost San Juan were derived. the Great Plateau. It thus becomes questionable fissuring with certain of the faults. member of the volcanic complex in this quadran­ The rocks of these stocks are mainly granular whether the association of these intrusive masses In the description of the economic features (p. gle is composed almost wholly of rhyolite, and in structure and are often rather coarse grained, with the surface volcanics of the San Juan is 15), by Mr. Purington, will be found statements indicates an important epoch, in which andesitic as in the case of the Stony Mountain gabbro. It evidence that they are products of eruptive activ­ as to the several systems of joints or fissures eruptions had practically ceased. There is no has been a belief among petrologists that such ity about one great center, or whether they are which are locally prominent in the Telluride means of knowing how fully the flows structures could result from the consolidation really independent of the typical volcanic mani­ quadrangle, and in his full report Mr. Purington and tuffs seen in the Telluride quad- of igneous magmas only at great depths depths festations of the San Juan. dwells at some length upon the origin of these rangle represent the whole series of epoch* measured by at least several thousand feet. To TERTIARY AND RECENT OROGKRAPHIC fractures. As the district studied is but a small MOVEMENTS. eruptions in this epoch, but it must be assumed assume that the rocks now seen in the Importance, portion of the San Juan region, no conclusive that there were still higher flows or tuff layers, summits of Mount Wilson, Mount Snef- The general problem. The area of the San statement can as yet be made regarding the rela­ now completely removed from the highest peaks fels, and other high peaks were consoli­ Juan Mountains seems to have been a land mass tion of these fissure systems to broad movements of the region. dated at even 3000 or 4000 feet below the surface from the time that the San Miguel<-> lake Orographic_ in the San Juan, nor can the fissures be classified As with the lower members of the local is to add to the volcanic complex of the San Juan was filled up by the great series of vol- J^fafiy^J- in sets of contemporaneous origin. That some of sequence, the Potosi flows and tuffs are so nearly an enormous amount of material. But unless it canic rocks. As no sedimentary beds termme these systems of fissures have been of much local horizontal and cover so much space that no infer­ is supposed that these granular rocks were formed exist by which the earth movements of Tertiary importance in determining the course of erosion ence of value can be drawn as to their source or very near the old surface they must be considered time can be differentiated within the mountain can not be questioned. extent in other parts of the San Juan. Dikes of as giving clear evidence of a former extent of the district, a detailed study of the whole region is glassy rhyolite are said to occur in Potosi Peak, volcanic pile of the San Juan, beside which its necessary before any definite statements can be DENUDATION OF THE SAN JUAN PLATEAU. but that these fissures were the channels of erup­ present dimensions become insignificant; and made as to the various movements of uplift or Character of the plateau. From present knowl­ tion for the thick flows of the same mountain whether the visible monzonites, diorites, or gab- subsidence by which the San Juan district has edge it appears that at the period of maximum does not seem possible. bros formed at 1000 or 10,000 feet below the been affected. That the area is now one of the development the accumulations from Dike eruptions. The three bedded members of surface, it is plain that the typical porphyries of most elevated in the United States, and that a the volcanic centers of the San Juan te°nt1oTseaxn the volcanic succession are cut by narrow dikes of the region, including the Kico and La Plata vast amount of denudation has already been must have formed a great plateau, pyroxene-andesite, especially in the northeastern mountains, consolidated at the same or greater accomplished, are the two great facts which testify much greater in extent than the area now covered portion of the quadrangle. These dikes cut every depths; so that the belief, finding most positive to the magnitude of the combined disturbances by the lavas and tuffs. The Telluride quadrangle, other rock in their paths, but there expression in the German school of petrography, since the beginning of the volcanic eruptions. now situated on the extreme western border of the is no certain evidence that the andes- ingtoaeiateg~ that great depth is essential to the formation of The San Juan area has, moreover, been uplifted volcanic complex, must once have been far within itic dikes belong to the same period of granular structure in large igneous masses, while as a great continental mass, and not by axial folds its limits, and we have here the evidence of enor­ eruption as the basic dikes associated with the the porphyritic structure belongs to higher zones like those of some of the Kocky Mountain ranges mous erosion, by which the abrupt western front diorite-monzonite stock of the Mount Wilson in the earth's crust, is clearly contrary to the facts of Colorado. of the San Juan Mountains has been carved out group. From the uniform character of the andes- here revealed. The Hayden map shows a lake-bed deposit in of the old plateau. Degradation of the volcanic ite dikes in the eastern portion of the quadrangle The petrographic complexity of some of these the valley of the Kio Grande above Wagon Wheel pile by ordinary agencies of subaerial erosion has it seems probable that they belong to some late stocks is worthy of much more detailed examina­ Gap, bearing the symbol of the Green Kiver doubtless been in progress since early Eocene epoch of activity in the cycle of the San Juan tion than could be given it in this work. The Eocene, but the reports give no information as to time, the work of many of the earlier intervals in volcano not otherwise represented in this area. one exhibiting the greatest variation is that of the the grounds for assigning the beds to that age. On the volcanic history being undone by succeeding Period of waning volcanic energy. "When the Ophir Needles, while a considerable diversity of the southern flanks of the San Juan the Puerco eruptions. entire San Juan region has been studied many rock varieties was also noted in Mount Sneffels and Wasatch divisions of the Eocene are present The bedded series of tuffs, agglomerates, and phenomena will undoubtedly be observed which and in Mount Wilson. Most of the changes in in the drainage area of the San Juan Kiver, and lava flows, still preserved in the higher ridges must be explained as belonging to the later composition appear gradual and are not explain­ they are to some extent upturned nearest the and peaks of the Telluride quadrangle, proves phases of activity commonly noted in volcanic dis­ able as due to distinct eruptions of different mountains. Since they do not come in contact beyond a doubt that a thickness of 3000 or 4000 tricts, such as local eruptions of peculiar lavas, magmas. In some cases sharp contacts were with the volcanic formations of the San Juan the feet of these rocks once covered the entire quad­ solfataric and fumarole action, hot springs, etc. found. No regular relation between the variation influence upon the latter of the movement affect­ rangle above the level of the San Miguel con­ The Telluride quadrangle, however, seems suffi­ and the form of the stock was observed. Some ing these Eocene deposits can not be determined. glomerate. The evidence of the large stocks ciently removed from the real centers of volcanic stocks, as that of Grizzly Peak and Rolling The discussions of orographic movements in indicates that a considerable further thickness of action to make it a matter of some doubt as to Mountain, are nearly homogeneous in mineral the Kocky Mountains by S. F. Emmons (Bulletin similar volcanic materials has been entirely how far the agencies which have produced composition throughout. No fragmental material Geological Society of America, Vol. I, 1890, pp. removed, so that it seems to the writer quite changes in the rocks, or have deposited secondary like agglomerate occurs in these stocks, and they 245-286) and K. C. Hills, (Proceedings Colorado within the bounds of reason to assume that there minerals in them, may be considered truly volcanic are not centers from which dikes radiate. These Scientific Society, Vol. Ill, 1890, pp. 362-458) may once have been 6000 or 7000 feet of volcanic agencies. Decomposition of the fragmental and facts do not allow of the supposition that present valuable generalizations as to the succes­ rocks present in this region. How far the vol­ of some of the massive rocks has taken place, repeated eruptions of different magmas took sion of movements recognized at various places, canics extended to the westward is totally but this would be a natural result of the percola­ place in any one of these conduits to a degree but in their application to the San Juan mountain unknown, but it must have been many miles, tion of surface waters. which can permit the idea that they are possibly region proper the conclusions are necessarily judging from the thickness still preserved in this throats of typical volcanoes. largely speculative. The unconformity at the quadrangle. LACCOLITHIC INTRUSIONS. base of the San Miguel shows that much of the The cause of denudation. The work of erosion RELATIONS BETWEEN THREE TYPES OF upturning of the Mesozoic beds adjacent to the on this, the western, side of the San Juan has The laccoliths of the Telluride quadrangle, ERUPTION. together with the small sheets and dikes near mountains, which, as seen on the Animas Kiver, resulted in the almost complete-L removal Erosion_ . on them and the granite-porphyry mass of Relation of Three very different phases of eruptive activity might be considered as the same movement that of the entire bedded series back to the ffif com- Howard Fork, are thought to have been l^SSSc seem to be illustrated in the Telluride quadrangle. affects the Puerco, is really in large degree post- abrupt front which has been described. pared' intruded after the surface eruptions of The products of eruption do not vary problemsasto Cretaceous, unless the San Miguel and the vol­ But erosion has not stopped with removal of the the bedded volcanics, but before the stock intru­ essentially in chemical composition; at teneous rocks- canic series are all of much later date than is now volcanics. The sedimentary rocks have been sions. The granite-porphyry mass penetrating least some of the stock diorites, laccolithic por­ believed. denuded over large areas down to a new plateau the San Juan tuffs affords direct evidence of this phyries, and surface andesites correspond closely Tilting of the volcanic complex. In the Tel­ level that of the Dakota sandstone and deep fact, and the presence of Mancos shales on the in chemical composition. But if all are derived luride quadrangle the two definite structural canyons have been carved in the harder rocks summit of Whipple Mountain seems to give infer­ from the same source of molten material, as might phenomena referable to Tertiary move- below it. In view of the eastward tilting of the ential testimony in the same direction. It does be inferred from this chemical composition, it ments are the tilting of the whole onemovement* definite region, it is probable that the San Miguel River not seem probable that these shales would have appears that the eruptions took place under dif­ bedded volcanic series, with the San at Sawpit, only 16 miles from its present head, been preserved at this point, 600 or 700 feet above ferent physical conditions ; that the forces impel­ Miguel at their base, and the faulting by which has cut at least 7000 feet, and possibly more than the plain of erosion upon which the San Miguel ling the eruptions were of different kinds, or of all formations seem to be affected. In the main 10,000 feet, below the surface which existed at beds rest in Kuffner Mountain, if the planation of greatly varying intensity if of the same kind. It valley of the San Miguel the conglomerate of that the close of volcanic activity. The penetration the region took place after the intrusion of the is not intended to discuss this problem at length name descends more than 1000 feet in the distance of the river to-day is only 8 miles beyond the diorite-porphyry. in this place, for it is hoped that light may be of 8 miles between Iron Mountain and Ingram mountain front. The Dolores Kiver has also The inference that the laccoliths are older than shed upon the connection between these several Creek. This is due to a gentle eastward inclina­ accomplished a great task of removal. On the northern side the various branches of crevassed neve ice, of a bluish color, which has retreat of the ice front the great delta-fan Other veins in the district have been developed the Uncompahgre and on the southern side those been already mentioned as occurring on the north remained as an obstruction to the drainage and by similar methods. It is often found impracti­ of the San Juan have pushed back the mountain slope of the high ridge east of Dallas Peak. This served as a dam, back of which the materials of cable to work the veins in any other way, since front and cut deeply into the sedimentary forma­ and similar masses of neve ice, reported by R. C. the present flat continued to accumulate. At the great altitude at which many of them outcrop tions. These streams have all worked under Hills as present at the head of Henson Creek, in present the river is engaged in the removal of the precludes the sinking of shafts at the outcrop. similar conditions and have produced similar the Silverton quadrangle, suggest that only a obstruction, and will doubtless eventually attack Snowslides, especially in the steep gulches, are of results, which contrast strongly with those slight change in climatic conditions has taken the alluvial deposits which have resulted from, common occurrence in winter, and the danger achieved by the Rio Grande, penetrating the place since the high basins of the region were the dam. from these, as well as the expense of transporting volcanic area from the east. From San Luis filled with ice from one year's end to another. Should any attempt be made to win the gold machinery and supplies to the high basins, makes Park to the head of the Rio Grande, in the Silver- The glacial debris to the north of the Wilson which doubtless exists in the great alluvial deposit it, in most cases, advisable to have the surface ton quadrangle, is 65 miles, and its fall in this group has not been represented upon the map. it will probably be found that the precious metal openings of the mines at as low levels as possible. distance is the same as that accomplished by the It consists of angular fragments of GIacial is distributed throughout its whole thickness, since The ore deposits of the quadrangle occur mainly San Miguel in 16 miles. varying size, evidently derived from d6bris- the sorting power of the streams which brought in veins which are filled fissures. The discus­ While the configuration of the San Juan Plateau the formations of the adjacent mountains. This the material into the supposed lake was rendered sion following will therefore relate, first, to the at the close of volcanic activity and the positions material seldom takes any of the familiar forms null as soon as their velocity was checked by the observed fissure systems, then to the veins, and of the rivers, which at once began their attack of glacial deposits, though some small retreatal standing water of the lake. last to the ore bodies found in the veins. upon it, are as yet matters of - speculation, it is moraines were noted, but is found in remnants WHITMAN CROSS, THE FISSURES. probable that two known factors have had great scattered over the ridges and hill tops in the area Geologist. influence in determining the course of the denuda­ which was covered by the ice. June, 1899. The word " fissure" should be distinguished tion of the area. The first of these is the climatic The surficial rocks on the east side of the can­ from the word "vein." Fissure means "crack," condition by which the western portion of the yon of Lake Fork, which have been mapped as with or without an appreciable amount of open San Juan has probably long received, as it now glacial bowlder deposits, have been somewhat of ECONOMIC GEOLOGY. space. " Vein," in a strict sense, means a fissure receives, a very heavy rainfall from the moisture- an enigma. Considerable areas of Mancos shale, DISCOVERT AND DEVELOPMENT OF THE which has been filled with ore. The word "fis­ laden currents coming from the southwest over and in places the Dakota sandstone, have been DISTRICT. sure " is often wrongly used in the sense of " vein." the arid lower country. The second factor of buried beneath an irregular thickness of the Whether or not the early Spanish explorers of Fissure systems. Many well-defined systems importance is the eastward tilting indicated in bowlder beds, and large amounts of similarly southwestern Colorado actually passed through of fissures occur within the area, single members the Telluride quadrangle, which, if long continued constituted debris are found upon the ridge west this district is uncertain. At any rate no traces of which are found at points widely distant from at a rate not too rapid for the cutting power of of the canyon and in the San Miguel Valley remain to indicate that exploration for one another. These systems are subject to classi­ the western streams, would stimulate their attack at the Keystone placer. The materials of the the precious metals was prosecuted mine'ioca- fication, and may be generally divided into five upon the retreating mountain front. If this rela­ deposit are various; blocks of San Miguel con­ before the middle of the present cen­ groups, having the direction indicated below. tive movement consisted in a subsidence of the glomerate and of volcanic breccia are mixed with tury, and the first active search for gold and sil­ N. 87° W. This system is especially well eastern area its effect in retarding the erosion of pieces of sandstone, apparently from the Dakota, ver in the Telluride district was in 1875. In that developed in the Mount Wilson area and to the streams on that side is evident. and with different varieties of igneous rocks, year locations were made on the vein now called eastward, on Sunshine and Yellow mountains. The degradation of the western San Juan area including two types of granular rock. Many of the Smuggler. At that time, and in follow­ Representatives of it occur more rarely on Silver is progressing to-day under the combined attack the bowlders are rounded, but others are angular ing years down to 1882, many locations were Mountain and to the northward. of several agencies. The most impor- A encies and subangular. The size of the fragments varies made and small amounts of the precious metals N. 38° E. This system has attained marked tant of these is the diurnal change in fhVpreslnt from coarse sand to bowlders several feet in were produced. Although the available data development in the Mount Wilson area and has temperature during a large part of the ime' diameter. concerning the early developments are of the most an especially remarkable manifestation year. The continued action of freezing and thaw­ The glacial deposits have not imposed any dis­ fragmentary character, it is probable that the on the divide between Navajo Basin ing in finely jointed rocks saturated with moisture tinctive topography upon the area in which they total product of the quadrangle previous to the and Bear Creek, just above the Special produces the enormous talus slopes represented have been observed; they lie irregularly upon a year 1882 did not exceed $50,000. Session mine. Crossing the long axis of the ridge in several illustrations of this folio. During the shale slope, and south of Turkey Creek they are The region did not attain importance as a source at about right angles, the system shows a winter months snowslides are frequent, and they considerably obscured by landslide material, of the precious metals until 1890, when the Rio steep dip to the west up io a point a few hun­ often carry rock masses with them, but the spring which seems to have come down over the bowlder Grande Southern Railroad was completed from dred feet west of and above the Special Session torrents from the melting of the snow are proba­ beds. In the gorge of Main Fork there is a kind Ridgway to the town of Telluride. It has since mine. From here east the fissures have a steep bly much more efficient agents of destruction. of coarse stratification of the materials, so that been extended south to Durango. Previous to easterly dip, so that a sort of broken anticlinal In the rainy season local "cloud-bursts" often layers of cross-bedded sand are found alternating the opening of railway communication, nearly all effect is given, the sides being very steep. As carry enormous amounts of debris far down the with bands carrying pebbles and bowlders. ore was transported on the backs of pack animals the fissures are not more than 2 feet from one slopes. During a period not far distant land­ The origin of the alluvial deposits in the flat to distant smelters, and in the same manner the another, the fissuring has the appearance of an slides of unusual magnitude occurred in this above the Keystone placer is thought to be machinery for a few mines was brought into the immense schistosity, or of a series of great lam­ region, and this agency is still active, both in intimately > connected with that of the district, at great expense. As a rule, however, inae of slate. breaking up the old slide masses and in detach­ placer itself and the bowlder beds only ore of very high grade could be selected for N. 53°-63° E. The fissures of this system, also ing fresh masses from, the cliffs. Glacial erosion which have been described. The mate- placer- shipping, and the product was necessarily small. especially prominent in the Mount Wilson area, of a time not long past is indicated in many of rials of the bowlder beds and of the alluvial beds During the last few years, development has dip steeply north, or stand vertical. Some of the the high amphitheaters, but the evidences of this were, however, derived from different sources. been rapid. Many mills have been erected, and subordinate ridges of the mountains, the develop­ action have been destroyed by more recent The former came from the drainage of Lake and in most cases only the concentrated Recent ment of whose topography seems in a measure agencies on most of the lower slopes. The Howard forks and from the basins adjacent to the sulphurets, instead of the crude ore, Pr°sress- dependent on the fissuring, also follow this direc­ streams are still cutting their channels in the can­ Gold King Basin, while the latter were derived are shipped for smelting. Besides the means of tion. The northeast systems of fissures are not yons and removing the fine detritus from their from the regions on either side of Main Fork. transportation afforded by the railroad, connec­ largely developed, although occasional representa­ head waters. The truth of the first assertion is made certain by tion between the valleys and many of the mines tives of both the above-mentioned systems may the occurrence of the diorite and granite-porphyry situated at high altitudes has been effected by be seen as far north as the Virginius Basin. GLACIAL PHENOMENA. bowlders in the Keystone placer. There is no wire-rope bucket tramways, operated by gravity. K 17° W.-N. 2° E. Fissures whose directions In common with all other high mountain dis­ available source for them except in the area indi­ Good trails, and in many cases excellent wagon fall within these limits are very common in the tricts of Colorado, the San Juan Mountains pre­ cated. Concerning the second statement more roads, connect the mines with the railroad. The northern portion of the quadrangle, especially on sent indisputable evidence of glacial action in the will be said further on. electric transmission of power generated by water Silver Mountain and its northern spurs. A prom­ more elevated portions, and extending down the The conditions which gave rise to these for­ has attained important development. Stamp mills inent set of joints following the north-and-south principal valleys for varying distances. Within mations seem to have been as follows: At the are operated by electric power at distances of direction is developed at Sawpit, in the very the Telluride quadrangle there is considerable time the accumulation of ice upon the high more than 10 miles, in a straight line, from the northwestern corner of the quadrangle. evidence of the former existence of glacial ice. ground was greatest there was a suificient mass generating station, near Ophir Loop, and at ele­ N. 25°-51° W., dip almost without exception Many of the amphitheaters aboVe timber line are in the deep gorge of Howard Fork to furnish a vations of 2000 or 3000 feet above the source of southwest. There are within this group four sets glacial cirques and contain small lake basins, stream of ice which filled the canyon of Lake the power. of fissures, namely: N. 25° W.,N. 36° W., N. 41° which in some cases are excavated in the solid Fork and reached to its junction with Main Fork. In the fall of 1896 about 400 stamps were in W., and N. 51° W. This group will not be con­ rock. Within some of these cirques, rock surfaces At its maximum the ice probably overtopped the operation in the district, and nearly 2000 men sidered in detail in this general account. The were seen to be polished and striated, and similar walls of the canyon, which was, of course, closed were engaged in mining and milling the ore. northwest fissures are of great interest from an scorings were observed in certain of the stream as a line of water drainage. During the summer Although for many years the principal product economic standpoint, and their principal develop­ canyons. Furthermore, deposits of glacial debris season, however, there must have been large vol­ was silver, gold now constitutes two-thirds of the ment occurs in the northeastern part of the quad­ are found which indicate the former existence of umes of water derived from the melting of the output. In 1896 the total amount produced was rangle, in the region immediately about and to an ice sheet extending out onto the mesa north snows upon the southern and western slopes trib­ over $3,000,000, about two-thirds of which amount the northeast of the town of Telluride. of the Wilson group of mountains, and of an ice utary to the drainage of Lake Fork, and these was gold. Since the first finding of ore in the From the foregoing it is evident that there are stream in the canyon of Lake Fork of the San waters must have found an outlet by flowing district the product has been steadily increasing, four general directions of fissuring : east and west, Miguel River. between the wall of ice on the west and the the region never having been, in any sense, a and northeast, best developed in the central and Distinct striation was noticed in the northern shale slopes on the east. It is through the "boom mining camp." A moderate estimate for southeast portions of the quadrangle; north and arm of Canyon Creek, not far above the Trust- instrumentality of such a stream that the bowlder the total product of the district is $25,000,000 up south, and northwest, best exemplified in the

RubyJ mill;' on Treasure Hill,' north- Glacial.,, . , beds are conceived to have been formed. In no to 1897. northeast portion. west of Stony Mountain; on the ridge striation- other way than by the action of running water Development is almost entirely by tunnels run Spacing of the fissures. The distances across east of Bilk Creek; and on Howard Fork at can the rounded form of many of the bowlders be into the steep sides of the mountains, and by over­ strike between the individual fissures of any one Ophir Loop, where the rocks are finely polished. accounted for. hand stoping. The tunnels are, of course, when system are exceedingly variable, and the observa­ Similar markings were seen in other places, but It seems that at the time Lake Fork was practicable, drifts on the veins, but in many cases tions tend to show that the occurrences may be in many of the basins the rock surfaces are filled with ice there was no corresponding ice adits of greater or less length have been run to grouped into zones of widely and narrowly spaced covered by a great amount of talus and soil, and stream in Main Fork, so that its valley cross-cut the veins. The longest of these adits is fissures. An occurrence observed in the Silver at best the comparatively soft volcanic rocks of must then have contained a deep lake, inGlacial Lake lakeFork what is known as the Revenue tunnel, which has Pick mine, which is regarded as, in a measure, Valley. the basin floors are not well adapted to the caused by the ice darn, into which the been run from the bed of Canyon Creek in a typical of the development of fissuring through­ preservation of striae. lateral stream emptied a vast amount of debris, westerly direction a distance of 7500 feet to cut out the region, is here recorded. In the drifts of Of interest in connection with these features of which received a rough stratification as it came to the Virginius vein. This it does at a distance this mine, as one goes southwest on the strike of the high glacial amphitheaters is the remnant of rest beneath the waters of the lake. After the vertically below the surface of over 2000 feet. the main vein, which runs to N. 40° E., dip SE., Telluride 15 one sees, crossing the drifts, fissures, practically width of the zone. The filled space within the the veins in the Mancos and McElmo are wide, known to displace three, and it probably displaces vertical, which strike N. 83° E. One notices that zone varies from the whole width between walls those in the Dolores narrow. In fact, with the four, of the northwestward-running veins, in each these fissures, from being 30 feet or more apart, to small seams less than one inch in width. marked exception-t of the Silver Chief NarrowKr case throwing the northern portion from 40 to 75 become nearer and nearer together, the spaces Usually the ore, especially its metallic contents, vein, the fissured zones of the upper ^ores1^! feet to the east. between them gradually decreasing to 3 or 4 follows one side of the vein rather than the other, rocks appear to have been reduced to ma lon' THE ORE DEPOSITS. inches, causing a sheeting or slating of the rock, generally the foot wall, while the accompanying single fissures in the Dolores. This sandstone while still farther on, the spaces increase again decomposing influences have so affected the rest underlies the region of Marshall, Savage, and The minerals of the veins. The minerals occur­ in width. This alternation is many times repeated of the rock included between the walls that the Virginius basins, where so large a part of the ring in the veins, besides gold and silver, are in the distance traversed by one of the longer resulting material, often called "gouge" matter, mining development has been done, but none of grouped as ores and gangue minerals. drifts. Space does not here allow the citation of is comparatively easy to work, and is found con­ the veins from which ore has been largely pro­ The minerals classed as ores, including other illustrations. Many instances have, how­ venient for blasting and "stripping" the vein. duced are as yet worked down to the horizon of here only those in which the precious ever, been seen, both over and underground, The width of the veins varies from 12 feet down, the sandstone, so it can not be said that fair oppor­ metals are in all probability chemically combined, where narrow and wide zones of fissures alternate, but an average of nearly 100 ore-producing veins tunities for comparison are given. Since, however, are galena, freibergite (argentiferous gray copper), the spaces between the fissures varying from examined is 3J feet. It is, however, impossible it is highly probable that a large amount of wear­ polybasite, proustite, pyrargyrite, stephanite, and several hundred feet down to an inch. to give a correct idea of the width of fissure zones ing down, representing a considerable vertical probably other of the rarer silver compounds, in It would seem most likely that where the which vary between such wide limits as do these. thickness of rock, has taken place since the fissur­ minute quantity. The foregoing are given as spaces between fissures are narrow the zones Many zones are 8 feet in width, while many are ing was formed, and since, even at the present ores of silver, since none of the tellurides or other themselves are narrow, and that where not more than 1 foot. stage, the sandstone lies at a depth of several of the possible rare compounds of gold occur single fissures are separated by consid- zonesFissures of van-in Structure of the veins. All gradations of ore thousand feet below the tops of the highest peaks, within the area, so far as has been determined by 11111 £ x j A able 8Pacln£- t/ O erable blocks ot unruptured country filling are seen in the fissured zones, from those it can not be supposed that the rupturing forces the present investigation. As is well known, the zones are necessarily wide. The observations in which the entire width is occupied No constant would have affected it so violently as they did however, no silver ore occurs in which there is have shown this to be the case. The zones of by ore to cases where the ore seam may structure- those rocks which lay nearer the original surface. not, in the free state, more or less gold. But narrowly spaced fissures rarely exceed 7 feet in be traced only with difficulty in the interstices Such observations as have been made on the veins these minerals, as, for example, galena, while they width. Where the spaces are wider, having, say, between the fragments of country rock. Local in the sandstone tend to confirm the above sug­ are to be considered as ores of silver, are, like iron 2 feet between each fissure, the zone is wider, ore chambers of limited dimensions, where the gestion. In the diorite the zones are perhaps as pyrite, merely the gangue or mechanical matrices being perhaps 70 or 80 feet. Where the broadest country rock has suffered more than usual crush­ wide as those seen in the volcanics, but the of gold. The metallic gangue minerals are, in intervals occur the width of the zone must be ing, and where the result is a breccia, with ore amount of open space which has been filled by addition to the above, iron pyrite, chalcopyrite, reckoned in hundreds of feet, sometimes thou­ for the cementing material, are of not uncommon ore is subject to much more variation. Local zinc blende, mispickel, magnetite, stibnite, and sands. The peculiarities observed with regard to occurrence. Very large "horses" are occasion­ bunches of ore often alternate with places where native copper. The nonmetallic gangue minerals the spacing of the fissures of any one system ally included in the veins. One such in the the veins have pinched to narrow seams. Reticu­ are quartz, calcite, siderite, rhodochrosite, dolo­ appear to warrant the suggestion that there is Mendota workings on the Smuggler vein is several lated or net-like structure is of common occurrence mite, fluorite, barite, sericite (white mica), biotite, exemplified here a sort of rhythmical recurrence hundred feet in length. From this size the in the veins in diorite. The Mount Wilson veins chlorite, amphibole, apatite, garnet, orthoclase, of alternate wide and narrow belts, which may be included fragments descend to microscopic pro­ are especially well defined, and make sharp cuts picotite, and kaolinite. due to a definite law. portions. Evidence is afforded by microscopic in the dividing ridges of the mountains. The attempt has been made to exclude from the Origin of the fissures. The fissures penetrate study, however, that the parallel fissuring of the Continuity of the veins in length. The example above lists all secondary products. An uncer­ afforded by the Smuggler vein, from which ore all the rocks exposed in the area, and were formed rock has not reached minute dimensions, as is the tainty * exists in the case of kaolinite.' Secondary_ later than the extrusion of the rhyolite and the case in ore-bearing rocks which have been reduced has been produced for a length of more the occurrence of which in the Tomboy minerals- intrusion of the diorite stocks. Although from to slaty or schistose form. The width of the sheeted than 2 miles, makes it evident that Veinsunusual of vein is proved, but the genesis of which is regarded the preliminary character of the present investi­ slabs of rocks and the space between the accom­ lodes of unusual continuity exist in the as doubtful. Of the secondary minerals the copper gation it is considered unwarrantable to assign a panying parallel fissures is probably never less Telluride district. Other veins have been worked sulphate, chalcanthite, of the Silver Pick and definite cause to the fissuring, there is little doubt than one inch. Very minute fissures are frequent, continuously for longitudinal distances of more Special Session mines is the most remarkable. that it was produced by dynamic action having its but they are irregularly disposed. Open vugs, than half a mile, while observations on the fissured Its presence is due to the constantly frozen condi­ source or sources in an area apart but probably that is, cavities lined with crystals, usually of zones in general show that a large number of them tion of the ground, allowing the percolating water not far removed from the field under considera­ quartz, are very common in the veins. One may continue for long distances in unchanged direction to carry but a small part of it away in solution. tion. observe gradations from this structure to that in and strength. The intersections of the narrow Small amounts of copper carbonate are not uncom­ If the fissuring be due to pressure acting as which the quartz crystals lining the two sides of fissured zones of systems, whose directions vary mon in the veins which bear copper pyrite. Lead above described the inference is that a consider­ the lenticular cavities interlock, and still further from one another by only a few degrees, often has carbonates and sulphates are usually present with able amount of open space would be formed along to solid filling by gangue minerals. an important bearing on the continuity or non- galena, as decomposition products. Massive the zones of narrowly spaced fissures, caused by Effect of the country rock on the veins. As has continuity of the lodes. This is especially the cerusite occurs in the flat ore beds at Sawpit, the crushing and grinding up of the country rock been already indicated, the fissure systems enumer­ case in Marshall and Savage basins, where the four replacement deposits in limestone. Limonite is by the repeated motion of the walls. Examples ated, and consequently the veins, pene- Veins best fissure systems extending N. 25° W., K 36° W., very common as a decomposition product of the of faulting along the fissures on a large or even trate all the rocks occurring within ^andlsmc N. 41° W., and N. 51° W. are so well marked. iron minerals, and the staining due to its presence perceptible scale have been rarely observed, but the area. There seems to be little Certain of the veins follow more than one set of may be seen in all portions of the veins above the there is reason to suppose that minute faulting of doubt, however, that a mechanical influence on the fissures, where the intersections occur, the ore permanent water level. It should be stated that the rock in connection with the fissuring was of both the nature and degree of development of the switching from one set to another as it found some of the vein minerals occur in very small widespread occurrence. fissured zones has been exercised by the rocks of space for deposition. In the Tomboy vein, amount, and have been detected only with the The presence of columnar joints in the lava the various horizons which they traverse. Thus observations have led to the conclusion that an aid of the microscope. flows, especially in the rhyolite which caps the in the case of the Smuggler and Tomboy veins it intersection of three of the narrow zones of fissures Distribution of the ore. In the Mount Wilson divides, may to some extent cause confusion in is evident from observation that the lodes are con­ has occurred, accompanied by an unusual amount district the character of the ore is much the same of grinding up of the country rock, and develop­ the study of the fissuring. The effects of subse­ tinuous from the breccias of the San Juan forma­ in all the veins. The Silver Pick and Mount , WiU,., quent disintegration have, in addition, masked the tion through the andesite and rhyolite fiows above. ment of open space. In this case the intersections Special Session mines show the most sondjstrict- original structure of all the rocks, especially those In the upper workings on the Smuggler vein, have been beneficial in effect, since a vein of development. The product is almost entirely which outcrop at the higher altitudes. however, where excellent exposures on the vein exceptional development has resulted. Several gold, which appears to be held mostly in the may be seen in both the rhyolite and underlying instances have been observed, however, where the arsenical sulphuret, mispickel. The pay streak THE VEINS. rocks, considerable differences are apparent. No intersection of a zone of widely spaced fissures of the vein lies on the foot wall, is from 4 to 12 Relation of veins to fissures. From the obser­ change was observed in the amount of fissuring with a lode has resulted in local impoverishment inches in width, and consists of straight, narrow vations made throughout the district it seems between the breccias and the overlying andesite, and sometimes in complete shattering and fraying bands of quartz, with iron and copper pyrite and hardly to be doubted that the lodes are but in the upper rhyolite, although the fissures out of the vein. The last effect cited is due to mispickel, and smaller amounts of calcite, galena, the narrow zones of closely spaced fis- w" are constant in direction, and although the zone the fact that the ore, instead of following the and zinc blende. The quartz is usually white and sures, which have been filled with ore. is equally wide, the amount of space now filled main fissured zone, has filled the single cross fis­ crystalline. Open vugs and comb structure are According to von Cotta's definition, which is gen­ with ore is much less, and the fissures themselves sures to such an extent that it is impossible to common. The principal sulphuret, mispickel, is erally accepted, a vein is a filled fissure. Thus, did not apparently afford as much open space as extract it at a profit. Thus in the investigation usually coarsely crystallized. The separate streaks in its strict sense, the term vein is limited in its did those below. It seems probable that the of the veins of this district, as regards their con­ of quartz sometimes have the character of ribbon application to the material of extraneous origin upper rock offered a greater amount of resist­ tinuity in length, it would seem advisable to study ore, having merely a parting between, while they filling the space between the two walls of a fis­ ance to the rupturing force than those below. with' considerable care the distribution of the fis­ are in other places separated by narrow streaks sure. In the case where fissured zones have been Although space does not permit a description sured zones and their intersections. of country rock. The veins are from 3 to 4 feet thus filled, it is somewhat difficult to decide of all the observed examples of comparative fis­ Faulting of veins LSeveral instances have been in width, but the pay streak is usually so narrow whether the term vein or the term zone of veins suring, the following general statements appear noted, especially in the Virginius and the Smuggler that, notwithstanding its high value, it is only

should be applied to the occurrence. For, as is to be warranted by the observations made. In mines,' where the veins swell or widen Faulting_ _ not where the ore can be handled on a large scale the case in the Telluride district, such filled fis­ going from the San Juan formation into the on steep dips and pinch where the dip common- that the mines in this region have been found sure zones generally consist of alternating bands San Miguel conglomerate below, the veins do not is flatter. As pointed out long ago by von Cotta, profitable. and lenses of ore and country rock. In the pres­ appear to become narrower, but they assume an such occurrences are indications of normal faulting On Yellow Mountain the veins, which generally ent paper it seems better to use the word vein in irregularly broken and brecciated structure rather along the fissures that is, a movement of the run nearly east and west, are almost all silver its broader sense, as referring to the whole lode, than the link-vein and banded character which hanging wall down with reference to the foot bearing, so far as hitherto developed. Yellow but the above remarks on the subject of definition they possess in the rocks above. In the Conten­ wall, or vice versa. Had the faulting been Like the veins of Mount Wilson, they Mountain- should be kept in mind. tion and Champion mines, and in the mines above reversed, the wide portions of the veins would lie almost wholly in diorite-inonzonite. The fis­ The narrow zones of closely spaced fissures are Ophir, on Silver Mountain, exposures of veins in have been on the flat dips; the narrow places, sured zones are from 3 to 6 feet in width, the pay well defined by limiting planes, outside of which the San Miguel conglomerate were seen. In along the steeper dips. Lateral faulting of the streak averaging about 2 feet. The ore consists no great amount of open space was developed. regard to the remaining sedimentary formations veins occurs where the ore of one vein is dis- of quartz, sometimes of dark color, usually carry­ Where ore-bearing solutions have deposited ore in which the veins have been seen namely, the tinctl}7 cut by that of another, but the observed ing zinc blende, calcite, and a mixture of the car­ along such zones, within the open space, these Mancos shales, the McElmo shales, and the occurrences suggest no definite conclusions. The bonates, rhodochrosite and siderite, accompanying limiting planes now form the walls of veins. It Dolores red sandstone it may be said that ex­ most marked displacement is along the vein galena; and freibergite, sometimes associated with is rarely the case that the ore under which term posures are so few and opportunities for com­ known as the Pandora, running about east and the carbonates, sometimes with barite. The silver is included, for the present, everything between parison so scant that deductions are not of great west, lying to the south of Marshall Basin, and values are in the galena and freibergite, while walls which is not country rock fills the entire value. In general it may be said, however, that to the north of the San Miguel Valley. This is an increase'in copper pyrite is often accompanied by an increase in the gold values. These veins veins have been worked, running approximately Manner of ore deposition. Probably no region beds at Rico, where traversed by veins, have been are also characterized by banded structure, not, north and south and east and west. They carry will afford better illustrations of the filling by ore heavily replaced by sulphurets, argentiferous to however, to such a marked degree as those in the almost exclusively gold values, and are much of interstitial space t in crushed and Deposition_. greater or less extent, and bedded Mount Wilson district. The quartz is often white, decomposed in all the exposures seen. The pres­ sheared zones of rock than the Tellu- gSJJiff deposits not unlike those at Leadville ffSSS1*1*8 ride quadrangle. The parallel fissur- opensPaces- 11 » IT, «i i seated lime- and comb structure is not uncommon. In places ence of sulphurets was not apparent in them. It have been lornied. It seems possible » zons.stone hori= the veins swell, forming chambers of ore of limited is evident, however, that considerable iron pyrite ing does not seem to be spaced more closely than that if these limestones are developed extent, which have the character of angular brec­ is present in the oxidized state, and it is probable one inch in any observed case. At the same time beneath the Telluride volcanics, similar ore cias of country rock cemented by ore. Such that the gold, now found in large measure free, there are, as shown by microscopic examination, deposits may exist in them in proximity to veins. veins occur in the Terrible, Silver Bell, Carribeau was originally contained in the pyrite, and that, veinlets of the most minute size existing in these It is not probable that the values in these deposits and other mines near these. consequently, in depth the gold will be found zones. These small veins occupy the irregular would pay for the sinking necessary to reach them, In immediate connection with the silver-bearing largely in the form of sulphuret ore. The Con­ interstices between the fragments into which the yet it cannot be denied that a possibility exists veins just described there occur veins of aurifer­ tention and Hamburg veins, in Bear Creek, are rock between the fissures has been crushed. The of the occurrence of workable ore in depth. ous quartz, having different physical examples of these lodes, on which a considerable presence of comb structure, of open vugs, and in Value of the Telluride mining district. The Ophir. characteristics. They consist of white amount of development has been done. general the evidence that crystals have formed region is one of precipitous topography. Most quartz, of saccharoidal nature, with disseminated Impregnations. In consideration of the gold unresisted in open spaces in the veins shows that of the veins now worked have been fine-grained auriferous pyrite and free gold. They deposits of Silver Mountain, of which the Suffolk substitution of country rock by ore has had but exposed by erosion, and their outcrops developed .1 -I » , .7 resources. have more the character of solid ore-filling between and Gold King mines offer the best examples, a little if any part in the formation of the veins. are in many cases visible tor long dis­ two walls than the silver veins. The Badger vein, brief prefatory note concerning the impregnations More convincing still that there has not been com­ tances across country. As the veins are usually on the lower southern slope of Silver Mountain, in the district in general seems in place. As the plete replacement of any portion of the country worked at considerable elevations above the val­ is of this character, and in the Terrible mine, on word impregnations is now used, its application rock is the fact that all fragments and horses now leys, transportation of supplies to and of ore from the west end of Yellow Mountain, such a vein lies is restricted to those occurrences where separate included in the ore have sharply angular corners, the mines is often difficult and expensive. Water parallel to and within a few feet of a vein carry­ crystals, usually of metallic sulphides, fill spaces, whereas if any complete replacement had taken is not plenty in the high basins, and it is usually ing almost entirely silver ores. however small, which have previously existed in place, such corners would have been rounded off. found inexpedient to mill the ores at the elevation In the northwestern part of the quadrangle, in the zones of rock forming and laterally bounding The phenomenon here referred to must not be of the mines. Wire-rope bucket tramways work­ the region of Marshall, Savage, and Virginius the walls of metalliferous veins. As a necessary confused with the partial replacement of the con­ ing by gravity have been found most expedient stituent minerals of the wall rocks, which action, basins,' there is also > a close association Northeastern.. iu , accompaniment of impregnations, channels through for ore transportation, and many are in operation. of the gold and silver veins. Even section- which mineral-bearing waters can circulate must as has been stated, was a probable accompaniment No sinking of shafts or pumping has as yet been more remarkable is the increase in gold values exist. But it is not necessary that such channels of the impregnations. resorted to, all development being done by means and the decrease in those of silver from north to should be visible, even microscopically. In many Values in the ore. Owing to the wide variation of drifts which are themselves open to the air, or south in several of the principal veins. It seems cases the paths traversed by the solutions are visi­ in the tenor of the ore in the Telluride district, are connected with adits. The Revenue tunnel, evident, from many observations^ that an increase ble, and seem to be filled with quartz, calcite, or estimates of only the most general nature can be 7500 feet long, which has cut the Virginius vein in gold values in these veins is accompanied by whatever material has served as the gangue. given. In the Mount Wilson gold veins the nar­ at an elevation of 10,800 feet, has proved a suc­ an increase in the amount of iron pyrite, both in Impregnations of the wall rock by iron pyrite row pay streak of the veins runs from $50 to $150 cessful venture, notwithstanding its great length the vein itself and as impregnation, of the wall- occur almost without exception in connection in gold per ton of ore. In the silver veins of and the moderate tenor of the ore in the vein. It rock. Thus in the northern end of the Smuggler with the veins of the Telluride quadrangle. Their Yellow Mountain the pay streak averages 75 seems likely that a portion of the veins of the dis­ vein the ore consists of quartz, generally of a development varies greatly in degree, and almost ounces silver and $3 to $12 in gold. The accom­ trict continue in length for considerable distances dark-gray or blue color, a rather small amount of inseparably connected with them is partial replace­ panying gold veins are said to run $17 to $20 in while others become broken up and thin out into iron and copper pyrite, calcite, and rhodochrosite, ment of the original minerals of the country rock gold. The gold veins of Bear Creek run $14.50 mere stringers, although further prospecting on with galena, stephanite, polybasite, pyrargyrite, by iron pyrite, white mica (sericite), and, more to the ton, milling ore, as an average of 22 pro­ the strike line beyond the cross zone of fissures galena, and zinc blende. There is usually a sub­ rarely, silica and chlorite. As an accompaniment ductive veins. It is especially difficult to repre­ along which the breaking up occurs would some­ ordinate amount of white quartz. Toward the of these processes there has often been developed sent by single figures the yield of the veins of times result in a continuation of the vein being south end the vein is occupied much more by hydrothermal decomposition of the rock, to which Marshall, Savage, and Virginius basins. On a found. Other permanent veins will doubtless be white, coarsely crystalline quartz, the ores of sil­ are due the brilliant red, white, and yellow colors milling basis, 20 ounces in silver and $9 in gold discovered. It is one of the best known laws of ver are less frequent, and a considerable amount by which the mineralized zones are now marked to the ton appears most representative. The yield ore occurrence that veins are formed in groups, and of iron pyrite is present in the vein. Moreover, at the surface. There can be little doubt that of the Tomboy vein is $20 to the ton in gold. In that where one or two veins have been found pro­ a much greater amount of decomposition appears the hot-water solutions from which Impregnation, the impregnated zones of country rock worked as ductive, others similar in character usually exist. to have taken place in the country rock, due to ore was deposited in the veins also deve^op"^ Ogold-ores on Silver Mountain the vyield is from $5* Origin of the ore. The ore now filling the veins the oxidation of a large proportion of the iron caused the impregnations and accom- near °phir- to $10 in gold. In the Sawpit deposits the ore in the Telluride district appears, from all the pyrite with which it was impregnated. These panying phenomena which now exist in the walls. shipped for smelting averages 11 ounces in silver evidence collected, to have been depos­ features are characteristic of all the veins which Nevertheless, the vein walls have, as it were, acted and $22 in gold, besides 15 per cent to 20 per ited from ascending hot-water solutions from a"cend= exhibit the change from silver to gold values as a septum for the solutions, causing them to cent in lead. which penetrated all portions of the toward the south. deposit by far the greater part of their metallic The fineness of gold in the district varies from fissured zones, wherever open space was found. In the Virginius vein silver values are in frei- contents, including the precious metals, in the .850 to .650. Data concerning the fineness were Surface waters, descending by capillarity, or pos­ bergite and galena. In the Trust Ruby mine, in veins themselves, while only a relatively small per not often available, consequently this estimate sibly in part by means of the fissures themselves, the bed of Canyon Creek, two veins, one bearing cent of the iron pyrite and the accompanying gold must be regarded as only partially representative. have taken into solution, in the form of sulphides, gold the other silver values, are intimately associ­ has gone into the walls. Thus it is in most cases Influence of the country rock on the ore. The alkalies from the constituents of the igneous rocks ated. In the Big Elephant or N. W. H. jr. mine, found that the pyrite of the impregnated wall rocks in which the greater number of the veins encountered in their paths. Getting hotter as in the bed of Savage Basin, an unusually wide rock is only slightly auriferous, and always less of the Telluride quadrangleJ- O have been Rocks_ , in. they descended toward the source of the magma vein consists of white quartz with disseminated so than the pyrite of the accompanying veins. found are the granular rocks of the ySJfaX* from which the igneous rocks have been derived, bunches of sulphurets, chiefly resinous zinc blende, On Silver Mountain, however, the deposit worked stocks along the central east-and-west found> the alkaline solutions probably became further which were said to contain almost entirely gold in the Gold King and Suffolk mines appears to portion of the area, and the heavy andesitic brec­ charged with sulphydric and carbonic acids values. The Tomboy vein (just over the line in be an impregnation which is auriferous to a work­ cia, tuff, or agglomerate, now included in the San derived from volcanic sources, thus becoming the Silverton quadrangle), consists largely of able extent. Veins running for the most part Juan formation, having its greatest development solvents for the metals, and for the silica, lime, white, coarsely saccharoidal quartz, with subordi­ north and south penetrate this portion of the in the northern half of the area. A few veins are etc., they gathered from the more basic portions of nate amounts of calcite, fluorite, kaolinite, sericite, mountain, but they are narrow, often having the, exploited in the overlying flows of andesite, and the magma. By these ore-bearing solutions the and small amounts of iron pyrite, copper pyrite, character of single quartz streaks not over 3 inches the upper workings on the Smuggler vein are in metals gold, silver, lead, copper, iron, and zinc galena, and zinc blende. The metallic sulphides in width. These frequently contain much free the rhyolite which caps the divides. No one vein as well as sulphur, lime, silica, and the alkalies, seem mostly confined to narrow streaks. The gold. It appears to be along the course of these has been worked throughout the whole section of were probably all brought up from the subter­ gold, so far as is known, is almost entirely free, veins that impregnation occurs, and belts of the igneous and sedimentary rocks exposed, although, ranean region to be deposited in various combi­ and its distribution bears no relation to that of country rock 5 to 10 feet in width are taken out as has been stated, there can be little doubt that nations as the waters approached the surface. The the metallic sulphides. From the foregoing it and milled as ore. Other mines in the vicinity the fissuring has affected all the rocks represented. gold quartz and the sulphides of the metals were will be seen that the veins of this most important are worked on the same character of ore. Impreg­ The evidence available in the field was not suf­ deposited in the veins themselves, while the pene­ part of the quadrangle vary greatly from, one nation as above described occurs especially in the ficient to justify conclusions as to the comparative tration of the wall rocks by the alkaline solutions another in their characteristics. In general it rocks of volcanic origin, the breccias and andesite. value of veins in the rocks above and those below. containing sulphydric acid resulted in changes in may be said that the silver is contained . The San Miguel conglomerate, where it outcrops By far the greatest product has come from the the form of the iron of the ferromagnesian silicates J t e Occurrence of in galena, freibergite, and the rarer themetals- along the slope of Yellow Mountain, has also been horizon of the San Juan formation. Productive of the rock and the deposition of potash combined sulphides and sulpharsenides, while the gold is impregnated by iron pyrite. A portion of the ore veins are, however, worked in the San Miguel in the silicate sericite. Carbonates, especially of either free in the quartz or mechanically combined mined in the Gold Crown appears to be merely conglomerate and the underlying sedimentaries. lime, were deposited in the walls, probably for the with sulphurets, usually iron pyrite. It has been this impregnated conglomerate. The impregna­ It has been proved by exploitation that in the most part from action of the percolating waters noted that the dark-colored quartz is most preva­ tion occurs in the material cementing the pebbles, stock rocks, the breccias, the volcanic flows, and on the lime feldspars. Silica was set free from lent in the silver veins, while the coarser white and is occasionally so heavily developed that it is in some cases in the sedimentary rocks, the values the bisilicates and feldspars, and has passed quartz is more characteristic of the veins carrying a question whether the ore body should not be are generally good. In view of the immense mostly away from the wall rocks, being present gold. The iron pyrite which accompanies the classed as vein filling rather than impregnation. thickness of the volcanics it seems likely that only in exceptional cases. As has been stated, the argentiferous minerals appears to be generally This mineralization of the San Miguel conglomer­ many years' work will be required to exhaust the gold appears to have been carried into the walls more finely crystallized than that with which gold ate was not observed except along the southern product of the veins within these rocks. The to some extent. Fluorite, which occurs largely is associated. The statement sometimes made slope of Silver Mountain. limited study of the region permitted in the pres­ in the Tomboy and some other veins, may be that the finely crystalline pyrite is richer in the The veins of the Valley View and Gold and ent reconnaissance does not justify the assertion accounted for if it be presumed, as Mr. Penrose precious metals than the coarse is not regarded Silver Chief mines to the north of the Telluride that veins which have been found good in the San suggests in regard to the Cripple Creek occur­ as generally applicable. The veins average 4 to Valley appear to correspond in general character Juan formation suffer impoverishment of values rence, that hydrofluoric acid, or other fluorine 6 feet in width, with a pay streak of from 2 to 3 to those of Marshall and Savage basins. In Bridal in the underlying sedimentaries. compounds in which silica forms a part, accom­ feet. As stated below, the tenor of values in the Veil Basin little development has been done, but At a distance of less than 1000 feet vertically panied the other elements in solution and, uniting precious metals is moderate. The strike of all the exposures of the veins as so far developed bear beneath the level of the Telluride Valley there is with lime, deposited fluorite in the veins. The these veins is northwest, the dips southwest. a resemblance in part to those of the Bear Creek probably developed the series of upper Carbon­ fluorite is not purple here, but of a light-green In the region about Bear Creek and on the region, in part to those of Yellow Mountain. The iferous limestones which have such marked promi­ color. Sericite, often spoken of as " talc," occurs northern spurs of Silver Mountain a number of ore-beds of Sawpit will be separately described. nence on Dolores Mountain, near Rico. These in the veins, sometimes in considerable amount, Telluride 17. and perhaps represents the form in which a por­ was an accompaniment of the ore deposition. In barren of any value cut some of the beds, and Although more than $100,000 has been taken out tion of the potash and silica of the ore-bearing the high basins and on the peaks the constantly penetrate the red sandstones beneath. Barren in alluvial gold, many times this sum has been solutions has combined. Small amounts of kaolin- frozen condition of the ground goes far toward "verticals" are now.recognized in several mining spent in the operations. The principal difficulties ite are occasionally mixed with this. Chlorite is retarding surface alteration, but at the same time districts as the mineralizers of adjacent ore-beds, experienced in working the gravels of the Tellu­ notably scarce. It is not likely that more than the continual melting and re-freezing of much and it is possible that these veins may have served ride valley were the large size of the bowlders and one general deposition of ore has taken place. In of the water that percolates through the mass in part as channels for the mineral solutions the unusual weight of the gravel itself. The local instances successive crystallizations, at inter­ causes rapid breaking down of the rock, and the which have brought up the ore. It is of import­ gravels appear to have been deposited in a lake, \ vals not far apart, of the ore and gangue minerals accumulation of fragments results in the forma­ ance also in considering the origin of these deposits whose origin was due to glacial conditions. Where are evident, but these cases are not frequent and tion of immense heaps of talus. to take notice of a dike of rather basic rock, 3 feet alluvial gold has been deposited in a lake bed it no correlation can be established. in width, which strikes about east and west and is not usually sufficiently concentrated to pay for THE SAWPIT DEPOSITS. No evidence in favor of the lateral secretion which occurs in close proximity to the ore-deposits. working. So far as present developments show, theory, in the narrow application advanced by In the northwest corner of the Telluride quad­ This dike, which is probably an offshoot of a much this lack of concentration has been one Sandberger, has been found. The hypothesis rangle, along the northeast wall of the canyon of larger igneous mass, may have been in part the of the difficulties experienced in the K wotting outlined above seems the most plausible in view the San Miguel River, the impure cal- Qnre assocU cause of the mineralization. It may be said, as of Telluride deposits. In those placers of the evidence at hand. In cases where tests for careous layer included in the horizon of ptlata,fd with lime= La the part of the district farther east, that there is which have been worked farther down the stream, the precious metals have been made in the La Plata sandstone has been par­ stone. a possibility of further replacement by metallic the additional difficulty has been experienced of the constituent minerals of igneous oreto°gii0e- tially replaced by ore, four-fifths of the value being sulphides of the beds of upper Carboniferous getting water to a sufficient height to wash the , . . ous rocks. rocks, in previous investigations ot ore- in gold. The ore bodies as they have been found limestone which it is reasonable to suppose under­ gravel, for the deposits are not found at the bearing districts, the basic minerals, hornblende, up to the time of this examination occupy the lie the Trias in this portion of the field. present river bed, but are remnants of terraced augite, biotite, etc., have been found to contain upper portion of the 8-foot layer of limestone. gravel beds lying at heights of from 30 to 150 feet PLACERS. the larger quantities. Much more basic phases Their dimensions are usually 100 to 200 feet in above the stream. The gravels are said to run of the Telluride igneous rocks than those gener­ length (east and west), 25 feet in width, and 3 Much of the gold of the San Juan region is in from 10 cents to 20 cents per cubic yard. The ally exposed are occasionally seen in the quad­ to 5 feet in height. They lie parallel to one a very finely divided state, and from all evidence character of the gold is generally fine, but not rangle as dikes, inclusions, etc., of small extent. another, the longest dimensions corresponding in it can not be doubted that large<=> quan-J- Sourcec of scaly. It does not seem probable that placer It appears probable that, in the magma wherein orientation to east-and-west fissures which form tities of it have been deposited along the gold> mining, according to the methods now generally the rocks had their source, a basic portion, of one of the systems here prominently developed. the courses of the streams draining the region. employed, will ever attain to much importance in which these fragments are representative, exists. Where ore has been taken out, cavities with well- The area over which it has been distributed is the district. This more basic portion is a possible source of marked walls, characterized by smooth, undulat­ probably large, and, indeed, alluvial gold is known the ore, since it does not seem necessary to go ing surfaces, are left. The ore has been for the to occur along the San Juan River and Grand COAL. beyond the limits of the rock magma for the most part formed by replacement of limestone by River hundreds of miles below the sources of In the southwest corner of the quadrangle the required constituents. The ore deposition was metallic sulphides, principally iron pyrite and their tributaries. It is probable that a large part Dakota sandstone contains lignitic coal. The bed probably not greatly subsequent in age to the galena. It now occurs, however, mostly in the of this gold was derived from veins in the San is from 20 to 30 inches in thickness, and its occur­ close of the volcanic eruptions, although it must form of oxides and carbonates of the metals, the Juan Mountains. The alluvial plain which fills rence is widespread. About 20,000 tons of the necessarily have taken place later than the fissur- lamellar structure showing rearrangement by the valley of the San Miguel from Pandora nearly coal have been mined. It is said to be fairly ing of the country. water. Masses of the original iron pyrite and to Vance Junction is a repository for much of the good coking coal, containing little clinker but a Surface alteration. The veins throughout the galena may be seen inclosed entirely by massive gold which comes from the sides of the mountains, large per cent of ash. district show remarkably little decomposition due cerusite and limonite. In general, a mixture of yet this ground has been explored with consider­ to surface agencies. Where the impregnations of secondary ferric material having the appearance able thoroughness without profitable results. In CHESTER WELLS PURINGTON, the walls by iron pyrite are strongest, surface of a light-brown mud is the richest in gold, while fact, ever since the district was first explored alteration of both the walls and the veins appears the subordinate amount of gray, massive cerusite placer mining has been carried on along the Assistant Geologist. most manifest. It is often difficult to distinguish carries the silver values. Free gold is never seen course of the San Miguel River for many miles this from the hydrothermal decomposition which in the ore. Vertical veins filled with calcite and beyond the limits of the Telluride quadrangle. June, 1897.