H. HIOKS ON PRE-CAMBRIAN ROCKS IN THE BRITISH ISLES. 59

The following specimens were exhibited on the table :-1. A large series of rocks from the pre-Cambrian formation of

St. David's Head. Exhibited by Dr. Hicks .2. Cut and polished agates. Exhibited by Mr. Needham.

ON SOKII: RECENT RESEARCHES AMONG PRE-CAMBRIAN ROCKS

IN THE BRITISH ISLES.

By HENRY HICKS, M.D ., F.G.S., V.P., Geol. Association, &c.

It occurred to me that as pre-Cambrian Geology has receivedof late years a considerable amount of attention in this country, itmay interest the Members of this Association to have placedbefore them some of th e results obtained from the more recentresearches amongst the rocks of that age. I will not attempt toreview the history of the earlier researches, which were mainlyconfined to Scotland, as you have already had the advantage ofthe elaborate paper by Mr. Hudleston, where these have been veryfully referred to.

Up to the year 1863, when I commenced to work amongst theolder rocks of St. Davids, the only rocks in England or Waleswhich had been even doubtfully assigned to a pre-Cambrian age,were some crystalline schists in Anglesea, referred by Prof. Sedg­wick to ., a distinct epoch from the other rocks of the district, andevidently older; " hence, doubtless, meaning that th ey were olderthan the Cambrian rocks in the neighbouring mainland inCaernarvonshire.

In his descriptions and sections he referred the schists only, aswas done also in Scotland, to this age, believing that the greatgranitic and felsitic patches, which we now refer also to that age,were composed of intrusive rocks of a much later date.

In the year 1864 Dr. Holl • referred the crystalline schists, form­ing the Malvern Hills, to a pre-Cambrian age; and in the sameyear Mr. Salter and myself described some rocks at St. Davids,which we also assigned to that age. At this time the only rocksmentioned as characteristic of the pre-Cambrian age, in any ofthe areas where they were known, were such as would be included

* "Quart. Journal Geolog. Soc.," vol, xxi., p. 72.

60 H. HICKS ON SOME RESEARCHES AMONG

under the name crystalline schists j therefore, when I commencedmy researches at St. Davids, my difficulties were considerable, forthough the evidence seemed tolerably clear that rocks of pre­Cambrian age occurred there, yet these appeared to presentcharacters so entirely unlike any that had been previouslydescribed as belonging to that age, that for some time I felt itnecessary to speak with considerable hesitation, fearing lest thefurther evidence, which it was thought necessary to obtain, wouldnot bear out the conclusions.

For instance, it seemed difficult to realise that the so-calledintrusive masses of granite and syenite could be metamorphicrocks, and, yet, though they appeared as if penetrating throughthe Cambrian rooks, there was no evidence anywhere of contact­alteration, or irregular intrusion into, or of their having producedany disturbance in the overlying rocks. The only solutionpossible seemed at first to be that they were igneous rocks ofpre-Cambrian age. Further evidence, however, seemed to showthat the latter view could not be the correct one in regard to themajority, and that after all we had here the metamorphic rocks,but unlike in several particulars and in a very massive state, ascompared with those previously found in other areas. Thesepeculiarities became subsequently known as characteristic, to agreat extent, of the Dimetian series, and enabled the groupafterwards to be recognisable by these characters in North Walesand elsewhere.

In addition to the granitic and syenitic ridges, there were manyothers described as composed of intrusive masses of felstones andporyphyries. That these rocks must, in the main, have had anigneous origin, seemed tolerably clear, but, on careful examination,they again did not appear to behave in the usual manner ofintrusions amongst newer sediments. Some also, classified underthese names, it soon became evident were mainly breccias orindurated ashes. The result of continued examination was to showthat the majority of the rocks, placed under these heads, in theneighbourhood of St. Davids, were also of pre-Cambrian age, andthese are now usually defined by us under the name-" ArvonianGroup."

In addition to these there were large areas of rocks described asaltered Cambrian. The alteration being supposed to be chieflydue to proximity to the so-called igneous masses above referred

PRE-OAMBRIAN ROOKS IN THE BRITISH ISLES. 61

to. Now, as these masses could not possibly, according to theview we held, have had any influence whatsoever on newer sedi­ments, it became evident at once that some other reason than thatadduced would have to be sought for to account for the presenceof such an altered series. On examination it was fouud that theserocks were entirely uulike those in the overlying Cambrian group,and, moreover, that whenever the junction between the two couldbe found, they never passed into one another, but that they wereseparated by a great thickness of conglomerates composed almostentirely of masses of rocks identical with those below, belong­ing to the metamorphic series. Hence it was that our third, orPebidian group, came to be differentiated. Such, in brief, may besaid to be the history of the discovery of the three groups at St.Davids j and as most of the data upon which we have dependedin our subsequent investigations in other areas, were derivedfrom these researches at St. David's, I purpose now to refermore minutely to the special or predominating characters exhibitedthere by each group.

DIMETIAN.

The rocks included under this name are coloured on the SurveyMaps as Granite or Syenite, intrusive amongst Cambrian rocks.On examination it is found that they everywhere show moreor less distinct lines of bedding, striking from about N.W. toS.E. They vary also, to some extent, in their mineralogicalcharacters, if examined at different points, and have apparently anorder of succession in which these changes occur at recognisablehorizons. The prevailing rocks in this group are of a granitoidcharacter, usually of rather a massive, but sometimes of a schistosenature.

Sometimes almost pure quartzites are found, but by far thelargest proportion contain an admixture of felspar of a white orpinkish colour. Specks of viridite usually occur more or lessthroughout, and sometimes give quite a tinge to the rock. Micaoccurs sparingly, for the most part, but there are occasionallymicaceous, chloritic, impure limestone and serpentinons bands.At some places also thin, compact, white bands of a more highlyfelspathic character occur. Some of the beds assume a brecciated

62 H. HICKS ON SOME RESEAROHES AMONG

appearance, the masses being generally angular or sub-angular,and in composition much like the associated rocks.

Speaking generally, the majority of the rocks comprising thisgroup are highly quartzose, of a granitoid or rather massivelygneissic nature, and usually easily recognisable by these characters j

their strike is from about N.W. to S.E.

ARVONIAN.

The rocks now included in this group I originally associatedwith the Dimetian, but in the year 1878 I separated them fromthe latter, under the above name.

On the Survey Maps they are coloured generally as felstonesand porphyries, usually intrusive amongst Cambrian or LowerSilurian rocks. They consist in reality of flows of rhyoliticlavas, alternating with felsitic breccias and hiilleflintas. Thestrike is from N. to S., and, hence, discordant to those newerrocks with which they are usually surrounded, as also to theunderlying Dimetians. Like the Dimetian, this is a highlyacid group, being mainly made up of the types of rocks known asthe quartzo-felspathic. But, instead of being like these, chiefly ofclastic origin, we have here a great series of acid lavas mixed upwith a comparatively small proportion only of rocks of a clasticnature. In colour these lavas vary from being very dark (almostblack) to a light grey, and from deep red or violet to flesh-colour.The flow-structure is usually well marked, and in many cases thespherulitic structure also. A large number are porphyritic, fromminute crystals of felspar or quartz. The halleflintas are moresiliceous looking than the rhyolites, and have a horny-lookingtexture and fracture. Under the microscope they are still moreeasily distinguishable. Their chief peculiarity, perhaps, consists inthe manner in which some of the quartz becomes separated awayinto nests, so as to give the rock a curious pseudo-porphyriticappearsnce ; whilst the intervening parts exhibit the appearance ofa micro-crystalline mass of quartz grains, with intervening felsite.The breccias usually consist offragments of lavas and halleflintas,like those in association with them, and the pieces angular.

This group, therefore, is characterised by being for the mostpart made up of- acid lavas, breccias, and compact siliceous rocksof the halleflinta type, and as having usually their strike in adirection from N. to 8.

PRE-CAMBRIAN ROCKS IN THE BRITISH ISLES. 63

PEBIDIAN.

Most of the rocks in this group differ from those alreadydescribed, though occasionally there is a certain amount of re­semblance remaining. Instead of the acid types prevailingas in the previously named groups, we find the basic types morelargely developed. Basic lavas and breccias now predominate overthe rhyolites, and the clastic rocks are more micaceous, chloriticand talcose. On the Survey Maps these rocks are coloured asaltered Oambrian, and partially as intrusive greenstones. On morecareful examination the so-called greenstones turn out to be bandsof indurated volcanic ashes, and contemporaneous basic lava-flows.Agglomerates and breccias occur in great thicknesses in the group,and the fragments are chiefly, except in the lowest beds, of a basiccharacter. Ohloritic, talcose, felspathic and micaceous schistoserocks occur also at various horizons, and occasionally purple andgreen slates. Serpentinous bands are also sometimes found, aswell as veins of jasper, epidote and asbestos. Some of the finerand more quartzose beds assume a gneissose appearance, andothers are porcellanitic.

The strike in this group is from about N.E. to S.W., and hencenearly in accordance with that in the overlying Oambrian rocks. Thatthis group, however, must have been in much the same conditionin which it is now found, before a grain of the Cambrian rocks wasdeposited, is perfectly clear from the fact that the conglomeratesat the base of the latter are very largely made up of rolled pebblesand rounded fragments identical with the rocks below. An actualunconformity between the two groups is also seen at several points.

This group consists of a far more varied series than the twoformer, and doubtless would exhibit a still greater diversity if fullyexposed, for it is perfectly clear that, in consequence of the rapidoverlapping of the sections by the Oambrian rocks, much ishidden from view.

Having thus sketched out the chief results obtained by the ex­amination of these ancient rocks at St. Davids, I purpose referringto some of the other areas, where the facts are almost equallyclear, before referring to those where the evidence may seemto some less conclusive, though, in my opinion, sufficiently satis­factory.

64 H. HICKS ON SOME RESEARCHES AMONG

Hal'lech Mountains.

Here, as at St. Davids, the Cambrian rocks occur in their fulldevelopment, and, until very recently, were supposed to have anunusual thickness. The estimates given had varied considerably, asit was supposed that the basal beds were nowhere exposed. Havingvisited the neighbourhood on several occasions and recognised agreat similarity in the Oambrian series to that I had' made out atSt. Davids, I felt, some years ago, convinced that a carefulsearch along the centre of the broken anticlinal would not onlyshow that the basal beds were there exposed, but moreover thatsome remnants of the old pre-Oambrian land would also be found.Last year, after examining a section I had not previously visited,accompanied by Mr. Spratling, I satisfied myself that myconvictions were fully warranted by the conditions exhibited;and, this year, further explorations along different lines inthis area with Prof. Hughes added numerous important factsconfirmatory of these views. Some of these I now purposereferring to. The Lower Oambrian rocks at St. Davids I havegenerally estimated at about 5,000 feet, whilst the lowest estimatethat has been given of the Harleeh rocks has been over 8,000 feetand from that up to 20,000ft. Now a difference even of 3,000 feetis considerable in areas so near each other, especially when weconsider that the sediments in each case give indications of havingbeen deposited under very similar general conditions. As, more­over, most of the beds are ripple-marked in each area this increasedthickness could only take place if a far more rapid depression tookplace here than at St. David's, with accumulations correspondinglymore rapid also. Looking at the whole series up to the fossiliferousMenevian beds, which, in passing, I may say are curiously likethose at St. Davids, I see nothing to show that the physicalconditions-even after allowing for the inequalities of the surfaceof the old pre-Oambrian land, which it is probable would be nearlyeffaced after depression, and an accumulation of 1,000 feet--wereotherwise than alike in both areas, which together formed but asmall part of a large area of depression extending not only overthe British Isles, but also over the greater part of Europe.

The higher ridges and plateaux would of course remain abovewater in 'the areas where they occurred, and the sediments wouldvary in proportion to proximity to these, but for general purposeswe find that over the Welsh areas these did not greatly interfere

PRE-CAMBRIAN ROCKS IN THE BRITISH ISLES. 65

with the order of succession. Subordinate beds of conglomeratescertainly do occur at St. Davids, as well as in the Harlech Moun­tains, at different horizons, showing, probably, a proximity to smallunsubmerged portions; but these cannot be confounded with thethick massive beds with large pebbles, which, in both areas, occurat the base of the whole Cambrian series. In these beds the frag­ments of the older rocks are sometimes almost angular, many ofgreat size, and all indicating that they must have been shoreaccumulations, and carried but a short distauce.

In this way we reason, and, I think, fairly, that at this periodnew physical conditions on a very large scale were taking placeover the British Isles, such, indeed, as have probably never sincebeen equalled, and that a fresh order of things must, then have setin. It is from this time forward also that nearly all the evidencehitherto relied upon in enquiries as to the real physical history ofthe globe, and of what is generally indicated as geologicalchronology, has been derived. That period which preceded it wasuntil lately looked upon as a blank, but I feel bold enough now tosay that this cannot possibly be the case in future, and that pre­Cambrian time will be found to have a history not less interestingthan that which has succeeded it, and as important to unravel inevery geological sense. The problems to be solved are, many ofthem, difficult, but this should only make us the more anxious toattempt by persistent labours, and by the assistance of the numerousnew aids which science is capable of commanding, to do our bestto solve them.

No better evidence of the character of the rocks which composedthe old pre-Cambrian land can be had than that derived from acareful examination of the various masses contained in thoCambrian conglomerates, for undoubtedly these, at least, musthave been derived from pre-existing rocks. For some time past Ihave been collecting evidence, from an examination of thesemasses from various areas, to arrive at some idea of the pre­vailing rocks which must have been exposed when these weredeposited, as this cannot fail to aid us in our search for the frag­ments of the old land which may now remain exposed. AtSt. Davids bits belonging to nearly all the rocks now recognised aspre-Oambrian may be found either in the conglomerates whichseparate the groups or in those at the base of the Cambrian. Inaddition to these, however, there are frequently masses which must

5

66 H. HI CKS ON SOME RESE ARCHES AMONG

either have been derived from a distance or from some rocks whichare not exposed in th e neigh bourhood at present. Th at the y mayhave been derived from a distance is quite possible, as previous tothe depr ession of the old land th e sur face was doubtl ess part iallycovered by much loose material conveyed from one part to anothe rby th e aid of ice, river s and oth er agencies. A s at St. Davids, soin th e Harlech Mountain s, we find that the conglomerates conta innot only abundance of fragments similar to the rock s below, butalso numerous others whose original locality must be sought forelsewhere. The pre- Cambrian series as exposed here approachesnearer to th at of th e Pebidian than to either of th e other groups,and it is with that group I propose to associate th e chief portion atpresent. The strike is general ly from about N.E. to S.\V., but 'itvaries snfficiently from th at in the Cambrian rocks to enable us torecognise how the overlapping takes place, especially as the dip isusually much higher in th e pre-Cambrian than in th e Cambrian.

Of th e fragments abundan t in th e conglomerates I may men­tion quartz felsite s and felspathic schists , and purple and greenslates and shal es, all of which are found in situ in th e im­mediate neighbourhood. Of oth ers not found at hand are gr aniteand gneiss rocks, mica-schists, talcose and chloritic rocks, quartzites,vein quartz, jasper, &c.

Caernarvonehire.

The next areas which I shall refer to are in Caernarvonshire­and here, again, it will be seen that the Cambrian and pre­Cambrian rocks occur in association. One of the se areas I de­scribed in a paper to th e Geolog ical Society in 1877. It ext endsfrom th e neighbourh ood of Moel T ryfaen by Llyn P adarn in aN.E. di recti on to Bethesda. In it are found quartz -felsites,rh yolites, felspathic breccias, and schis tose rocks, similar to th osedescribed in the Arvonian group at St . Davids ; but along theflank of these other rocks occur, which are more like members ofthe Pebidian group, and in a pap er in 1879 I endeavoured toseparate the two groups. In tracing the Cambrian conglomerates,which occur almost continuously along the sides of thes e rocks, wenotice particularly that upon whatever series they rest, a propor­tionate increase of pebbles identical with the rocks of that seriesare found. For instan ce, at Moel Tryfaen, where schistose rocksmainly occur, the pebbles in th e conglomerates are chiefly of a

PRE-CAMBRIAN ROCKS IN THE BRITISH ISLES. 67

schistose natnre j whilst at Llyn Padarn, where quartz-felsitesand rhyolites occur, the pebbles are very largely made up ofrocks of that character. This indicates, to my mind, that we havehere undoubtedly an old coast line, and that these were the truebeach accumulations on that old shore.

Another important area in Caernarvonshire was partially re­ferred to by me in my paper in 1877, and also by Prof. Hughes inhis paper at the same meeting. It extends in a N.E. directionfrom Caernarvon to Bangor. At the former place granitoid rocks ofthe Dimetian type, with a N. W. strike, are found; between theseand Bangor quartz-felsites, rhyolites, breccias and hallefiintas, likethose in the -Arvonian group at St. Davids, occur, and with a N.and S. strike. Nearer to, and about, Bangor, agglomerates, breccias,porcellanitic rocks, chloritic schists, &c., are found, which havea N.E. and S.W. strike, and which are identical in character withmany of the rocks found in the Pebidian group. Cambrian con­glomerates are found resting on the different groups at variouspoints, and, like those in the other areas referred to, containmasses of these in proportion to their proximity or otherwise tothe several series.

Here, therefore-in two areas, at least, in Caernarvonshire-c-wehave, it appears to me, very strong proof of the presence ofportions of the old pre-Cambrian land, and of the accumulationswhich took place around its shores.

I may now, perhaps, presume that the evidence which I havebronght forward, and which I might have greatly strengthened byreferring to the very careful microscopical labours which havebeen bestowed upon these rocks by Mr. T. Davies, and by Prof.T. G. Bonney, will be considered sufficiently convincing by mostpersons in regard to the areas already referred to.

I fear, however, it will not be considered equally clear in regardto some of those which I purpose now referring to, foralthough the evidence to my own mind might be equally satis­factory, yet it frequently happens that it cannot be readilydemonstrated. Moreover, cases arise where the facts are ample torender their pre-Cambrian Age probable, though the evidence maynot be considered conclusive.

In my paper in 1870 I mentioned several areas in the Lleynpromontory, where I considered pre-Cambrian rocks were to beseen. In the mountains called the Rivals, I recognised rocks so

68 H. HICKS ON SOME RESEARCHES AMONG

identical in character with those at Llyn Padarn, Moel Tryfaen,and neal' Bangor, that I ventured to associate them with theArvonian rocks of thc pre-Cambrian. I did this, moreover, onother grounds besides identity in mineral characters. In examiningthe surrounding rocks, I found that all were of the type ofsediments known as dark ferruginous slates and flags, socharacteristic of the late Oambrian or very early Silurianperiod, as generally exhibited in North and South Wales,whilst in the Rivals themselves nothing but rocks of a highlysiliceous type occurred. Moreover, it was soon seen that thelatter consisted largely of rhyolitic lava-flows and breccias, andthat they conld not have been intruded into, or have been inter­stratified with, the surronnding argillaceous slates and flags.One important fact which contributed to my arriving at thisconclusion was that even on the Survey Maps they had found itimpossible to show these and other masses in the area to be inter­stratified with the slates, as they had done with some in theArenig and Bala districts, but had been compelled to indicatethem as intrusive masses breaking through the slates. Anotherimportant fact arrived at during this examination was that someof the surrounding slates were found to contain fossils belongingto the Tremadoc group, and hence to be considerably olderthan previously supposed. As no contemporaneous lavas ofthat age were known anywhere else in North Wales, thisevidence seemed of some importance. Moreover, the intrusivetheory could not possibly account for the conditions observed.There was no contact-alteration to be seen, no irregular penetra­tion, and the rocks themselves, for the most part, were not of thetypes considered usually as intrusive. They have been, in somecases, traversed by dykes of a later date, and by someothers which I believe to belong to pre-Cambrian times, butwhich cannot be easily differentiated from intrusive rocks of a laterperiod.

The only theory which seemed to me, therefore, possible wasthat these rocks, so identical in their mineralogical and physicalcharacters with those found elsewhere, as typical of some pre­Cambrian groups, must also belong' to that age, and that thenewel' rocks which were now in contact with them had beenbrought into that position by faults. In many cases, also, therewas clear evidence to show that the junctions were lines of

PRE-CAMBRIAN ROCKS IN THE BRITISH ISLES. 69

fault. Some, however, may raise objections to this theory, andargue that the faults necessary would be enormous. To this Ireply that faults of equal and greater magnitude are quitecommon in the other pre-Cambrian areas, and that it is not at allunusual to have one edge of the pre-Cambrian fragmentcovered with Lower Cambrian rocks, whilst the other edge is incontact with those of Lower Silurian age.

If we remember that the pre-Cambrian rocks before they weredepressed to receive any of the Cambrian sediments were notonly in an indurated state, but also highly metamorphosed, wemay readily see how it must have taken place, that during thevarious changes to which the crust has been subjected sincethat time, fracture after fracture much have occurred-sometimesparallel to, sometimes across the bedding, and sometimes evenredisturbing connections previously produced, perhaps, by thesame cause.

A clear recognition of the probable effects produced by therepeated contractions of the crust, on these older metamorphicrocks, covered, as they were, by great thicknesses of super~

incumbent sediments, is, perhaps, of all things, the most neces­sary when we commence to explore amongst them.

::;ometimes a small fragment only of the old land is seen;bounded by faults and surrounded by comparatively recent sedi­ments. In another place a large piece occurs, and the newerrocks have been dropped into the midst of it at different points, alsoas the result of faults. Such occurrences as these must alwaysrender the explorations difficult, but certainly not less inter­esting to the really enquiring geologist.

Towards the point of the Lleyn promontory a considerable area iscoloured on the Survey Maps as altered Cambrian, bounded on theeast by Syenite. Our examinations soon demonstrated that theso-called Syenite was in no way the cause of the alteration in theassociated rocks, but that it really was itself of metamorphicorigin and of Dimetian age, and that the so-called altered Cambrianrocks consisted of a series of breccias, volcanic ashes, and schistsmainly of the type of the Pebidian rocks at St. Davids.

The condition of things in this promontory was found to be sosimilar to that in the promontory of St. Davids that it could notfail at once to suggest the correlation of two of the groups. Onfurther examination, rocks of theArvonian type were also found i

70 H. HICKS ON SOME RESEARCHES AMONG

SO we have here again the three pre-Cambrian groups repre­sented in a comparatively small area.

Anglesea.

As already mentioned, some of the schists in Anglesea were,many years ago, described by Prof. Sedgwick as forming the baseof the Cambrian series, not perhaps as a pre-Cambrian group,but at all events as the oldest rocks in the district.

The Geological Surveyors, however, did not agree with this view,and arrived at the conclusion that these schists were merely thealtered representatives of the Cambrian, and probably in part ofthe Silurian, rocks found on the mainland in Oaernarvonshiro, Myrecent explorations amongst these rocks compel me to agree ratherwith the views of Prof. Sedgwick than with those of the Survey;but I have ventured to go farther, and have included amongst thepre-Cambrian rocks others which had been previously supposed byall geologists who had explored them to be intrusive rocks of amuch later date. I have also obtained much evidence which hasenabled me to attempt to correlate the several series found inAnglesea with those in the other areas described. Near thecentre of the Island, running in a north-east direction, a largepatch is coloured as granite in the Survey Maps. ThisI found, in 1878, to be composed mainly of a series ofgneissic rocks, similar, for the most part, to those composing theDimetian group in Caernarvonshire and at St. Davids. It couldnot possibly, therefore, be an intrusive granite of later date thanthe surrounding series, as snpposed by the Survey, nor could ithave in any way tended to produce the metamorphic change inthat series supposed to have been induced by it. On the one side,also, Silurian rocks were found in an entirely unaltered state, andmore recently Prof. Hughes has been able to prove conclusivelythat some of these unaltered sediments must be Lower Tremadocrocks, and hence of Upper Cambrian age. My interpretation atpresent of the older rocks in Anglesea is that the granitoid gneiss,already mentioned, is a part of an important axis, in which theoldest rocks in the island are found-that along one side this isflanked by rocks .of the Arvonian type, and that these aresucceeded by the schists and breccias, which have .a N.E. andS.W. strike, and which are, for the most part, of Pebidian age.

PRE-CAMBRIAN ROCKS IN TIlE BRITISH ISLES. 71

That there are at least two other axes of older date thanthe rocks with the N.E. strike is more than probable;one of these occurs in the neighbourhood of Holyhead andanother near the Menai Straits. Here and there Cambrian andSilurian rocks seem to have been dropped amongst the olderrocks by faults, as in the Lleyn promontory, but these aregenerally easily recognisable, even when not fossiliferous, bytheir unaltered condition, and by being chiefly of the type of theargillaceous sediments characteristic of the Cambrian and Silurianrocks on the mainland. That the metamorphic rocks in Angleseaare all of the types of rocks recognised elsewhere as typical ofthe pre-Cambrian rocks, there cannot be a doubt. Moreover, Iventure to state that there is hardly a rock amongst them ofwhich I have not seen specimens in the Cambrian conglomerates.The rocks of Dimetian type have the normal strike of that group.Those also which resemble the Arvonian have their strike, and thevery varying series characteristic of the great Pebidian groups havethe strike everywhere more or less evident in it. That some ofthe rocks found here have not been discovered in the other areasis not at all to be wondered at, for in no place can it be said thatwe have seen them in their full development-especially as theyare nsually quickly overlapped by the Cambrian rocks. Thatthe highest series, however, has the normal strike of the Pebidiaugroup, and that fragments of every type in that series arefrequently found in the Oambrian conglomerates, are, to mymind, very strong proofs that it belongs to that periodin the history of the globe, when the Pebidian rocks weredeposited.

Shropshire.

In the year 1877, the important announcement was made byMr. S. Allport that the Wrekin ridge and another area to theN.W., coloured as greenstone in the Survey Maps con­sisted almost entirely of a "series of ancient vitreousand semi-vitreous lavas, with their associated agglomeratesand ashes," and that the term greenstone was the mostinappropriate that could possibly be applied to them. He alsosays that they" all belong to a highly acid type, and have not theslightest resemblance whatever to greenstone or any other rocks of

72 H. HICKS ON SOME RESEARCHES AMONG

the basic series." In regard to the age of the series he supposedthem to belong to the" older contemporaneous volcanic series, soextensively developed in the Lower Silurian district of Salop andRadnor." In the same year, Dr. Callaway announced that a"part of the so-called quartzite betweeu the shale (Shineton) andthe Wrekin represents the Hollybush Sandstone of Malvern," andthat he had" satisfied himself that the so-called greenstone of theWrekin and neighbouriug areas is largely composed of beddedrocks." Since that time, Dr. Callaway has paid much attention tothese rocks, assisted by Prof. Bonney, and the result has been thatthey are now classed with the pre-Cambrian rocks. Hithertothey have described two groups only from that area-viz., theDimetian and Pebidian-but after carefully examining many ofthe specimens collected, and also the rocks in the field, I feeltolerably convinced that there are representatives of the threegroups, and that some of those now classed by them with thePebidian must, on physical and mineralogical evidence, be assignedto the Arvonian.

The Malvern Hills.

Since Dr. Holl published the results of his labours in 1864, verylittle additional information concerning the rocks composing thesehills has been forthcoming. Last year, however, Dr. Callaway an­nounced, at the field meeting of this Association, that he had re­cognised, in a spur from the Herefordshire Beacon towards theS.E., a series of rocks very unlike those composing the mainportion of the ridge, and that they were in appearance much likesome of the Pebidian rocks which he had seen at St. Davids. Incompany with many members of the Association, guided by Dr.Callaway, I had the pleasure of examining these rocks in the field,and, I may say, formed at once the same conclusion that Dr. Cal­laway had-that they must belong to that group. Since then hehas published the results of further examinations, which are inevery way confirmatory of his previous conclusions. As to therocks which compose the main portion of the ridge, I can atpresent only say that, on the whole, they more resemble theLewisian rocks of the N.W. of Scotland and the Hebrides than allYother rocks I am acquainted with ; but that, withal, it is probablesome representat.ives of the Dimetian rocks are also be foundthere.

PRE-CAMBRIAN ROCKS IN THE BRITISH ISLES. 73

Charmoood Forest.

The older rocks in this area have of late years been very care­fully studied by Messrs. Hill and Bonney, of Cambridge, and, asthey are now supposed by these authors to be "probably pre­Cambrian," I venture to refer to them. In their first paper theyassociated them with the Lower Silurian rocks. At that time,however, I ventured to suggest that they would ultimately turnout to be of pre-Cambrian age, from an examination of the speci­mens collected by those authors.

In this area the rocks are described as consisting of " slaty andgritty beds, of thick masses of coarse agglomerate of a rhyolitictype, though less glassy than those which occur in the W rekin,and of still more extensive deposits, which seem to be composed ofvolcanic materials slightly rolled and arranged by water." By theGeological Survey these rocks have been called Cambrian; but, asI have already shown in other areas, it will be observed from theabove descriptions that they are in no way like those rocks, butresemble in a very marked degree some of the pre-Cambrian rocks,especially those belonging to the Pebidian Group.

Cornwall.

For some years past I have believed that pre-Cambrian rockswould be found in Cornwall, and it appears !lOW that this is likelyto be proved ere long to be a fact. The researches of Mr. Collinsin the Meneage Peninsula, and of Mr. A. R. Hunt along the coastof South Devon and Cornwall, have brought to light some newfacts which are, in my opinion, almost conclusive of the presence inthose areas of rocks of pre-Cambrian age. I must, however, statethat neither of these authors have as yet committed themselves tothe view that these are pre-Cambrian rocks. Still, Mr. Collins hasstated that there are some of "pre-Silurian age," and Mr. Hunthas granted the possibility of the rocks he has examined being pre­Cambrian. I have seen some of the specimens collected by the latter,and believe that among them there are types only known in the pre­Cambrian rocks in this country. At present, however, it would be un­safe to theorise upon the evidence available, but no harm can arisefrom referring briefly to the facts. It is well known that the Eddy­stone Lighthouse is built upon rocks of a gneissose type-somemassive, others more schistose. These gneisses appear to be of the

74 H. H ICKS ON SO}IE RES E ARC HE S AMONG

true type of the older or granitoid kind , hence rend ering it pro­bable that, in th e Channel at least , a rid ge of th ese older rocksoccurs . Mr. Hunt, when reading his recent paper at th e BritishAssociation, exhi bited num erous masses which he had dr edged upoff th e coas t of South Devon and Cornwall , and amongst th em avery large numb er were found to be gneiss rocks. Some pieces ofa conglomerat e were also found, and in the se distinct fragment s ofolder rocks occurred. Th ese, as sugges ted by Mr . Tawn ey-towhom th ey were first submitted-appeared so exceed ingly like aconglomerate near Cae rna rvon, in North 'Vales, tha t it would beimpossible almost to distinguish bits of th e one from the other. Thehornblendic and serp entinous rocks of Cornwall are so well-knownthat I need only refer again to th e fact that Mr. Collins has separ­ated them from the Silu rian rocks, and that he believes the latterare unconformable to, or faulted against, the former.

If this be the case, it seems to me only reasonable that we shouldassociate them with the metamorphic pre-Cambrian rocks which th eymost resemble, rather th an with th e Silurian or Cambrian rocks,which, as we hav e already shown, are usually so entirely unlikethem.

S cotland-s-North- JVest Coast.

But few researches seem to have been made amongst th e pre ­Cambrian, or so-called fundamental gneis s, rocks in Scotland oflate years, and in regard to some areas, such as the Hebrid es andthe ex t reme North-W est Coast, little is known beyond what is givenin th e wall -known papers by Pr of. Nicol and Sir R. Mur chison.

H aving, however , on different occasions visited some of theother areas, I purpose referring briefly to the results obta ined, andthe conclusions arrived at . Th ese, I fear, will not be foun d toaccord in some important par ticulars with the views of previouswriters . Some of th e results have been alr eady pu blished,· butothers have been eith er but briefly touched upon or not re­

.ferr ed to at all. Th e so-called fundam ental rocks along th e North­W est Coast of Sutherland and Ross have been by mostauthors considered as belonging to one group, and designatedunder the names of Laur enti an, Lewisian, or Heb ridean. Inexamining some of th ese areas, particularly about Ga erloch in Ross,I was struck with the general wan t of resemblanc e of the rocks to

• "Quart. Journal Geolog. Soc.," 1878, and "Geolog. Mag.," 1880.

PRE-CAMBRIAN nO CKS IN THE BRITISH ISLES. 75

those which hav e been usually described as chara cteristic of th oseareas. I nstead of the very massi ve granitic gneisses and hornbl endicrocks commonly mentioned, I found th e majority to be rather t hin­bedded schi stose rocks, t he gneisses to contain usually black micain stead of the hornblende described as characte rist ic of them, and tobe, on the whole, far more quartzose than was usuall y supposed tobe th e case. Moreover , chloritic and other distinc t rocks occurredplentifully in some places. That all these also were undoubtedly ofpre-C ambrian age was rendered perfectly clear by their positionand by the finding of abundan t fragm ents in th e immediately over­lying Cambrian conglomerates and breccias .

It seemed to me, th erefore, probable that two distinct groupsat least are represented in th ese western areas, and th e micro­scopical examinations of th ese rocks which I have been able tomak e, with the valuable assist ance of Mr T. Davies, has tendedstrongly to confirm th ese views. I believe, moreover, that th ere isa discordance in the strike of th e two groups. The oldest of these­consisting of the massive horubl endic g neisses with a west-north­west strike (usually)-1 would refer to the Lewisian group, whilstth e more quartzose or gra nitoid types, with some associated schists,with a N.N.\V. strike, I would refer to a newel' group, and I fleeno reaso n at present why they may not be associated with th eDimetian, which of all g roups at preseut known they most resemble.

As in \Yales, so here t he Cambrian conglomerates which rest uponth ese old rocks must be looked upon as offering most valuable evi­dence of the condit ion of th ings when they were deposited . They nowrest almost horiz ont ally upon the old floor, and the unevennesses ofth e su rface of th e latter are much as they were pri or to th e de­position of the overlying sedime nts, and are filled up usually by thelatter. I noticed at once the ge nerally more brecciated characterhere of th ese Cambrian sediments th an in the majority of th ose in\V ales; but, in both , the fragment s are frequently so angul ar th at Ifelt compelled to believe th at some agent iu addi tion to ordinarymarine action had aided in heaping them up. In a form er paperalso I ventured to suggest icc as the probable agent, and more re­cent ly I find Prof. Geikie has suggested the same. Th ese facts areint eresting as indicating some of the physical conditions of th e sur­face of the old pre-Cambrian land, and as I have shown in formerpapers that volcanoes also were stu dded over th e surface, and th atrai n and river action was evident ever.ywhere, we seem to be tolerably

76 H. HICKS ON SOME RESEARCHES AMONG

able to arrive at some idea of its physical condition. That its generalsurface was very similar to what might have been observed inseveral succeeding epochs, and that the atmospheric conditions werealso very much alike, seems highly probable. It appears to methat the chief blank in the history still remaining is as to the stateof vegetation and of animal life at the time. That such a landsurface must have been covered by vegetation seems more thanprobable, and that animal life had attained to a high degree of per­fection is clear from the abundance of life which we know to haveexisted in the surronnding Cambrian seas. The various fragmentswhich make up these breccias and conglomerates tell us clearlywhat rocks must then have occurred in that or some adjoining areas,and their condition. I have recently paid considerable attentionto these fragments, and I find that they belong to several types.We have the older gneiss rocks frequently represented; but, in addi­tion to these, some slaty and schistose masses, certainly very unlikethose usually associated with the older gneiss groups. For instance,micaceous, talcose, chloritic and purplish slates and schists are found,such as hitherto have been looked upon as characteristic only ofgronps supposed here to be newer than the conglomerates. Thatsomeof these may have been derived from rocks in intimate associa­tion with the older gneisses may be possible; but if so, then I holdthat most of the evidence hitherto relied upon to separate the so­called newer metamorphic rocks from the older is of no avail, andwill have to be given up. In addition to the above-mentioned frag­ments, bits of granite, qnartz-felsites, quartzite, quartz and jasperalso occur, though not so abundantly as in some of the Welshareas. This evidence, it appears to me, conclusively shows thatrocks of the character of the fragments found in any of the ad­joining areas may, lind probably are, of pre-Cambrian age, if thereis not very strong evidence to the contrary.

In the Geological Survey Maps, all the metamorphic rocks to theeast and south-east of a line extending from Loch Carron, by LochMaree, to Loch Eribol are coloured as altered Silurian. In theyear 1878, after carefully examining some sections in these areas, Iarrived at the conclusion that many at least of these so-calledaltered Silurian rocks were of pre-Cambrian age, and that therowas no evidence whatever to show that they were, as supposed,newer than the Cambrian conglomerates. I endeavoured to showthat in some places the supposed passage was due to a deceptive

PRE-CAZ,IRRIAN RO CKS IN THE B RITISH ISLES. 77

overlap, and in others to faulted junctions. Moreover, I describednumerous rocks from the central areas, to show that they differedin no particula rs from many of th e rocks foun d in the westernar eas immediately under th e conglom erates. In th e early part ofthis year I again visited some areas fur ther south, in a line withtho se I had previously examined, and the results throughout wereonly confirmatory of my pr evious supposit ions.

SOUle of these I purpose now referring to. The sections I hadpreviously examined were in lines nearly E. and W. acrossRoss Shire, from the coast about Gaerloch and Torridon, by LochMares, to the mountains about Auchnansheen. The lin e I took thisyear was considerably fnrther south, commencing in the mountainsat th e head of Loch Shiel in Inverness, by Loch Eil, to FortWilliam, and afterwards southward across the Grampians in twodifferent directions.

Glen Finnan, Loch Shiel, to the Caledonian Canal.

From the descriptions which had been given by Sir R. Mur­chison and ot hers of this line of section, I felt that no better onecould be selected for testing the accur acy of my previous sectionreferred to, from Loch Maree to Ben Fin, exactly due north of thisline. I was, however, well aware that I should not meet here withthe newer rocks found in th e western portion of that secti on, andwhich had a strike from N.E. to S.W. j but I anti cipat ed th e pro­bability of meeting with newer rocks further east, if, as I had sug­gested, the rocks of Ben Fin and to th e South formed a portion ofan axi s of pre-C ambrian rocks, and in this I was not dis­appointed.

In the mountains at the head of Loch Shiel the rocks aremostly rath er massive gneisses, and much like some of thosedescribed by Mr. Davies and myself in the" Geological Magazine"from Gaerloch and Ben Fin. I have submitted all thespecimens collected to MI'. Davi es, and he entirely con­firms my diagnoses of th em. Th ey are true gneisses, in whichthe minerals are all intimately crystalline, and they con­tain two felspars, usually white, but frequently of a pink orreddish colour, someti mes two micas, the predominant one beingbiotite, a fair proporti on of quartz, and some sphene and garnet.Alternating with th ese are some thick horublende bands, with a

78 H. HICKS ON SOME RESEARCHES AMONG

gneissose foliation , and containing usually a small proportion ofmica, quartz and felspar, and an abundance of sphene and garnct ;also. black mica schists, and a gneiss in which there are largelenticular masses of mica. Some of the beds are much contorted,but in the more massive ones the strike is clearly seen, and it isg enerally in a direction from about N. W. to S.E., and hence in agree­ment with that prevailing in the true gneisses at Ben Fin and in th e\Vestern areas. At no point in this area did I find any rockwhich was not completely crystalline, and in walking over themountains towards Loch Eil I found the same st rike prevailing,and the same alternations of rocks as tho se above menti oned until1 reached Glen Fionn. It is impossible to conceive that the serocks can be altered Cambrian and Silurian sediments, as suggestedby Sir R. Murchison and Prof. Geikie, or that they can have beenchanged into this condition since Silurian times by any process of"elective metamorphism. Indeed, the evidence is now almostoverwhelming that no such great changes have been produced inany sedimentary rocks of great thickness, or occupying a largearea, which have been deposited since pre-Cambrian times. Imaintain, therefore, that these rocks about Glen Shiel are un­doubtedly pre-Cambrian, and that they cannot in any way bedifferentiated from the pre-Cambrian rocks along the west coastabout Gaerloch, either by their mineral or physical characters.·At Glen Fiona, at the head of Loch Eil, a mass of granite isseen, apparently striking in a N.E. direction, and immediatelybeyond this, and reaching for some distance towards Fassfern, greygneisses prevail. These differ considerably from those fartherwest, and strike from N.E. to S. W. with an average dipof about 45° to the N. W. 'I'hey alternate with a corrugatedmica-schist and with quartzose bands which are spotted witha greenish micaceous mineral, These I look upon as newerthan the Loch Shiel series, and probably faulted down against the

• In the excellent collection of rock specimens in the Jermyn Street}luseum, which I have had the pleasure of repeatedly examining, throughthe kindness of Mr. Rutley , some good specimens of granitoid gneissesoccur, labelled Strontian, Argyleshire. This place is nearly du e South ofLoch Shiel, and it is evident that they belong to the same axis of pre­Cambrian rocks. Others in the same Mnseum from Loch Duich show theircharacter further north. 'I'he specimens from these several points, wbichare all true gneisses, prove beyond doubt that there is a great thick.ness of su ch rocks repeated in folds running partially across the axis,and that th ey are not a local development dependent upon some specialcause.

PRE-CAMBRIAN ROCKS IN THE BRITISH ISLES. 79

latter. They should probably be classed with the Pebidian rocksof Anglesea, and with others to be further referred to in the morecentral parts of Scotland. At Fassfern they are overlaid byquartz rocks, dipping at a very low angle to the S.E. Thedetrital origin of these is still most marked, and they may be saidto be comparatively unaltered, though the fine admixture of quartzgrains with bits of felspar may at first lead one to think that therehad been considerable change. They seem here to occupy a depressionsimilar to the one on the opposite side of the axis, and they may be,in my opinion, the equivalent beds to those fonnd underlying thelimestone at Glen Laggan, and which are found at several pointsalong the west side to curve upwards, as at Loch Doule, LochCarron, &c., to overlap the axis on that side. Between Fassfernand Kilmallie they bend round so as to dip to the N. W., and nearthe latter place again overlap some grey gneisses and mica schists.Some of these, as seen in a quarry on the roadside west ofKilmallie, appear to be a kind of talcose gneiss, but the majorityare grey, highly micaceous, thin bedded gneisses and mica schists.I look upon them as forming a portion of the same series as thosedescribed between Glen Fionn and Fassfern. At Banavio, onthe side of the Caledonian Canal, there is an exposure of a graniticrock, consisting of coarse-grained red felspar, chiefly orthoclase,quartz (sometimes in continuous bands) and a soft greenishmicaceous mineral. This mass appears almost identical incharacter with that described in my previous paper (and morefully since by Mr. Davies in the "Geological Magazine") fromGlen Laggan, Loch Maree. As there, also, its general strike isfrom N.E. to S.W., and the quartz bands seem also to arrangethemselves in that direction, as in the case of the Glen Lagganmass, I look upon this as primarily an intrusive rock, but ofpre-Cambrian age, and hence to be classed properly with rocks ofthat age. Along the line described some dykes of diorite werealso met with.

Fort 'William and Glen Nevis.

Immediately nnder Fort William, and extending in a line fromN.E. to S.W., chloritic schists and rather massive chloritic rocksare found. They dip at a high angle to the S.E., and attain inplaces a very considerable thickness. In the mountain to the S.E.

80 H. HICKS ON SOME RESEARCHES AMONG

towards Glen Tarbet they are overlaid by quartz rocks, and theseare succeeded by a black calcareous shale, which decomposes veryreadily where exposed, leaving on the surface some very beautifulcrystals.'*' Resting on the latter are dark slates and shales, pene­trated by dykes of a greenish grey felspar-porphyry. I was guidedover this interesting section by Mr. Livingstone, of Fort William.A similar succession to that just described is met with also inmen Nevis. The chloritic rocks, with some alternating bands ofcompact quartz rocks and quartzose schists, are found at theentrance, and these are succeeded by the quartzites towardsClaggan, the calcareous shale and slaty series being well exposedin the hill S.W. of Glen Nevis House. Though the strike in thechloritic series is almost identical with that in the overlying shales,slates and quartz series, I yet believe that the latter rest uncon­formably upon the former, which, from their general appearanceand their state of alteration, I am inclined to associate withrocks found in other areas in the Pebidian group of the pre­Cambrian rocks. I believe that there are some important faultsalso in this immediate neighbourhood which have somewhat inter­fered with the succession, for the newer beds are tilted up to anunusually high angle, and stand apparently endways, against thebase of Ben Nevis.

Ballachulish, Glen Coe, and Black Mount.

In travelling along the road from Fort William to Ballachulishthe chloritic rocks already mentioned are found to extend con­tinuously to the entrance into Loch Leven. Here they are coveredby a series of quartzites which dip at a low angle to the S.E., andthese are again succeeded by slates and shales.

The section along the south side of Loch Leven is a most in­structive one, and shows clearly that the Silurian rocks are here

* I have submitted this rock to Mr. Davies for examination, and hewrites concerning it as follows :-" On prolonged digestion with acid thewhole of the lime carbonate is broken up, the resulting rock being a softmuddy shale which is apparently somewhat carbonaceous, and containsmuch of a eolourlesa talcose or micaceous mineral. The diverging blade.shaped crystals have been originally tremolite, but they are now whollyreplaced by talc and a little calcite. I am inclined to think that the cal.careous portion is not of subsequent introduction, as the shale, after thedestruction of the calcite, becomes soft and easily pulverizes between thefingers to an almost impalpable mud. It has not suffered much changesince its deposition."

PRE-OAMBRIAN ROOKS IN THE BRITISH ISLES. 81

contained 1n two distinct basins. The section westward from theBallachulish Hotel to the Ardsheal Peninsula has been carefullydescribed by Prof. Harkness." He describes the "syenite" as"flanked on the coast by metamorphic rocks in the form of quartzrocks and limestone. On the east of the Ardsheal Peninsula thesyenite is seen forming the hill of Benivair, against which, on the.west, there occurs a mass of quartz rocks, which, when in proximityto the syenite, contains crystals of felspar, and becomes almost aporphyry. On its N.W. side it becomes more distinct inits stratification, and is succeeded by a grey Iimestoue about twelvefeet in thickness. Above this limestone the dark grey slates areseen dipping in the same direction, but on passing over the Ard­sheal Peninsula the inclination of these slates becomes reversed.On the western side the quartz rocks again make their appearance,dipping under the slates, and these quartz rocks repose againstanother mass of syenite which forms the extreme west of theArdsheal Peninsula." The above description shows clearly thatwe have here a true synclinal fold of the quartz rocks, limestoneand slates. Prof. Harkness believed that the " syenite" here wasan intrusive mass, and that it altered the overlying sandstones.From what I have already mentioned of the section at Loch Eil,it will be gathered that there is the possibility that, like the granitemasses there, it is of pre-Cambrian age, and that the sandstoneswere really deposited upon it, and that the crystals of felsparmentioned in the quartz rocks were derived from the denudation ofthe granite. It is coloured as granite in Prof. Geikie's map, andit is described carefully by Mr. J. A. Phillips in the" Quart. Journ,

Geol. Soc." t On examination I could find no. evidence of itshaving produced alteration in the Silurian rocks, and the includedmasses so plentifully met with in it are fragments from the oldgneiss group, being usnally bits of gneiss with biotite and sphene.The sections along the shores of Loch Leven towards Glen Coeshow that there is here another well marked synclinal fold. Onthe north shore between the Ferry and Onich the quartzites andsucceeding beds are clearly seen, dipping to the S.E. for some dis­tance, and eastward, opposite the Ballachulish slate quarries, thedip is reversed to the N. W. in the slates, and beneath these againin the quartzite group.

* .. Quart. Journ. Geol. Soc.," Vol. xvii, p. 266.t Ibid, Vol. xxxvi, p. 14.

6

82 H. HI CKS ON SOME RESEARCHES AMONG

On the south side a similar order is clearly traceable aR we traveleastward from the Ballachulish Hotel, the dip at first being to theS.E., and afterwards reversed to the N. W. The slates here arenot more highly altered than are some of the Arenig slates inWales, and I was tempted to spend some time in searching forfossils, but, unfortunately, to no avail. Indeed, tbe whole of therocks exhibited in this trough are so exceedingly like the lowerSilurian rocks of ·Wales, that one is almost inclined to classifythem with either the Arenig or Llandeilo groups. They bave beentraversed by numerous dykes of igneous rocks, and approach thegreat granitic masses, and yet we see no evidence of conversioninto true gneiss rocks. How this fact is to be reconciled with theviews held by Sir R. Murchison , Prof. Geikie, and others, that thewhole of the metamorphic rocks in the North Western and CentralHighlands, including true gneiss rocks, &c., are only alteredSilurian rocks, I am quite unable to conceive. We have in theseall kinds of detrital materials, from quartz, mica and felspar tomixtures of these with argillaceous deposits in very varyingdegrees, and yet there are no indications anywhere of conversioninto gneiss. There are beds with alternating Iaminas of variouskinds of deposits, but these show no true change, and nothingapproaching to that complete change or crystallisation throughoutin all the minerals which is characteristic of true gneisses. Onentering Glen Coe we lose the quartz rocks suddenly, as if theywere faulted against or overlay unconformably the rocks which lieto tbe east, and occupy the glen. In the glen, as we ascend,we meet with numerous bands of dark green igneous rocks,probably basaltic lavas, some banded felsites, porphyries, and agranite like that at Ballachulish. Alternating with these aregneiss rocks and jaspery-Iooking bands. There are also somebrecciated rocks . The whole group calls to mind the Arvonianrocks of North and Sou th Wales, and the resemblance to these ofBorne of the rocks is most remarkable. I think there can be nodoubt that, after excluding some recent dyk es, we bave here apre-Cambrian group of lavas, breccias, hiilleflintaa and gneissrocks, upon which the Silurian rocks bave been deposited uncon­formably. In the Black Mount area granites and gneisses prevail.The granite here is much like that met witb at Banavie and alongthe Caledonian Canal, and again recall s strongly the granite ofGlen Laggan, Loch Maree, It contains the same" large grained

PRE-CAMBRIAN ROCKS IN THE BRITISH ISLES. 83

crystalline association of quartz, and a reddish orthoclase withsome microcline," and "the same greenish, soft and talc-like"mica. It is also frequently banded. In addition to the graniteabove described, other varieties are also met with-some large­grained with a white felspar, and some fine-g-rained of a lightpinkish hue. There are also some granitoid gneisses, and adark fine-grained moderately massive gneiss is very prevalent inassociation with the granites between the Black Mount and LochTullich. According to Murchison the gneiss is ., occasionallyhornblendic," and Nicol mentions along the same line, "gneiss,qnartz rocks, and mica-slate, the last often containing hornblendecrystals." All these beds are highly inclined and sometimes con­torted. On the road between Loch Tullich and Tyndrum, nearOrchy Bridge, an entirely different group of rocks is again metwith. They are called by Murchison and Geikie "quartzose flag­stones," and are stated to dip to the N.W. at an angle of 15°-25°here, and in the adjoining mountain of Ben Do at from 10°_] 5°,and afterwards to bend round so as to dip to the S.E. at a" higherangle than on the north-western side." It was impossible not tosuspect at once in examining this area, and in tracing the con­tinuous line exhibited by these sandstone beds from mountain tomountain to the N.E., that we hall here a broken anticlinal ofthese rocks, and that the granites and gneisses of the Black Mountarea and the rocks of Gleu Coe formed the floor over which thesehad been deposited, and afterwards the axis over which they hadbeen bent. We have, undoubtedly, here a great pre-Cambrian axis(though, as I shall further show, probably not the only one) i~ theCentral Highlands, and it can be traced for a considerable distanceto the S.W., as well as to the N.E. I examined it along GlenSpean and Loch Laggan to the N.E., and I there found the samegranites and gneisses composing the axis, and the same flaggysandstones dipping away from the axis, as described near FortWilliam and Loch Leven on the N.W. side, and in the range ofmountains on the S.E. side. As the axis is traced to the N.E., itis quite evident that a greater breadth of pre-Cambrian rocksbecomes gradually exposed, and the schistose series, characteristic,I believe, here of the Pobidian group, with crystalline limestone andserpentinous bands, prevails over a considerable area, and is wellexposed along Glen Garry, Glen Tilt, &c. These schists have astrike almost identical with the Silurian rocks, and may therefore,

84 H. H ICKS ON sOME RE SEARCHES AMONG

by physical evidence alone, be confounded with th em. But as faras petrol ogical evidence is capable of differentiating th em, it isclear that they are entirely unlike in th eir state of alteration andin most of their mineral characte rs. I feel convinced, th erefore,that all these must be included in the pre-Cambrian rocks, andthat they are overlaid unconformably by the quartz rock s to theS.E., these quartz rocks also being contemporaneous sedimentswith those at Schiehallien and in the line of mountains to theS.W., including Benchalader and the mountains to the North ofTyndrum.

Tyndrum to Callender.

In the area immediately to the south and east of Tynd rum Inn,th e pre-Cambrian rocks again app ear uncovered by newer deposits.Here they are chiefly mica schi sts, with a dull silvery lustre, andhighly micaceous and quartzose gne isses. About Loch Dochart,according to Murchison and Geild e,· th ey are much gnarled andtwi sted, dipping in vari ous directi ons from N. E. to S.E.

Th e gn eisses and mica schists in th is neighbourhood may be Raidto resemble , in some respects, the Dimetian rocks of Gaer­loch and Ben Fin, especially the thinner seri es in that group.Th ey contain an abundance of garnets and the two micas, but thebiotit e is generally in small proportions only. In a line from heretowards the N . E . th ey ar e again plentifully exposed, and on th enorth bank of Loch Tay are interstratified with hornbl ende slates,also exceedingly lik e th ose found at th e Kerrie falls near Gaerloch.tNear Cri anlarish and Lu ib, and also along Loch Tay bands of acrystalline limestone occur "with num erous gr een ser pentinousand talcose int erlaminations." A t first sight these much resemblethe gn eiss rocks, and ar e frequently much crumpled.

To the south of this lin e schistose and slaty chloritic rocksbecome more abundant in association with micaceous rocks,and the mountains Ben More, Ben Voirlich, and Ben Ledi consistlargely of chloritic rocks in association with micaceous schistsand gneisses. These may be well seen in the railway cuttings,

* "Quart. Jour. Geol. Soc.... Vol. xv ii., p. 218.t Some specimens of the hornblende r ocks from Loch Tayare to be seen

in the J ermyn Street Museum. also gneiss es from the Silurian, markedArroshan, Loch Lomond.

PRE- CAMBRIAN ROCKS IN THE BRITISH ISLES. 85

and along the roadside between Killin Station and Loch Lubnaig,and particularly in the mountain Ben Ledi, on the west ofthat lake. The rocks in the areas last mentioned everywherestrongly recall to mind the pre-Cambrian rocks of Wales, especiallythose in Anglesea and in the Lleyn promontory, which I have in­cluded amongst the Pebidian group. The hard, massive, andschistose chloritic rocks also strongly resemble many of the rocksin that formation at St. Davids, and as the strike in these and intheir associated schists and gneiss is invariably the same, I thinkthere can be no doubt that they should be grouped together in thatformation. I'he tolerably persistent strike from N.E. to S.W.,which coincides generally with that in the Cambrian and Silurianrocks, has undoubtedly here, as in Wales, caused these rocks to beallied with those deposits, and the petrological evidence has usuallyin consequence been entirely overlooked. Now that these havebeen separated by stratigraphical and petrological evidence inWales, the time seems at hand when a similar attempt should bemade in Scotland, and the results obtained by this recentexamination of many areas in that country have fully convincedme that this is quite possible, and that it must ere long beaccomplished.

Ireland.

Of late years but little has been done in Ireland to unravel thehistory of the metamorphic rocks, but I am glad to find that thereis some tendency there at the present time to attack the questionafresh. If the evidence I have brought forward in regard to therocks which occupy most of the north-west Highlands of Scotlandis in any way conclusive, then I think it will be readily granted thatthe rocks which have been by so many authors, including the mem­bers of the Geological Survey, correlated with them in the north­west of Ireland, may be treated by the same kind of evidence.From specimens I examined with Dr. Sterry Hunt, which he hadreceived from Mr. J as. Thomson, of Glasgow, from the intermediateislands, especially Islay, I have no hesitation in saying that therocks are in every particular to be correlated with pre-Cambrianrocks from the west coast of Scotland and from Wales. . Dr. Huntalso examined the rocks along the shores of Lough Foyle, and

86 H. H I CKS ON SO~rE RE SE ARC HE S AMONG

t hese he correlates with the Huronian or P ebidian group. Hence,if these facts are taken in association with th e former research es ofP rof. Ha rkn ess, it seems to me impossible not to arriv e at the con­clusion that the metamorphic rocks along the north-w est of Irelandmust be of pre-Cambrian age. Recently an interesti ng communica­tion by Mr. G. H. Ki nahan, of th e Geological Survey of Ireland , to"Nature," has tended much to st reng then th ese views. H e doesnot actually place these rocks with the pre- Cambrian, but he saysthat th e evidence goes to show that in th e County Tyrone, " thereare metamorphic rocks which were upt urned, contorted and meta­morphosed and denuded pri or to the overlying fossiliferous 1 Pome­roy rocks ' having been deposited. Th e fossils in th e latter wouldpoint to their being Cambro-S ilurian." Considering that the se hadhitherto been classed with the so-called newer Metamorphic series,of supposed Silurian age , in Scotland, this fact, placed before us byMr. Kin ahan, is of th e utmost import ance. H e mentions oth er

areas also where these so-called newer metamorphic rocks und erlieunaltered rocks with Cambro -Silurian fossils, as in north-west Mayo,and in south-eas t ""rVexford. I must confess, however, that in someof th e other areas, where I also consider the met amorphic rocks tobe chiefly of pr e-Cambrian age, he still supposes them to be oflater date-to have, in fact, been metamorphosed /I at th e close ofCambro -Silurian times"--and others " subsequent to th e dawn ofUpper Silur ian tim es." It is, however, satisfactory to find it statedin his concluding remarks that "it seems possible th e metamorphicaction prior to Cambro-Silurian, may have been gre at er than thatsubsequent to it." This is really what I have maint ained for solong a t ime, viz. : that metamorphic action on a large scale, andaffecting great thi cknesses of strata, has not taken place in theBri tish areas since pre-Cambrian times.

In 1878 I exam ined in company with Dr. Ster ry Hunt andother s the metamorphic rocks along the coast of Dublin andW icklow, and the unaltered Cambrian rocks in proximity tothem. Th e met.amorphic series are her e intruded into by greatmasses of granite which send out many large veins, but these donot appear anywh ere to pass into th e neighbouring Cambrian rocks.If thisis really ~he case, I cannot come to any other couclusionthan th at the metam orphi c series and the intrusive granite are ofpre-Cambri an age, and tha t the Lower Cambri an rocks of Bray

PRE-CAMBRIA.N ROCKS IN THE BRITISH ISLES. 87

Head rest unconformably upon, or are perhaps partly faultedagainst these, and, moreover, that they were derived chieHyfromthe denudation of the metamorphic schists and granite.

It seems to me, therefore, that we have very strong evidence toshow that in Ireland, as in Scotland, and in England and Wales,fragments of the old pre-Cambrian land are frequently exposed,and may readily be found, and that the rocks composing them. canbe identified usually by their mineralogical and physical charactersfrom the newer sediments which either surround them, lie uponthem, or are entangled amongst them.

ORDINARY MEETING.

JANUARY 7TH, 1881.

W. H. HUDLESTON, Esq., M.A., F.G.S., F.O.S., Vice-President,in the Chair.

The donations to the Library since the previous meeting wereannounced as usual, and the donors received the thanks of theAssociation.

The following were elected Members of the A-ssociation :­

Henry T. Wakefield, Esq.; Henry W ordley, Esq.; and A. T.Roberts, Esq.

Messrs. Spratling aud Braby were elected Auditors of the Ac­counts for the year 1880. [The latter gentleman being unable toserve, Mr. Cheadle subsequently took his place.]

The following paper was then read:-

ON THE ZONES OF TBlt CHALK.

By Prof. J. MORRIS, M.A., F.G.S.