187

ORDINARY MEETING.

JULY 6TH, 1883.

HENRY HICKS, E sq., M.D., F.G.S., President, in the Chair.

The minutes of the previous Meeting were read and confirmed,but, the Librarian being absent, th e donations were not announced.

The following were elected members of the Association :-H. W.Choat, Esq., W. Cole, jun., Esq., J. W. Cole, Esq., and ArthurHodgson, Esq.

Mr. Hudleston then took the chair, while t he President read thefollowing paper ;-

"On th e geology of the district to be visit ed during thelong excursion. "

The following paper was then read :-" On Glacial distribution in Canada," by the Rev. David Honey­

man, D.C.L.Publicati on of Dr. H oneyman's paper is postponed.

ON THE GEOLOGY OF THE DISTRICT IN NORTH WALES TO BE

VISITED DURING TIlE LONG EXCUHSI ON, JULY 23-28, 1883.

By H ENRY HI CKS, M.D., F.G.S., Presid ent.

It would perhap s be imp ossible to select a district more full ofgeological interest than th e one chosen for thi s Summer's E xcur­sion. It contains an 'unusual numb er of formations in a com­paratively limited space, and some of th e most important pro­blems which it is th e desire of geolog ists at present to solve, largelydepend upon the interpretations furn ished by th e research es whichhave been carried on in this d istrict of late years. It has receivedthe best at tent ion of many of our gr eatest geologists, and from anearly period in th e history of the science. Sedg wick , De la Beche,H enslow, Smyth, Ramsay, Phillips, and Selwyn were among theearlier, and Hugh es, Bonney, Callaway, Roberts, and :Marr, th e morerecent explorers. Yet I am bold enough to st at e, that th oughall the se have done most excellent work, they are far from havingexhausted the field, and enthusiastic members of the Associati onmay, if th ey desire it , st ill find here am ple work and highly st irnu-

L3

188 H. HICKS ON THE GEOLOGY OF NORTH 'VALES.

lating food for their mental energies. On some very importantpoints, the members of what may be called the older school differedvery much in their views from the newer school of geologists,though amongst the older some, such as Sedgwick and Phillips,may be claimed as holding a kind of intermediate position. Itwill not, however, for the purposes of this paper, be necessary forme to refer to all these differences, but rather to call attention onlyto the main distinctions which now separate the two schools; for itis but fair that an allowance should be made for the disadvantagesunder whichthe members of the older school laboured when comparedwith the advantages the others have derived from recent researchesin other areas, and such aids as the advance in chemistry andmicroscopyhave been able to afford. The viewsheld by the membersof the Geological Survey, as published on the maps at present avail­able to us, are sufficiently near to those which I have ventured to callthe older school, and it is to the differences which exist between theSurvey and the so-called newer school that I propose chiefly to refer.It is absolutely necessary that I should do this, as many of themembers will doubtless purchase the Geological map of the dis­trict, and it is but fair that our reasons for differing from the inter­pretations there given should be fully explained. In the accom­panying map large areas of Archsean (or pre-Cambrian) rocks areshown, while in the Survey map the oldest rocks of the district arestated to be the Oambrian. Again, rocks we call Archeean (pre­Cambrian) are marked in the Survey map as metamorphosedrepresentatives, in some places of Oambrian, in others of Silurianrocks, and in several areas as intrusive rocks of a comparativelylate (Silurian) date. The crystalline character of many of theschistose rocks also is attributed by the Survey to effects due toproximity to these so-called igneous intrusions.

Sedgwick and Phillips many years ago express ed the opinion thatsome of the schists in Anglesea were probably older than the trueCambrian, but this view was strongly opposed by the members of theGeological Survey. It was not, however, until the year 1877 thatfacts were presented to the Geological Society in papers by Prof.Hughes and myself to show that some of the so-called igneousmasses were of older date than the Oambrian, and that the meta­morphism of the adjoining strata was in no case due to them.These masses had been supposed also, by Sedgwick and Phillips,to be intrusions; therefore the discovery by Prof. Hughes and myself

H. HICKS ON THE GEOLOGY OF NORTH WALES. 189

that they were part of the Arehsean series, like rocks of a-similarcharacter that I had previously described at St. David's, gave the cluethat was necessary to make the geology of the district intelligible.Since then the papers by Professors Hughes and Bonney, Dr.Callaway and myself have supplied further information fromdifferent parts, and the result has been that a good base line for theCambrian has been recognised over much of the district, and thatthe metamorphic rocks of Anglesey and Caernarvonshire, as wellas the more crystalline rocks referred to above, are now by manyplaced amongst the Archeean rocks. The latter also have been sub­divided into groups. The oldest of these, resembling the Dimetianrocks of St. David's, occur at and to the N.E. of Caernarvon (Ionmap), and in Anglesey to the N. and S. of Llanfaelog. These arechiefly granitoid rocks, but some true gneisses also occur amongthem in Anglesey. To the E. and N.E. of this ridge of granitoidrocks in Anglesey are some hiilleflintas (No.2) like those found inPembrokeshire in the Arvonian group. In Holyhead Mountainare some quartz schists, which I think should also be included inthis group (Arvonian). The parts marked 2, between Oaernarvonand Bangor, and to the N.E. and S.W. of Llyn-padarn, chieflyconsist of quarts-felsites, The position of these rocks, whether atthe base of the Pebidian, or more intimately allied, as at St.David's, with rocks of the Arvonian group, is still an open question.I am inclined to associate them with the Arvonian rocks, thoughothers think they should be placed with the Pebidian, In No.3 Iinclude the mica-schists, chloritic rocks, and serpentines of Anglesey,and the schistose and brecciated rocks of Bangor. Whether allthese can be grouped together, or whether it may not be necessaryto separate the true schists from the more brecciated rocks, is still amatter for future decision. That they are newer than the granitoid,gneissic and hiilleflinta series is, I think, pretty certain. In a formerpaper read before the Association* they are divided into two groups-the lower called Menai, as distinct from the Bangor series. Itwill be observed that very large areas are occupied by these pre­Cambrian groups, consequently the unravelling of these rocks isamong the most important questions in the geological inquiry re­lating to these parts of Caernarvonshire and Anglesey. These rocksformed a part of the earliest land out of which the British Islands

* •Proc, Geol. Assoc.,' Vol. vii, pp. 59 et seq.

190 H. HICKS ON THE GEOLOGY OF NORTH WALES.

have been evolved, and all facts which can be collected from their ex­amination tend to fill up gaps in the most important period of geolo­gical history. Much information has been added oflate, but this hasonly given a taste for more, and we can only hope that the visit oftheAssociation will give a fresh impetus to the desire for a furtherknowledge of these rocks. That this old land had an uneven sur­face, that it was studded with volcanoes, and that its plains andvalleys were covered with much loose material, derived from the dis­integration of its then indurated and crystalline rocks, probably bythe aid of such active agents as ice, rain and rivers, are facts whichhave already been gathered from these examinations j but muchmore yet remains to be unravelled.

A great deal has been said of late about the basal conglomeratesof the Cambrian, and in the sections prepared by Prof. Hughes, forthe excursion, these conglomerates are pointedly referred to.

The evidence afforded by these conglomerates, in reference to someof the most important physical questions connected with that periodin the world's history, is of very great importance.

It will be observed, by referring to the map and sections, that weconsider that these conglomerates lie unconformably on the under­lying rocks, that they creep over different members of the Archreanseries, and that they are seldom found reposing upon these rocks forany distance at the same horizon in their order of succession. Thisevidence points to an enormous break at the base of these conglom­erates, and shows that new physical features of very great importancein regard to the geological history of the British Isles date from thisperiod. We therefore claim that these conglomerates should markthe commencement of the Palseozoic age so far as it relates to theseislands. When we study these conglomerates in different areas, wesee that they generally contain fragments of the rocks upon whichthey repose, and that the latter must have been in much the samecondition as they are at present when these fragments were torn offfrom the parent rocks. Moreover, these basal conglomerates arefound to lie at such varying horizons as to point clearly to thegradual subsidence of a very uneven old land surface. The membersof the Association will have several opportunities of verifying thesefacts for themselves.

The rocks above the conglomerates are carefully described in SirA. Ramsay's Memoir of North Wales (last edition 1881), andespecially in the excellent appendix by Mr. Etheridge, F.R.S. In

H. H ICKS ON T HE GEO LOGY OF NORTH WALES. 191

ascending order we meet with the Lowest Cambrian ( Llanberis andHarlech groups) to the south of Bangor, but probably here not infull development. The great areas of these rocks are along theborders of the Archsean rocks to the north and. south of LlynPadarn, On the shore of Llyn Padarn and Llyn Peris are thewell-known slate quarr ies-the Glyn quarry on the south, andthe Llanberis great quarries on the north , the Glyn quarry being inbeds at a lower horizon than the others. The Pe nrhyn slate quarryis apparently on the same horizon as the Llanberis quarries, butabout four miles further north.

At present fossils have not been found in the lower groups in thisarea. Curiously, also, fossiliferous Menevian beds have not beendiscovered, but unless these have been thrown down by faults,they must, I think, occur here, as in Merionethshire to thesouth; for there are no indicat ions of any sudden physical changeat this point in the succession in this area. Moreover, the over­lying Lingula flags are much like those in the other areas, andth ey are also fossiliferous. The latt er are supposed to be thinn erhere than in th e neighbourhood of Port Madoc, but otherwisemuch like the rocks in th at area. In the last edition of Sir A.Ramsay's work it is stated that in the beds not carefully workedin these areas we may possibly meet with not only the Menevianbeds, but also the Tremadoc and Arenig groups. He therestates that since the discovery at St . David's and elsewhere ofth ese groups he believes that their distr ibution is less local than hesupposed when the first edition was printed. The break which heplaced between the Tremadoc rocks and the Lingula flags he nowalso acknowledges, on the strength of the evidence brought forwardfrom St. David's and elsewhere, to be a mistake, but he stillbrings forward some evidence in support of his view that thereis a physical break between the Tremadoc and the Arenig Groups,where Prof. Lapworth places the lower boundary for his Ordovician.The Arenig rocks have been worked by Prof. Hughes in Anglesey,and by Mr. Marr near Caernarvou, and there can be no doubtthat equivalent beds occur there to those which I called Arenig atSt. David's, representing rocks originally classed as Arenig byProf. Sedgwick near th e Arenig Mountains, though at that t imea distinguishing fauna had not been found in them. Prof. Ramsayhas dwelt fully on this question, and has in my opinion done sowith fairness towards th e contending views.

192 H. HICKS ON TilE GEOLOGY OF NORTH WALES.

The next overlying rocks, the LJand eilo series, have not so farproved of great importance in thi s area, nor have they as yet beencarefully separated here from the other groups.

The Bala rocks of Snowdon ar e well known to all those whohave ascended that mountain, and the int erstratifying volcanicbands and the intrusive rocks in this group will doubtl ess bedescribed by Prof. Hughes in his report of the excur sion.

Upper Silurian rocks are not likely to be touched during theExcursion, but may be met with, by th ose who wish to extend theirresearches, by visiting the neigh bourh ood of Conway, and partiallyalso in Anglesey (according to Prof. Hughes.)

The Old Red Sand stone occurs to th e south of Dulas Day inAnglesey, and the Carboniferous series along the shores of MenaiStrait, also at the north-east corner and in a line nearly acrossAnglesey in the direction of Malldraeth Marsh. (S ee Map) .

Permian Rocks are stated also to occur at Malldraeth Marsh.The glacial deposits about Snowdon, &c., are of considerable in­terest, and marine sh ells ar e sta ted by Ramsay to have been foundat heights of from 1,100 to 1,400 feet above the sea.

EXCURSION TO THE MEDWAY VALLEY FROM:MAIDSTONE TO AYLESFORD.

SATURDAY, JULY 14TH, 1883.

Director : - J. LOGAN LOBLEY, F.G.S.

(Report by THE DIRE CTOR.)

As the Medway Vall ey, in the neighb ourhood of Maidstone, hasbeen on several occasions visited by the Associat ion, and itsgeological structure described in the I Proceedings,' '" it will besufficient here to briefly record th e route taken and the sectionsvisited during this excursion.

Previous to leaving the town of Maidstone, where the partyassembled, a visit was paid to the Natural History Museum, whichis so well placed in a fine old Manor House. Mammalian remainsfrom the Post-Pliocene beds of Aylesford form an importantportion of an extensive general geological collection, especiallyrich in the fossils of the local formations. Much time and greatcare have been given by th e Curator, Mr. Bartlett, to the preserve-

• Vol.iii,p. 88; Vol.iv,p.603; and Vol. vi,p. 392.