ECONOMIC GEOLOGY

WITH WHICH IS INCORPORATED

THE AMERICAN GEOLOGIST

VOL. III JULY-AUGUST, x9o8 No. 5

GEOLOGICAL ASPECT OF THE LODES OF .'

DONALDA. MACALISTER(Assoc. R. S. M.).

INTRODUCTION. Although the west of mining regionof Cornwalland Devonshirehas not recentlybeen in a flourishingcondition, its past fame entitles it to a claim on the attention of those inter- ested alike in the genesisof tin and copper ores and in the antiquityof mining. During the last few years a sectionof the field staff of the GeologicalSurvey of has beenengaged in remap- ping the Cornishpeninsula and as this work is nearly completed the most recentgeological information concerningthe region is available. There is quitean extensiveliterature dealing with the west of Englandgeology and mineralogy,and as this has been acknowledgedin the Memoirsof the GeologicalSurvey it will be unnecessaryto refer to it here?

HISTORICAL.

The earliest records of the connection of Cornwall with the ' Communicatedby permissionof the Director of the GeologicalSurvey of England and Wales. aThe GeologicalSurvey Memoirs dealingwith Cornish Mining are as follows: (x) "Geology of the Lands End District"; (2) "Geology of Camborneand "; (3) "Geology of "; (4) "Geology of and ." Other memoirs are in course of preparation. 363 364 DON,4LD ,4. M,4C,4LISTER

tin trade are the extracts from the lost writings of Pythias by the Greek historians Strabo and . Although it is believedthat a century before the voyage of Pythias, the Carthaginian Himilco had visited Cornwall with a view to stimulating the tin trade betweenthat country and the continent,there was in the time of Pythias an overland trade betweenCornwall and Marseilles--possiblyalso a sea-bornetrade between Cornwall and Cadiz. When Pythias was in Cornwall he was informed by the in- habitants that the tin, after being cast into ingots, was con- veyed to the island of Ictis, which was accessibleto foot passen- gers, at low tide. From here it was taken to the mouth of the river Loire and thenceto Marseilles. As to the identity of the islandof Ictis there has beenconsiderable discussion in the past, but recentlyMr. ClementReid has shownhow the might have been the Ictis referred to by Pythias/ as at that time it is possiblethat there was a natural causewaybetween the isle and the mainlandwhich has sincebeen worn away by the scourof the sea. This view is combatedby Mr. T. Rice Holmes who, on historicalgrounds, inclines to the other suppositionthat the old Ictis and St. Michael's Mount are the same. •' In either case the antiquity of the tin mining in Cornwall cannot be doubtedand with fluctuating activity the production of tin has beencontinuous to the presenttime. The tin mines in the twelfth and thirteenth centuries were under the controlof the Kings of England who grantedprivi- legesfor working them, but for the last 3oo yearsthe production of both tin and copperores has beenof nationalimportance.

TOTAL YIELD OF TIN AND COPPER FROA{ CORNWALL. An exactest{mate of the total metalliferousyield o,f the county of Cornwall is practically{mpossible to obta{nowing to the ab- senceor uncertaintyof the olderrecords. In a brochureby the presentwriter publishedin i9o 7 by the GeologicalSurvey, an estimateof the total known productionof the m{neralsof Corn- '" Tl, e Island of Ictis," ,4rchceologia,Vol. 59, 19o5. • "Ancient Britain and the Invasions of Ca•sar," I9o7. GEOLOGICALASPECT OF LODES OF CORNWALL 365 wall was made within the limits of a possibleerror of Io per cent., as follows:

Total Produce of Metallic Tin from Cornwall. Between •2o• and •749 A. D ...... 7H,99o tons Between •75o and •9o5 A. D ...... 755,90otons (from black tin containing 65.o2 per cent. of tin). (O'f283,ooo tons of tinyielded between I2oI andI6oo A.D. 75,oootons were obtainedfrom alluvial'deposits, and of 573,8oo tons producedbetween I6oI and I8oo A.D., 473,ooo tons were obtainedfrom alluvial deposits.)

Total Produceof Metallic CopperFrom Cornwall. Between •5o• and •725 A. D ...... 20,000 tons. Between •726 and x9o5 A. D ...... 883,35o tons (from ore containing 7.89 per cent. of metallic copper). For the amountsof ores yielded from the minesof Cornwall individually the reader is referred to the Memoirs already mentioned. For over I5o years Cornwall has had formidablerivals in the tin-fieldsof the East Indies (Bangka and Billiton islands) and while still holding a high place in the world's productionof tin ore its output is exceededby Bolivia, Australia, Siam and Malay --the last country producingabout ten times the quantity raised in Cornwall. ßIn additionto oresof tin and copperCornwall has yieldedcon- siderableamounts of argentiferous-galena,zinc ore and iron py- rites, arsenic, wolfram and of iron ores, ochre and umberø Smaller amountsof bismuth,antimony, nickel ore and uranium have beenyielded as by-productsor in a small way.

GEOLOGY.

GENERAL GEOLOGICAL DESCRIPTION OF CORNWALL. With the exceptiono,f a few undeterm{nedpoints the geology of Cornwallhas beenrecently worked out by the Geological Survey. The small key map (Fig. 28) showsthe general arrangementof the Palaeozoicsedimentary rocks of which the 366 DONALD •I. M•IC•ILISTER GEOLOGICALASPECT OF LODES OF CORNWALL 367 regionis composedtogether with theigneous masses which were intrudedat differentperiods during the disturbanceswhich have upheaved,folded and cleavedthem. The sedimentaryrocks of thewestern part of the region,south of an east and west line situated north of , are of Lower Palaeozoicage as determinedby trilobitesin the regionsouth of St. Austell,and consistof argillaceousslates of purplishand green color with fine sandstones.The rocksare thoroughly cleaved,folded and overthrust,and contain interbeddedquartz lenticles,and in thosebeds which consistof alternatelyhard sandyand soft shalybands there is crush-brecciaor pseudo- conglomeraticstructure due to the fracturingof the harderbands which under pressurebecame crushed into lenticularfragments separatedby softer argillaceousmaterial. The Lower Palaeo- zoicsh•.ve been separated into five subdivisionsnot indicatedon the map. These rockshave a strike of E. 3øo N. and are sup- posedto have as a wholean anticlinalarrangement. • The sedimentaryrocks of central and' east Cornwall are mainly of Devonian age and consistof shales,sandy beds, grits and calcareousbands with an east-and-weststrike. The series is much folded and cleaved, the Lower Devonian rocks occurringon the south,and the Middle and Upper on the north. In Watergate Bay, north of Newquay, the sandy and argil- laceousLower Old Red seriesis seento occupythe axis of an anticline,the overlyingLower Devonianbeds occurring on both the north and south side of that feature. A similar anticlinal structureis discoverablein the Upper Devonianseries in the dis- trict on the west of Kit Hill in East Cornwall. The Carboniferousrocks of the region compriseboth the Upper and Lower Divisions. The Lower Carboniferousshales and radiolariancherts, as well as the Upper Devonianshales, are overlain unconformablyby the Upper Carboniferousplant- bearinggrits andshales. XThe discoveryof Devonianconglomerates near the just• north of the Lizard serpentineappears to confirm this suppositionsince, according to Mr. J. B. Hill, they are correlatable with the Devonian shales and sandstones north of Truro. 368 DONALD A. MACALISTER

The igneousrocks of the region comprisethe greenstones, lavas and schalsteins,granites, quartz porphyriesand mica traps. Under the term greenstoneare includeda seriesof pre-granitic basic intrusions comprising diabases,proterobases, porphyrite and epidiorite, most of which have been shearedand more or less altered, particularly near the granite. The lavas are typically representedby spilite---fine-ffrainedrocks belonging, accordingto Dr. J. S. Flett, to the Essexitegroup and charac- terized by exceptionalamounts of albite feldspar. The schal- steins occur in patchesin both the Upper Devonian and Car- boniferous rocks of East Cornwall. The igneousrocks of Post-Carboniferousage do not as a rule show any foliation or shearingsince they were intruded,only at the close of, and as a relief to the great Post-Carboniferous movementsto which the folding and cleavageof the Palaeozoic sedimentsare due. The most important of these are the five granite intrusionsshown in the figure, togetherwith their fine- grained hypabyssaltypes, the elvans or quartz-porphyrydykes and the comparativelyrare mica traps or minettes.

STRATIGRAPHICAL TABLE.

SEDIMENTARY ROCKS.

Upper ? Culm grits and shales Middle CARBONIFEROUS

Lower Culm shales and radiolarian cherts.

Upper, Middle and Lower Devoniangrits, calcareous slates and shales DEVONIAN Lower Old Red shalesand fine sandybeds (Dartmouth Beds) GorvanHaven beds UPPERSILURIAN ( Wenlock)

Veryanslates, sandy beds and quartzitesand thin limestonesand ORDOVICIAN cherts Portscathoslatesandsandstones -

Falmouth slates CAMBRIAN ?

Mylor slate•and Dodman(?) Phyllites

...... [1 ...... II - - - r I Iii I II ..... I GEOLOGICALASPECT OF LODES OF CORNWALL 369

IGNEOUS ROCKS.

Granite

Quartzporphyry (EIvan) LATE OR POST-CARBONIFEROUS

Mica trap

Tuffs and lavas CONTEMPORANEOUS CARBONIFEROUS Greenstones

UPPER AND MIDDLE Pillow Lavas and Schalsteins. Greenstones DEVONIAN

Pillow Lavas

Greenstone(hypabyssal) OLDER PALAEOZOIC Gabbro

Serpentineof the Lizard, with granuliteintrusions

It is with thesegranite masses that the mineralwealth of the west o.f England is connected. The metamorphicrocks include the hornblendeand mica schistsand quartzites of the Lizard; and the contact-alteredrocks in the vicinity of the granite masses.

THE LODES. The stippled areas on the map indicate the chief tin and copperdistricts of Cornwall and South . It will be noticed that in all cases the lodes occur concentrated in districts con- nected with the granite massesor their metamorphic contact zones,but this positionis generallyfurther characterizedby the presenceo.f post-granitic dykes of quartz-porphyry (F-Ivan), which, like the lodes,have directionssimilar to that of the cleav- age of the sedimentaryrocks which they traverse. As a whole the oresappear to representthe final phaseof plutonicactivity in the region,the sequenceof eventsbeing: (t) Intrusion of the granite masseswith thermal metamorphismof the slates sur- roundingthem, (2) intrusionof Elvans, (3) formationof the tin and copperlodes accompanied by extensivemineralization of bothsedimentary and igneousrocks. 370 DONALD A. MACALISTER

LODE STRUCTURES. The tin and copperlodes as a whole may be describedas fissuresof varyingsize and form, which contain a varietyof ores of tin, copperand other metalliferousminerals together with characteristicveinstone accompaniments. The structure of the lodes indicate to some extent their mode of origin; and while someof themare merejoints or cracksof the thicknessof a knife blade,others are of great width and con- tain much fragmentalcrushed materiM indicating that they are probablyof the nature of faults. The variety of structureswhich the vein stonesmay assume under suchcircumstances is fully representedin Cornwall and as a wholemay be classifiedinto one or more of the following groups: I. Fissure,or seriesof close'parallelfissures filled with metal- liferous and other minerals. 2. Mineralizationof the walls of the lode by impregnationor metasomaticreplacement. 3- Mineralization of crushedrock or brecciascontained in the lode. In the first classare includedall simple infilled fissures,to- gether with the large workable ore-bearing massesknown as stockworksin which a seriesor group of closeparallel fissures often crossedby anotherseries, is worked out as onelode. Stock- works in the granite, killas and elyan are common in all the districtsof Cornwallbut are not the mostprofitable types of lodes. Under the secondheading come all the metalliferousportions of the lode which occur as metasomaticreplacements of the country rock. As a rule this impregnatedzone varies in width from a few inches to a foot or more. It does not yield the highest percentageof mineral but can be profitably extracted with the true lode material. Most of the lodes are associated more or lesswith this altered band, but abnormaldevelopments known as "Carbonas" sometimes occur. The best known ex- amplesof the Carbonasare thoseof St. Ives and other placesin West Cornwall in which long, wide; irregular bodiesof schorl, quartz and tinstoneoccur as an alterationproduct of the granite GEOLOGICAL ASPECT OF LODES OF CORNWALL 371

in the vicinityof smallfissures running at right anglesto the ordinary lodes. Underthe third headingcomes the mostimportant of all the lode structuresin Cornwall,namely the lodescharacterized by brecciasand other crushedmaterial of the country rock. In someimportant mines the Widthof thesebrecciated zones varies from a few feetup to manyfathoms. The lodesnear andRedruth are typical examples of mineralizedbreccias but this typeof lodeoccurs in modifiedform throughout Cornwall. The lodesof the regionbetween Camborne and Penzancegenerally contain brecciatedveinstones, but movementsin the lode have resultedin slicken-sidingand the roundingof much of the material so that it has now the form of a conglomerateto which the term crush-conglomerateis applied. This leadsto a variety of lode structures,composed of brecciasand conglomeratesce- mentedby tin stone,copper and iron pyrites,zinc blende,and other•minerals, the wholebeing traversed by stringsof quartz, chlorite, fine-grainedtourmaline, etc., and constitutingthe veinstones. In some brecciatedlodes in which very little movementhas taken placethe rock has merelybeen comminuted and the lode consistsof a massof interlacingstrings of lodemineral. In somedistricts the lodeswhere they traversethe slaty rocks ("Killas ") show abnormal developmentsarising out of the structure of the killas itself. These are the so-called "floors" which althoughoccurring in the St. Just and other mining dis- tricts are mostly typically developedat the "Park of Mines" at the westernend of the St. Austell granite masswhere the tin ore occursin interbeddedlayers forming lateral offshootsof narrow veins which cut acrossthe cleavageof the slates. The whole zone in which the layers of tin ore occur was removed bodily.

RELATIONS OF THE ORES OF TIN AND COPPER TO THE GRANITE. The tin and copperores occur together in intimateassociation, but in someimportant casesthese ores occupydifferent horizons in the lodes. The best known case is that of the lode on the 372 DONALD A. MACALISTER northern edge of the Cambornegranite, extending from Cam- borne to near Redruth, a distanceof over three and a half miles (the Dolcoath Mine Lode). In this lode the ores of copper mixed with tin ore occupythe upperpart of the lode and extend from the surfaceto varying depthsdown to about2o0 fathoms below surface. Below this the principal bodiesof tin ore occur to a depth of 500 fathoms from surface. In this casethe upper part of the lode is in killas and the lower part is in granite. This zonal arrangementis exhibited in several important mines in Cornwall, among which may be mentionedLevant Mine (near St. Just) where the tin and copperores occurtogether in a long "chute" in the shearedkillas and greenstonefor a mile sea- ward, parallelwith the subterraneansurface of the granite and at right anglesto its margin. The copperores at the edgeof the granite in Tresavean Mine, near Redruth; the tin and copper minesnear St. Austell and other places,illustrate the localization of the ores in certain horizons, the ores of tin and copperoccur- ring either mixed or apart from one another and generally not far from the granite. In a more general way the Cornish mineral areas are desig- nated tin or copperdistricts according to the relative proportions in which these minerals occur, but all the districts contain both classes of ores.

•^TURV. OF Ta• WX•S•O•V.S. The veinstonespresent a large variety in structure and com- position,from the simplebanded or sheetedstructures in simple fissuresin whichminerals have been successively deposited, to the complexmineralized breccias characteristic of the largest lodes in the county. In examining the veinstonesunder the microscopeit is seen that they are merely a repetitionof the lode structurescarried to a microscopicdegree of fineness,so that it is unnecessaryto describethem again. In the arrangementof the mineralsand in other featuresthe study of the lode material by the microscopeaffords the in- formationnecessary for formingan ideaas to their origin. The oreswhich enterinto the compositionof the lodematerials GEOLOGICAL ASPECT OF LODES OF CORNWALL 373

comprisecassiterite, stannite, copper pyrites and dark sulphidesof copper, oxides of copper, Fahlerz, carbonate and silicate of copper and native copper; mispickeland other "arsenicalores; wolfram, scheelite;phosphates, arsenate and carbonateof lead; silver ores, zinc blende,bismuthine, cobalt and nickel ore, man- ganese,pitchblende and other ores,among which are embraced various ferriferous minerals. Among thesewill be recognizeda number which have been formed from alteration(oxidation, etc.) of previousminerals. The othersare stable,and are practicallyincapable of beingde- composedunder ordinary conditions. In describingthe oresit is importantto recollectthat they fall intotwo groups.In the firstgroup is includedthe original mineralsof the lodessuch as cassiterite,wolfram, copperand iron pyrites,mispickel and variousother sulphides.The second group embracesthe oreswhich originatedas a result of the de- composition,migration and redeposition of the primaryminerals, includingsuch ores as cuprite,melaconite, native copper, mala- chite,chrysocolla and varioussecondary sulphides of copperand other metals. Thesesecondary ores are confinedto the upperparts of the lodesin a zonewhich may extend to a depthof 20o fathomsfrom surface. They are easilyrecognized to havebeen the last min- eralsdeposited in the lodesas theyoccur in crackstraversing other minerals, or as incrustations. Some of the carbonatesof ironor coppertogether with pigotite are of suchrecent origin as to be foundencrusting the wallsof old levels. The veinstones,or non-metalliferousgangues, characteristic of thetin andcopper lodes of Cornwallare quartz,chlorite, tour- maline, fluorspar and kaolin with other less common minerals. In describingthe microscopicstructures of the veinstonesit is convenientto divide them into two groups. In thefirst group is includedthe material occurring as a depositin openfissures and, in the second,the veinstonesderived 'by the alterationof the ßadjacent country rock. Of the materialdeposited in the openfissures little needbe saidsince the structures of the veinstones are so largely dependent 374 DON,4LD .4. MAU dLISTER

on the forms of the openingsin which the ores were deposited, as already described. From the relations of the various ores to one another, how- ever, it appearsthat the order in which the mineralsarrived was as follows: I. Tin ore, wolfram, sulphidesof copper, zinc and arsenic; quartz, fluorspar,chlorite and tourmaline. 2. Copperores and other sulphideswith possiblysome silver and lead; quartz, chlorite, fluorspar. 3- Silver-lead ores and carbonates;quartz and fluorspar. The veinstonesderived from the alterationof the countryrocks are of more interest sincethe mineralogicalchanges which have gone on in the rock through the effectsof mineralizing vapors or solutions enable conclusions to be drawn as to the nature of the reactionswhich took place. In additionto silicificationthe granite near the lodesmay be tourmalinized,propylitized and greisenized. The killas or slaty rocks show developmentof silicifiedtypes and tourmalinizedbands, while in greenstonesgarnet is often formed.

PN•-U•AT0•.¾TIC AI.T•.R•XION •D •-X•S0•SIS. I. Granite.--Granite is the best rock in which to study the changes brought about by pneumatolysissince the coarse- grained nature of its minerals enablesthe changesto be readily identified. Tin ore which occursin suchfine pseudomorphsafter porphy- ritic crystals of orthoclaseat I-Iuel Coates near St. Agnes is commonlyfound throughout Cornwall as a partial or complete replacementof the ordinary felspar of the ground mass,while sulphidicminerals such as copperpyrites are frequently found along with tinstonein the decomposedparts of the samecrystals or in smallcracks travers. ing the mass. The changeswhich the mineralsundergo are briefly: (I) A conversionof felsparinto lithia-bearingmica or hydratedmica (lepidoliteor gilbertitc); but also mainly throughthe actionof moisture,the felsparby lossof silicateo.f potashis extensively,in GEOLOGICAL ASPECT OF LODES OF CORNWALL 375 someplaces, converted to chinaclay, as in the St. Austell region. The silica liberated during such reactionsis depositedelse- where with the formationof tough silicifiedrock types. Where boron vapors have been active there is a conversion of felsparinto blue tourmalinewhich occursgenerally as fibrous aggregates. Fluorspar,quartz, tourmaline,copper pyrites, tin- stone,topaz, kaolin and pale hydratedor lithia mica all occuras secondaryproducts after feldspar,through the actionof original vaporscontaining fluorine, boron, tin, lithia, sulphurand much moisture. The mica of the granite is either bleached,with separationof oxide of iron, and conversionto hydrated pale varieties or it may be convertedto chloritewith considerablemigration of the material. In the ore districts the conversion of biotite to brown tourmalineis universal. These brown crystalsare frequently fringed with later growthsof acicularblue tourmaline. The chloritic material and the brown tourmaline are the char- acteristicalteration productsof the biotite. ' With the exceptionof considerablecorrosion of the original quartz of the granite this mineralis often quite unaffected. Much secondarysilica is producedby the alterationof the rock. This occursthroughout as a granular mosaicwhere the rock is altered to any extent. 2. Killas.--The veinstonesoccurring in the contact altered sedimentssurrounding the granite masseshave a few features of specialinterest. The alteration types include mica schists,and flinty biotite hornfels, andalusite hornfels with exceptional cordieritc and pinitc. The more gritty bandsof the rocksare convertedinto quartzitictypes. In the originally calcareousgrit bandstough bandedcalc-hornfels is producedby contactmetamorphism, with developmentof epidote,axinite, pale pyroxeneand garnet. The modificationsof thesealtered rocksin the vicinity of the lodei are principallythose produced by silicification,tourmalini- zation and impregnationwith ores. Where the slaty rocksare impregnatedwith tin-ore and sul- 376 DONALD A. MACALISTER phide of copperthe mineralsoccur in small cracksor along cleavageplanes. As a rule the walls of lodestraversing slates are much crum- pledand shearedand therehas been an extensiveinfiltration of quartz which occursin small lenticlesfollowing the contorted cleavage of the slates. One of the most beautiful and conspicuousof the metasomatic alterationsof the killas is that effectedby tourmalinization. In the more massiveshales the developmentof tourmalinehas taken placefor a distanceof severalinches and evenfeet on eitherside of the veins; but where they are bandedand consistof alternate thin seamsof grits and shales,the tourmalinehas only beende- velopedalong the impure argillaceouslayers, leaving the grit bandsquite unaffected. As the rocksare muchfolded on a small scale, gnarled, shearedand brecciated,all the contortionsand generalstructure of the rocksare preservedand accentuated.In the specimensexamined the alterationof the sedimentsby pneu- ma,tolysis has been superimposedon that of contact meta- morphism. In the calc-hornfelsbands instead of tourmalinewhich is only sparinglydeveloped axinite as a productof penumatolyticaction on lime-bearingrocks occursin abundancein associationwith amphibole,pale and green pyroxene,epidote and garnet. These minerals are well developedin the district north of St. AusteI1 and in the cliffs at St. Just. 3. Greenstone.--Like the calc-hornfelsthe greenstonesgive rise to lime-bearingminerals such as axinite, fluorspar,epidote, pyroxeneand amphibole,and garnet. Tourmaline is uncommon. The garnet occursin the form of veinswhich traversethe green- stonelike an ordinary veinstone,and in associationwith it quartz and pyritic mineralsare found. By the dim light of the miners' candlesthis rock has occasionallybeen sent to surface in mistake for tin-ore in Levant Mine (St. Just). These garnetiferousveinstones are known to occurin Dolcoath Mine, at , also on the north of Dartmoor and other places;while they occurabroad in Arizona, I-Iungary,the Urals, New South Wales and Queenslandand in Sweden. In the de- GEOLOGICAL ASPECT OF LODES OF CORNWALL 377

velopmentof the garnet there has probablybeen an addition of silica to constituentsderived from the greenstone,while the changestook placeunder thermal conditions.

GENESIS OF TIlE ORES.

The nature of the minerals of the lodes indicates that the vaporsand solutionsfrom whichthe oreswere derivedcontained, in addition to the metals, such materials as fluorine, boron, sulphur,silica, lithia, probablya little carbondioxide and much moisture.As pointedout long ago by Daubrdeand •lie de Beaumont the period at which this type of ore depositswere formed followedclosely on the consolidationof the granite mass, the solutionsbeing withdrawn from the magmaas it crystallized. In the form of fluorides, sulphidesand other compoundsthe metals and silica were extracted in this way at high temperature and depositedin fissuresin the cooler rocks accompaniedby the typical mineralization already mentioned. As elaboratedby ProfessorVogt, the Cornish ores belong to the subgenetictype, "copper-tin" lodes,or tin lodes character- ized by large amountsof copperand other sulphidicores. The presenceof tourmaline recalls the tourmaline copper veins of Mexico on the one hand and the tourmaline wolfram veins of Tirpersdorf (Bohemia) on the other; while the occasionalpres- enceof argentiferousgalena in the lodessuggests a genetictype having affinitieswith the Bolivian silver and tin lodes. The secondaryconcentration of the sulphidicores in the upper parts of the lodes has already been referred to. The actionof surfacewaters containing oxygen or carbondioxide in solutionhas decomposedoriginal sulphides of the lodescausing them to be carried downwards in the lodes in the form of sul- phatesor carbonates.Meeting with iron pyrites or other sul- phidesthe sulphateswere reducedand the ore precipitatedas sulphide. Secondaryenrichments of copperores in theupper parts of the lodeshave yielded in somecases extremely rich bodies of copper ore. 378 DONALD A. MACALISTER

THE SILVER-LEAD VEINS. The principal lead districts of Cornwall were those of the centraland easternpart of the county. That of centralCornwall occurson the north of Truro while the east Cornwall region is situatedin the parishesof Menheniotand St. Ives. In Devon- shire the parish of Beerferris was once an important lead pro- ducer but like those of Cornwall has been abandoned many years. The lead districts are situated in the killas at some distance from the granite masses. The ores occur in a seriesof approximatelynorth and south cross-coursesor faults of tertiary age crossingthe general strike of the rocksat right anglesand thus belongingto quite a differ- ent set of movementsto that to which the tin and copperlodes owe their origin. The richest district has been that of Menheniot and St. Ive, but no lead oreshave beenraised from it sinceI884. The yield of the district between I848 and I884 was as follows: I848 to x884 Dressed lead ore...... 92,oootons. Metallic lead obtainedfrom the ore.... 63,600tons. Silver (parted from lead) ...... •,93o,ooo ounces. The metalliferous minerals found in association with the lead ores are zinc blende,some copperore, rich silver ores of various kinds, iron pyritesand mispickeland in one casea few tons of wolfram. The veinstoneis composedof quartz, chalybite,cal- cite and fluorspar. These occur in successivesuperimposed layers in the lodes,the lead ores being intermediatein age be- tween the first and last of the depositsof the veinstones. The brecciated nature of some of the lodes is a feature worth remarking. In their geneticrelations to the lead lodesof the world they may be regardedas falling into the sulphidicsubgroup charac- terized by quartz, carbonatesand fluorspar.

IRON LODES. Like the lead lodesthe iron ores occur in crosscourses, but one of the most important of them--the Perran iron lode, near New- quay---hasa directionof its own. GEOLOGICAL ASPECT OF LODES OF CORNWALL 379

Little needbe saidof theselodes. They are generallyof great width but somewhatvariable in this. In the upperpart they are brecciatedand composed largely of haematiteand limonite with a little manganeseore, while in depththere is a considerableamount of chalybite. Comparedwith othersources of iron supplythese lodesare of no great industrialimportance.

ALLUVIAL DEPOSITS OF TIN. ThroughoutCornwall and Devonshirethe streamsand upland basinsor peat mossesdraining the mineral districtshave been extensivelyworked in days gone by for tin ore. Lately there has beensome attempt to work the samedeposits for wolfram which like the tin ore occursnot only as a shoadmaterial., but in the alluvial flats where wolfram lodes are found. The best known alluvial depositsare those on the north of St. Austell (the Goss Moor, Criggan Moor, Red Moor, etc.), all of which are wide shallow hollows, excavated in what is tentativelyconsidered by Mr. Reid to be an ancientmarine shelf sharpenedup in Pliocene times at above a level of 400 feet.• The largestof theseflats is the GossMoor, one and one-fourth miles in width and three miles in length. The angle of slopeis about 00-3 ø' towards the stream which drains it so that the gradient is that of torrential conditions. The material in these moors consistsof peat, hard subangularstones and finer sands and clays, the tin ore occurringin channelsat the baseof the deposits. The detritusappears to mergelaterally into the broken decomposedsurface rock which coversthe whole of the west of Englandand whichappears to havebeen formed under special cli- matic conditions,probably similar to thoseof the steppes--cold and dry. As the district has never beenglaciated the superficial fragmental rock has been allowed to accumulate,and under specialconditions has crept or beenwashed by rainsdown the slopesinto hollows,whence much of it has found its way into the moors above mentioned. The alluvial deposits of the rivers draining these flats some- times attain thicknessesof 3¸ , 40 or 5¸ feet and consistof alter- • See Lands End Memoir. Explanatory of sheets35x and 358, •9o7, p. 68. 380 DONALD A. MACALISTER hating bedsof sands,clays and layers of peat. At the estuaries the tin ore has been worked below sea level at the bottom of the ancientsubmerged valleys. In someof the covesround the coast the beach sands have been treated for tin ore. To the strangerthere may be someambiguity when speaking o.f the stream-tin. The stream-tin depositsjust describedare natural alluvial depositswhich have been abandonedfor many years and are only worked occasionallyin a 'smallway; but there is an important industry known as the stream-tin works for sav- ing the tin ore from the tailings or wastethrown into the rivers from the dressingfloors of the mines. As much as œ4o,ooo worth of tin ore a year has beencollected by the extensivedress- ing plants or stream-tin wo.rks situated on the rivers coming from the active mining districtsof Camborneand Redruth.