99

ORDINARY MEETING, JUNE 7TH, 1872.

The Rev. THOMAS WILTSHIRE, M.A., F.G.S., &c., President, inthe Chair.

The following Donations were announced :-

"Memoirs of the Geological Survey of England and Wales,"vol. iv., "The Geology of the London Basin," by William Whit­taker, B.A., F.G.S., from the Author.

" Thesaurus Siluricus, the Flora and Fauna of the SilurianPeriod," by J. J. Bigsby, M.D., F.G.S., from the Author.

" Sketch of the Geology of the Neighbourhood of Banbnry,"by Thomas Beesly, F.C.S., from the Author.

" Report for 1870 of the Geological Survey of Ohio," from theOhio State Survey.

"Thirty-ninth Annual Report of the Royal CornwallPolytechnic Society, 1871," from that Society.

"Quarterly Journal of the Geological Society," from thatSociety.

" Abstract of the Proceedings of the Geological Society," fromthat Society.

"Proceedings of the South Wales Institute of Engineers,"from that Institute.

The following were elected Members of the Association :­

George Blumberg, Esq.; James Campbell, Esq.; WilliamChapple, Esq.; Edwin Litchfield, Esq.; Samuel Rowles Pattison,Esq., F.G.S.; Mrs. Robins; Miss Jane C. Seel; W. T. Stone,Esq.j Thomas Warburton, Esq. ; and Cecil Ray, Esq.

The following Papers were read :-

1. ON THE CLASSIFICATION OF THE CAMBRIAN AND SILURIANROCKS.

By HENRY HICKS, Esq., M.R.C.S., F.G.S.

Geological science is so progressive, and new discoveries are sofrequently made, that it can scarcely be expected that the classifi­cation of any portion of the series of stratified rocks can re­main long without the necessity for considerable change andmodification: the classifications, therefore, of the Cambrianand Silurian Rocks, by Professor Sedgwick and Sir R. Mur­chison, made many years ago, and sufficiently correct at that

H

100 H. HICKS ON CAMBRIAN AND SILURIAN CLASSIFICATION.

time, are now in several respects unsatisfactory. Even at pre­sent, when so much still lies hidden from our view in unexploredregions, any attempt at a classification must be more or less im­perfect, though it is satisfactory to find that fresh discoveriesleading towards a natural arrangement are continually being made.

Systems of classification are necessary, if only for working pur­poses, and doubtless cannot be dispensed with on that account j

still, it must not be forgotten that all lines of division, whereverplaced, are more or less arbitrary, for we have only one greatnatural system to deal with, and, therefore, as Nature from timeto time unfolds herself to us, so these lines have to yield andgive way to others.

We know also that there are numerous breaks in the succession evenat present, and we avail ourselves of these accidental circumstancesfrequently for purposes of classification; but as these have arisenchiefly from faults in the strata, denudation, and such causes, wemust expect to be able to fill up many of these gaps as our know­ledge of hitherto unexplored and imperfectly explored regionsbecomes more satisfactory.

It is important also to bear this fact in mind in examining theorganic remains of succeeding groups; for, however dissimilar theforms in these groups may appear, this must not be looked upon asa want of accord in nature, but as evidence only of the absence, bysome accidental cause, of deposits, which, had they been present,would have shown how unvaryingly true Nature has always been toher laws.

In the accompanying Table the groups of strata which are known,collectively, to form at present the Lower Paleeozoio rocks of thiscountry are placed in their natural order of succession; and sideby side the classifications of these groups, as adopted by ProfessorSedgwick, and by Sir R. Murchison, along with the one now proposed.

All rocks not newer than the Bala Limestone were placed byProfessor Sedgwick in his Cambrian, which was divided by himinto an Upper, Middle, and Lower portion; the Upper consistingof the Bala group, and the Arenig or Skiddaw group; the Middle,of the Tremadoc, and Ffestiniog groups; and the Lowe?', of theHarlech, and Longmynd or Bangor groups. His Silurian includedthe beds from the base of the May Hill group to the top of theDownton Sandstone, the Bone Bed, and Upper Ludlow.

By Sir R. Murchison, the Cambrian was made to include only

H . HICKS ON CAMBRIAN AND SIL URIAN CLASSIF IC ATION. 101

th e beds contained in Professor Sedgwick's Lo wer Cambrian, viz.,th e Longmynd or Bangor group, and the Harlech group, thewhole series above th ese being ' called Silurian, or rather, in hismost recent work, P rimordial S ilurian and Silurian, the nameP rimordial S ilurian being adopted by him after the publi cation ofth e researches by Mr. Salte r and myself in the Menevian group inW ales.

Murchison's classification varied considerably from t ime to time(the last being th at published in "Silur ia," in 1868, and whichcorresponds with th e one in th e Table), and apparently not so muchwith a view to a more natural arrangement as to include all fossili­ferous rocks of th e early formations in his Silurian, leaving theterm Cambrian only to what he in 1868 called th e sterile basesof th e fossiliferous rocks; th ese" sterile bases," however, even inthat year proved to be them selves richly fossiliferous.

Th e divisions now proposed differ in several particulars fromthose of Professor Sedgwick, and Sir R. Murchi son, and are anattempt at a more natural arrangement in accordance withP alreontological evidence. This arrangement has alr eady beenadopted to a great extent by Sir Charles Lyell, in his " Student'sManual of Geology," 1871, and by Mr. SaIter and myself inseveral papers.

r have left out from th e Table the rocks which of late yearshave become known to us under the names p re-Cambrian orLaurent ian, as they have yielded no organic remains at presentin th is country. Moreover, th eir positi on is easily recognised, andno difficulty is likely to occur with th em. They appear generallyas quartzifcrous or g ranitic rocks, with a st rike from N.W . to S.E.,and, hence, discordant to that of the overlying Cambrian stra ta ,which invariably in this country has a str ike from about N.E. toS.W. Th ese p re-Cambrian rocks occur as ridges in the strata inth e neighbourhoods of St. David' s, and Malvern, CharnwoodF orest, the H ebrides, and western parts of Scotland.

In the Table the lowest gro up is the Longmynd, so namedfrom the Longmynd hill s of Shropshire. This group is usuallydivid ed into an upper and a lower portion, the former sup ­posed to be equivalent t o th e Harlech Grits, and th e latter tothe Ll anberis Slat es. There is no evidence, however, at presentt o show that th e Ll anberis Slates are older than the HarlechGrits , for the difference in th eir lithological characters may easily

102 H. H ICK S ON CAMBRIAN AND SIL UR IA N CLAS BIF IC ATION.

Wenwbe explained, as stated by Sir C. Lyell,

by lookin g upon them as deposits of finemud thrown down in th e same sea, onthe borders of which the sands of theH arlech Grits were accumulating. Thegreat thickness of the stra ta in theLongmynd hills has na turally led authorsto believe in th e probability of a moreancient series occurring there than ispresent in th e H arlech district , but thisview seems not borne out by actual facts , ~

sinee there is every probability of repeti- ~

ti ons of the strata in the L ongmynd dis- ===

trict, as is the case at St . Davi d's; more- ::t:Sover , after an examination of th e beds 0tr:

at these places, I am inclined to believetn"

that in no case does th e actual thicknes s 8of the series, without repetitions, attain ~.-,:t o nearly what has been usually g iven A

to it in th e Longmynd hills. The series ~

so well exposed in coast sect ions at St . uiDavid' s is evident ly id entical with that 6in North Wales, and Shropshire; the ~

lith ological cha racters and th e order of zsuccession being strangely alike, and ;::S

~

even th e thickness, when any particular .::::series is exposed to view, shows only a ~

overy slight difference. Th e lowest beds ...at St. Da vid's, in my opimon, are cer- 0

ta in!y of equal antiquity to any in th e 13Longmynd hills, and have the ad vantage t;of showing a base resting on a pre-Cam- @br ian ridge, such as is nowhere shown inth e Longmynds, or in Nor th Wales. Theaccompanying section explains the relationof these beds to the pre-Cambrian islandsat St. David's.

The life of this period, so far as we atpresent kn ow, consisted of annelids, crus­taceans, brachiopods, pteropods, and8ponges. From 3000 to 4000 feet ofthese beds, in natural succession, ha ve

H . HICKS ON CAMBR IA N AND SI LU R IAN CL ASS IF IC AT I ON. 103

yielded fossils at St . David's, where as yet th ey have beenchiefly examined. Th e Longmynd beds in Shropshire, in NorthWales, and also in Ireland, have also yielded a few indications of life,but these districts need fur ther exploration. Resting on the se rocksat St. David's and in th e H arlech distri ct , we have a series of greyand blue flags and slates, with some sandstones at the ilpper part ;and designated by Mr. Salte r and myself the" Menevian Group."Th ese beds are richly fossiliferous, and contain genera liketh ose in th e Longmynd Group below ; several species are alsocommon to th e two groups. Between th ese two groups th eboundary line separating Cambrian from Silurian was platedby Sir Roderick Murchison, and it is still retained here by themembers of the Geologic al Survey. Lithologically there is a gooddeal of difference in th ese two groups, for the lower or Long­mynd is almost invariably made up of green, purple, red, andgre y rocks, and conglomerat es ; but palreontologically th ey are verynearly allied, and it would be in every respect wrong to place sostrongly marked a divisional line between two groups so intimatelyconnecte d by th eir org anic contents. Sedgwick 's line of divisionbetween the Middle and Lower Cambrian also comes in here, andfrom following lithological characters only. It may be grunted,however, that th ere was some excuse for this boundary line,for at the tim e it was made no fossils had been found in theL ongmynd Group.' Henceforth, however , it will be impossible toretain this line, and I have th erefore suggested tha t these twogroups should go togeth er to form Lower Cambrian, and tha t th eline of division should be above instead of being below the Mene­vian Group. Th e genera and species of the Menevian group, also,are so th oroughl y disti nct from the immediat ely overlying rocks ofthe Lin gula Flag group that the bounda ry seems to fall naturallyat thi s spot .

17 genera and 25 species have been found in th e LongmyndGr oup, and 23 genera and 52 species in the Menevian Group.Twelve genera and eight species are common to both groups,and such genera as P aradoxidee, Conocorpphe, and Microdiscusare amongst them. This proportion is far greater than we usuallyfind between two group s so dissimilar in lithological characte rs,and comprising so gre at a thickness of strata.

Th e next group we come to consists of what bas usually beencalled the Lingula Flags, and so named from the abundance of

104 H. HICKS OY CAMBRIAN AND SI L URI AN CLASSIFICATION.

Lingulella Dauisii found in some of the beds, chiefly the middleones. This group has been divided into an Upper, Middle, andLower series, each characterised by genera and species peculiarto that zone. On the whole, considering the thickness of the strata,which must be over 5000 feet, this group is exceedingly barren oforganic remains. The Lower or Maentwrog beds contain only fourgen era and eight species j the Middl e or Ffestini og beds six genera andsix species ; and th e U pper, DolgeIIy or Malvern Shales, ten generaand eighteen species. The lithological characters are bluish or gr eyarenaceous slates for the Lower, grey arenac eous and micaceousHags for the Middle, and soft black and blui sh slates for theUpp er. The genera and species increase greatly in these Upp erbeds, as is usual with fine muddy deep sea deposits, though thethickness of the Upper beds is not more than 600 feet. We havein these last a return to conditions similar to tho se which followedthe shore and shallow water deposits of the Longmyndsduring the depositions of the Menevian Gronp ; but sufficienttime had elapsed to change th e prevailing forms of life, andyet not to an extent that they did not retain some resemblanceto their ancient ancestors. All th e trilobites of the LingulaFlags are allied to Olenus, and there are none in any waySilurian in character.

Following the Lingula Flags are the Tremadoc rocks. Theseoccur chiefly in the neighbourhoods of Pottmadoc, in NorthWales, and of St. David's, in South Wales. They rest con­formably on the Lingula Flags, and usually graduate by in­sensible degrees from them. They are more than 1,000 feet inthickness, and consist of dark earthy slates and flaggy sandstones.In the table they form the upper group of the Cambrian, a positi ongiven to them by tbe late Mr. Salter, and afterwards by Sir C.Lyell, and others. Mr. Salter, along with Messrs. Homfray andAsh, of Portmadoc, examined th ese rocks very carefully in NorthWales, and proved that they formed an intermediate series inwhich Cambrian gen era seemed to commingle in about an equaldegree with Silurian genera. This has been: born e out since bythe researches conducted in the neighbourhood of St. David's, inSouth Wales, where a considerable addition to th e fauna has beenmade; but here the trilobites, which in these early rocks are thesurest indices of the age of the strata, are more strongly primordialin type, and tend to connect the group still more closely to theunderlying series.

H. HICKS ON CAMBRIAN AND SILURIAN CLASSIFICATION. 105

The Arenig Group, which comes in next in the Table, is dividedinto an Upper and a Lower series, both characterised by a distinctassemblage of fossils, The Lower series occurs chiefly in theneighbourhood of St. David's, and is of about 1,000 feet in thick­ness. The fossils consist of numerous graptolites, especiallydendroid forms, and trilobites of the genera Ogygia, Trinucleue, and.kJglina. All the forms are thoroughly Silurian in character, andthe series marks a definite line for the base of the Silurian groups.The Upper series comprise the well known Skiddaw Slates. Uponthe Arenig comes the Llandeilo Group, the lowest rocks recognisedby Sir R. Murchison in the typical Llandeilo district. This groupin the Table is divided also into an Upper and Lower series, markedout by distinct faunas.

The Bala or Caradoc Group now comes in with its exceedinglyrich fauna, and it forms in the Table the upper member of ourLower Silurian, and also of Professor Sedgwick's Upper Cambrian.Sir R. Murchison added to the Lower Silurian the LowerLlandovery rocks, but Sir Charles Lyell in his" Student's Manualof Geology," 1871, preferred "to regard them as the base of theUpper Silurian, to which group they are linked by more thantwice as many species as to the Lower Silurian." The evidence ofProfessor Hall and others, in America, tends to prove, however,that ultimately it will be necessary to make a Middle Siluriandivision, and though the group is scarcely important enough, ashitherto known in this country, to warrant such a division, yetshould a spot be found where the present unconformity is filledup, a series may be discovered, like that in the American continent,sufficiently large to enable it to be done.

The higher groups, Wenlock and Ludlow, which complete theTable, are each capable of being divided into at least two groups,with a considerable thickness of strata and well defined faunas.

2. ON THE SILURIAN ROCKS OF THE ENGLISH LAKE DISTRICT.

By H. ALLEYNE NICHOLSON, Esq., M.D., D.Sc., M.A., F.R.S.E.,

Professor of Natural History and Botany in UniversityCollege, Toronto.

Considering the extent to which the Silurian series of the Eng­lish Lake District has been studied, it is somewhat remarkable thatthere should still be considerable difference of opinion as to the