Page images

and serpentine. But perhaps their most intelligible system” is not filled with serpentine but with dolomite, sections are those which they present in Bavaria and which seems to show that the cavities must have existed Bohemia between the valley of the Danube and the lead before either dolomite or serpentine were introduced into waters of the Elbe. They are there divided into two the substance. Dr Carpenter contends that the disposition well-marked groups-(a) red gneis3, covered by (6) grey of these passages in his decalcified specimens is very regular, gneiss. According to Gümbel the former (called by hin and quite unlike any mineral infiltration with which lie is the Bojan gneiss) may be traced as a distinct formation acquainted. associated with granite, but with very few other kinds The opinion of the organic nature of Eozoon has been of crystalline or schistose rocks, while the latter (termed supposed to receive support from the large quantity of che Hercynian gneiss) consists of gneiss with abundant graphite found throughout the Archæan rucks of Canada ịnterstratification of many other schistose rocks, graphitic and the northern parts of the United States. This mineral limestone, and serpentine. The Hercynian gneiss is occurs partly jn veins, but chicfly disseminated in scales overlaid by mica-schist, above which comes a vast mass and laminæ in tlic limestones and as independent layers. of argillaceous schists and shales. Gümbel some years ago Dr Dawson estimates the aggregate amount of it in found in the marbles associated with the younger gneiss one band of limestone in the Ottawa district as not less what he considered to be an organism of the same genus as than from 20 to 30 feet, and he thinks it is hardly an the Dozoon of Canada, to which reference will immediately exaggeration to say that there is as much carbon in the be made. He named it Evzoon Bavaricum. More recently Laurentian as in equivalent areas of the Carboniferous a similar substance was obtained in the Archæan scries of system. He compares some of the pure bands of graphite Bohemia, and named by Fritsch Eozoon Bohemicum. to beds of coal, and maintains that no other source for their

AMERICA. - In North America Archæan rocks cover a origin can be imagined than the deoxidation of carbonic acid large part of the continent from the Arctic Circle south by living plants. In the largest of three beds of graphite wards to the great lakes. They appear likewise, as in at St John he has found what he considers may be fibrous Europe, along the central parts of prominent mountain structure indicative of the existence of land-plants. chains, as in the Rocky Mountain range and that of the Still further evidence in favour of organized existence Appalachians. They have been carefully studied in Canada, during Archean time in the North American area hns been where the late Sir W. E. Logan, Director of the Geological adduced from the remarkably thick and abundavt masses Survey of the Dominion, estimated their depth at about of iron ore associated with the Laurentian rocks of Canada 30,000 feet, but neither their top nor their base can there and the United States. Dr Sterry Huut las called attenbe found. He named them the Laurentian system from tion to these ores as proving the precipitation of iron by their abundant development along the shores of the St decomposing vegetation during the Laurentian period on a Lawrence. They have been divided into two series—(1) more gigantic scale than at any subsequent geological epoch.' a lower formation more than 20,000 feet thick, consisting Sone of the beds of magnetic iron ranged up to 200 feet chiefly of granitic, orthoclase gneiss, with bands of quartz- in thickness. Large masses also of hæmatito and titaniferrock, schists, iron-ore, and limestone ; and (2) an upper ous iron, as well as of iron sulphides, occur in the Canadian formation fully 10,000 feet thick, composed also, for the Archæan series. These · great bands of iron ore run most part, of gneiss, but marked by the occurrence of southward, and form an important feature in the economic hands of Labrador felspar, as well as schist, iron-ore, and geology of the Northern States of the Union. limestone. The


division has been stated to lie un- Above the Laurentian rocks in the region of Lake Huron conformably on the lower. Mr Selwyn; however, has lies a vast mass of slates, conglomerates, limestones, and recently pointed out that this is almost certainly not the quartz-rocks, attaining a depth of from 10,000 to 20,000 case, but that the limestone-bearing series rests conformably feet. They are termed Huronian. No fossiis lave yet upon a massive granitoid gneiss, to which he would restrict been found in them; but they must be much younger than the term Laureutian, classing the limestones in the next or the Laurentian rocks, on which they rest unconformably, Huronian system (Nat. Hist. Soc. Montreal, Feb. 1879). end from which they have been in part at least derived.

In one of the Laurentian limestones of Canadı, specimens have been found of a remarkable mixture of

II. PALÆOZOIC. calcite and serpentine. These minerals are arranged in altervate layers, the calcite forming the main framework Under the general term of Primary or Palæozoic are now of the substance with the serpentine (sometimes loganite, included all the older sedimentary formations containing pyroxene, &c.) disposed in thin, wavy, inconstant layers, as organic remains, up to the top of what is termed the if filling up fattened cavities in the calcareous mass. So Permian system. These rocks consist mainly of sandy and different from any ordinary mineral segregation with which muddy sediment with occasional intercalated zones of limehe was acquainted did this arrangement appear to Logan, stone. They everywhere bear witness to comparatively that he was led to regard the substance as probably of shallow water and the proximity of land. Their frequent organic origin. This opinion was adopted, and the structure alternations of sandstone, shale, conglomerate, and other of the supposed fossil was worked out in elaborate detail by detrital materials, their abundant, rippled, and sun-cracked Dr Dawson of Montreal, who pronounced the organism to surfaces marked often with burrows and trails of worms be the remains of a massive foraminifer which he called as well as the prevalent character of their organic remains, Eozoon, and which he believed must have grown in large show that they must have been deposited in areas of slow thick sheets over the see-bottom. This opinion was con- subsidence, bordering continental or insular masses of land. firmed by Dr W. B. Carpenter, who froin a large suite of As regards the organisms of which they have preserved the additional and better preserved specimens, described a casts, the Palæozoic rocks, as far as the present evidence system of internal canals having the characters of those in goes, may be grouped into two divisions—an older and a true foraminiferal structures. (See FORAMINIEERA.) Other newer :-the former distinguished more especially by the observers, notably Profesors King and Rowney of Galway abundance of its graptolitic, .trilobitic, and brachiopodous and Möbius of Kiel, leve opposed the organic nature of fauna, and by the absence of vertebrate remains; the latter Eozown, and have endeavoured to show that the supposed by the number and variety of its fishes and amphibians, the canals and passages are merely infiltration veinings of serpentine in the calcite. In some cases, however, the “canal

1 Geology of Canada, 1863, p. 573.

[ocr errors]

Upper. {s: Tremadoc slates.

uisappearance and extinction of graptolites and trilobites, great upward extension were more forcibly urged than ever. and the abundance of its cryptogamic terrestrial flora. But these claims could now be urged on palæontological

evidence such as had never before been produced. AccordCAMBRIAN.

ingly there has arisen a general desire among the geologists

of Britain to revise the nomenclature of the older rocks. This name was applied by Sedgwick to the rocks of Though as yet a common accord of opinion has not been North Wales (Cambria), where he first investigated them. reached, there seems a strong probability that ultimately the Their base is there nowhere seen, so that, though they boundary line between the Cambrian and Silurian systems attain a great depth, some part of their total mass must be will be drawn above the primordial zone along the base concealed from view. They pass up coutinuously into the of the great Arenig group or Lower Llandeilo rocks of base of the Silurian system. Considerable diversity of Murchison. All his Silurian strata of older date than these opinion has existed, and still continues, as to the line where rocks will be classed as Cambrian. the upper limit of the Cambrian system should be drawn. According to this classification, the Cambrian system, as Murchison contended that this line should be placed below developed in North Wales and the border English counties, the strata where a trilobitic aud brachiopodous fauna begins, consists of purple, reddish-grey, and green slates, grits, sandand that these strata cannot be separated from the overly- stones, and conglomerates. Its true base is nowhere seen, ing Silurian system. He therefore included in the Cambrian yet even the visible mass of strata has been estimated to only the barren grits and slates of the Longmynd, Harlech, reach the enormous thickness of 25,000 feet. By for and Llanberis. Sedgwick, on the other hand, insisted on the larger part of this vast depth of rock is unfossiliferous. carrying the line up to the base of the Upper Silurian rocks. Indeed it is only in some bands of the upper 6000 feet, or He thus left these formations as alone constituting the thereabouts, that fossils occur pleatifully. By fossil eyi. Silurian system, and massed all the Lower Silurian in his dence the Cambrian system may be divided into Lower and Cambrian system. Murchison worked out the stratigraphi- Upper, and each of these sections may be further subdivided cal order of successiou from above, and chiefly by help of into two groups, as in the following table :organic remains. He advanced from where the superposition of the rocks is clear and undoubted, and for the first Cambrian

3. Lingula flags. time in the history of geology ascertained that the “transi

of Wales.


2. Benevian group. tion-rocks” of the older geologists could be arranged into

1. Harlech and Longmyud group. zones by means of characteristic fossils as satisfactorily as

1. Harlech and Longmynd Group.—This includes purple, red, the Secondary formations had been classified in a similar and grey flags, sandstones, and slates, with conglomerates. These

strata attain a great thickness, estimated at 4000 feet in South Wales, manner by William Smith. Year by year, as lie found bis but more than 8000 in North Wales. They were formerly supposed Silurian types of life descend farther and farther into lower to be nearly barren of organic remains; but in recent years, chiefly deposits, he pushed backward the limits of his Silurian through the researches of Mr Hicks at St Davids, they bave yieldeil system. In this he was supported by the general consent

a tolerably abundant fauna, consisting of 30 species. Among these

are 16 species of trilobite (Paraloxides, Plutonia, Microdiscus, Palæoof geologists and palæontologists all over the world. Sedg

pyge, Agnostus, Conocoryphe), four annelides (Arenicolitcs), a. wick, on the other hand, attacked the problem rather from sponge (Protospongia), five brachiopods (Discina, Lingulclla), two the point of stratigraphy and geological structure. Though pteropods (Theca), &c. Many of the surfaces of the strata

in some he had collected fossils from many

of the rocks of which he parts of this group are marked with ripples, sun-cracks, and rain. had made out the true order of succession in North Wales, water and shore-conditions of deposit. 14 of the 30 species, accord.

pittinge as well as with trails of worms--indicative of shalloir. he allowed them to lie for years unexamined. Meanwhile ing to Mr Etheridge, F.R.S., pass up into the Menovian group, Murchison had studied the prolongations of some of the and 7 continue into the Lingula fags. same rocks into South Wales, and had obtained from them

2. Menerian-Group.—This subdivision has been proposed for a

series of sandstones and shales, with dark-blue slates and flags, the abundant suite of organic remains which characterized dark-grey flags and grey grits, which are seen near St David's his Lower Silurian formations. Similar fossils were found (Benevia), where they attain a depth of about 600 feetThey pass abuodantly on the continent of Europe, and in America. down conformably into the Harlech group with which, as just Naturally the classification proposed by Murchison was

stated, they are connected by 14 species in common. The Menevian

beds have yielded upwards of 50 species of fossils, of which 24 adopted all over the world. As he included in bis Silurian

are confined to the Menevian, whìle 18 pass up into the lower system the oldest rocks containing a distinctive fauna of Lingula


. Among these the trilobites are specially prominent, trilobites and brachiopods, the earliest fossiliferous rocks Some of them attained a great size, Paradoxides Davidis being were everywhere classed as Silurian, and the name Cambrian nearly two feet long. But with these were mingled others of was discarded by geologists of other countries as indicative that while the trilobites had attained their maximum size at this

diminutive size. It is noteworthy also, as Mr Hicks has pointed out, of a more ancient series of deposits not characterized by early period, they are represented among the older Cambrian rocks peculiar organic remains, and therefore not capable of being by genera indicative of almost every stage of development, “froin elsewhere satisfactorily recognized. Barrande, investigat- the little Agnostus with two rings in the thorax, and Microdiscus

with four, to Erinnys with twenty-four,” while blind genera occur ing the most ancient fossiliferous rocks of Bohemia, distin together with those having the largest eyes. Upwards of 30 guished

by the name of the “ Primordial Zone" a group of species of trilobites have been obtained from the Menevian beds, strata underlying the Lower Silurian rocks, and containing a the genera Agnostus (7 species), Conocoryphe (7 species), and Parapeculiar and characteristic suite of trilobites. He classed doxides being specially characteristic. Four species of sponges (Proit, however, with the Silurian system, and Murchison tospongia) and some annelide-tracks likewise occur. The mollusca

are represented by 6 species of brachiopoda of the genera Discina, adopted the term, grouping under it the lowest dark slates Lingulella, and obolella; 5 pteropods (Theca) have been met with. which in Wales and the border English counties contained the earliest entomostraca (Entomis) and the first cystidean (Protosome of the same early forms of life.

cystites) yet discovered occur in the Menevian fauna. Investigations during the last twelve years, however, slates and flags, with bands of grey flags and sandstones, attain in

3. Lingula Flags.--These strata, consisting of bluish and black chiefly by the late Mr Salter and Mr Hicks, have brought some parts of Wales a thickness of more than 5000 feet. They to light a much more abundant fauna from the so-called received their name from the discovery by Dir E. Davis (1846) of primordial rocks of Wales than they were supposed to layers

. They rest conformably upon, and pass down into the

vast numbers of a Lingula (Lingulella Davisii) in some of their possess. These fossils were found to be in large measure

Menevian beds below them, and likewise graduate into the Tredistinct from those in the undoubted Lower Silurian rocks.

madoc group above. They are distinguished by a characteristic Thus the question of the proper base of the Silurian system was re-opened, and the claims of the Cambrian system to a

Hicks, Quart. Journ, Geol. Soc., xxviii. 174.




suite (78 species) of organic remains. The trilobites include the land afterwards Hay Cunuinglan, pointed out that they genera Agrostus, A napolcnus, Conocoryphe, Dikelocephalus, Erinnys, underlie parts of the schistose rocks of the northern High Olenus, and Paradoxides. The earliest phyllopods (Hymenocuris) and heteropods (Bellerophon) occur in these beds. The brachiopods lands. The discovery by Dir C. W. Peach of Lower Siluriay include species of Lingulelo (L. Darisii), Discina, Obolella, and shells in the overlying limestones showed that the massivo Orthis. The pheropods are represented by three species of Theca. red sandstones of western Ross and Sutherland could not Several annelides (Cruziama) and polyzoa (Fencstella) likewise occur. According to a careful census by Mr Etheridge, the Lingula counties, but must be of older date than part of the Llan

do paralleled with those of the eastern tracts of those flagg may be grouped into three zones, each characterized by a peculiar assemblage of organic remains. The Power division deilo rocks of the Lower Silurian period. Sir R. durchison contains 37 species, of which 9 are peculiar to it. The middle zone classed them as Cambrian-an identification which has Lingulella Davisii

) pass down into the loiver division, 1 (Kutorgia rocks of the north-west Highlands and much of the Lower has yielded 5 species, of which (Coņocorupko buccphala and much support in the lithological resemblance between these cingulata) into the upper, and 2 (Lingula squamosa and Bellerophon Cambrensis) are peculiar. The upper zone has yielded 40 Cambrian system of Wales. species. Of these 9 pass up into the Tremadoc beus, while 2 In the south-east of Ireland masses of purplish, red, and (Linguldla lepis and L. Darisië) continue on into the Arenig group 4. Tremadoc Slatcs.—This name was given by Sedgwick to e

green sliales, slates, grits, quartz-rocks, and schists occupy group of dark grey slates, about 1000 feet thick, found near

à considerable area and attain a depth of 14,000 feet withTremadoc in Carnarvonshire, and traceable thence 'to Dolgelly: out revealiug their buse, while their top is covered by ui). Their importance as a geological formation was not recognized conformable formations (Lower Silurian and Lower Carboni. until the discovery of a remarkably abundant and varied fauna in ferous). They have yielded Oldhamia, described originally them. They contain the earliest crinoids, star-fishes, lamelli. branchs, and cephalopods yet found. The trilobites embrace 14

as a sertularian zoophyte, but now regarded by many genera, anong which, besides some, as Agnostus, Conocoryphe, and palæontologists as an alga; also numerous burrows and Olenus, found in the Lingula flags, we meet for the first time with trails of annelides (Histioderma Hibernicum, Arenicolites Angelina, Asaphus, Cheirurus Neseurdus, Niobe, Ogygia, Psilo. didymus, A. sparsus, Haughtonia pæcila). No Upper cephalids, &c. "The same genera, and in some cases, species, of Cambrian forms have been met with in these Irish rocks, brachiopods appear which occur in the Lingula flags, Orthis has described 12 species of lamellibranchs from the Tremadoc beds unconformability at their top. being regarded as equivalent Carausii and Lingulella Davisii being common forms. Wir Hicks which are therefore placed with the Lower Cambrian, the of Ramsay Island and St Davids, belonging to the genera Cleno

cro- to the interval required for the deposition of the intervening (lonta, Palaarca, Glyptanca, Davidia, and

Modiolopsis. The cepha: formatious up to the time of the Llandeilo rocks, as in the lopous are represented by Orthoceras scriccum and Cyrtoceras north-west of Scotland. Some portions of the Irish Campræcox; the pteropods by Thcca Davidii, T. operculata, and Conuo laria Homfrayi; the echinoderms by a beautiful star-fish (Palas brian series have been intensely metnmorphosed. Thus ou terina ramsegcusis) and by a crinoid (Dendrocrinus Cambrensis)." the Howth coast they appear as schists and quartz-rocks;

Careful analysis of the fossils yielded by the T'remadoc in Wexford they pass into gneiss and granite. In West beds suggests a division of this formation into two zones. Galway Mr Kinahan has described a vast mass of schists, According to a census by Mr Etheridge, the Lower Tremadoc quartz-rocks, and limestones (8000 feet and upwards) pass, rocks have yielded in all 56 species, of which I pass down ing up into schistose, hornblendic, and unaltered rocks coninto the Lingula flags and 10 ascend into the Upper taining Llandeilo fossils, and he agrees with Griffith and Tremadoc zone, 31 being peculiar. The Upper Tremadoc King in regarding these as probably Cambrian. He suggests beds contain, as at present ascertained, 33 species, of which that they are Upper Cambrian, which would imply that 9 are peculiar, and 13 or possibly 15 pass up into the Upper Cambrian rocks pass conformably into the Llandeilu Arenig group. It is at the top of the Upper Tremadoc formation without the occurrence of the thick Arenig rocks strata that the line between the Cambrian and Silurian of Wales. In a difficult country, however, broken by faults systems is here drawn. According to Professor Ramsay, and greatly metamorphosed, an unconformability might there is evidence of a physical break at the top of the easily escape detection. Tremadoc beds of Wales, so that on a large scale the next CONTINENTAL EUROPE.— According to the classification succeeding or Arenig strata repose unconformably upon adopted by M. Barrande, the older Palæozoic rocks of everything older than themselves. Die Etheridge also shows Europe suggest an early division of the area of this con that the palæontological break is nearly complete, only tinent into two regions or provinces,--a northern province, about 7 per cent of the fossils of the one series passing embracing the British Islands, and extending through North over into the other. Out of 184 known Arenig species, not Germany into Scandinavin, on the one hand, and into more than 13 are common to the Tremadoc beds underneath. Russia on the other, and a central-European province, Besides these important facts the character of the Arenig including Bolemia, France, Spain, Portugal, and Sardinia fauna strongly distinguishes it from that of the formations Bohemia.—The classic researches of M. Barrande have below, and further supports the line of division here given to the oldest fossiliferous rocks of Bohemia an extra; adopted between the Cambrian and Silurian systems. ordinary interest. He has made known the existence there

In the north-west of Scotland a mass of reddish-brown of a remarkable suite of organic remains representative of and chocolate-coloured sandstone and conglomerate (at least those which characterize the Cambrian rocks of Britain 8000 feet thick in the Loch Torridon district) lies uncon- At the base of the geological formations of that region lie formably upon the fundamental gneiss in nearly horizontal the Archæan gneisses already described. These are overor gently inclined beds. It rises into picturesque groups laid by vast masses of schists, conglomerates, quartzites! of mountains which stand out as striking monuments of slates, and igneous rocks, which have been more denudation, seeing that the truncated ends of their com- less metamorphosed, and are singularly barren of organic ponent flat strata can be traced even from a distance forming remains, though some of them have yielded traces of anne parallel bars along the slopes and precipices. The denuda. lides. They pass up into certain grey and green fissile tion must have been considerable even in early Silurian shales, in which the earliest well-marked fossils occur. The times, for the sandstones are unconformably overlaid by organic contents of this zone (Étage C) form what M. quartz-rocks and limestones containing Lower Siluriau Barmnde terms his primordial fauda, which contains 40 or fossils. No trace of organic remains of any kind has been more species, of which 27 are trilobites, belonging to the found in the red sandstones themselves. They were at one characteristic Cambrian genera–Paradoxides (12), dignus time regarded as Old Red Sandstone, though Macculloch, *us (5), Conocoryphe (4), Ellipsocephalus (2), Hyılrocephalus

(2), Arionellus (1), Sao (1). Not a single species of any Hicks, Quart. Joun. Geol. Soc., xxix. 39.

one of these genera, save Agnostus (of which 4 specics





Other Crustarea

er | Pteropods.


O O Brachiopods.


Sponges. how | Bryozoans.

Total by Countries



3. Scandinavia Ac and



[ocr errors]



2 18 5

2 25

appear in the second fauna), has been found by M. Barrande higher than his primordial zone. Among other organisms

Molusks. in this primordial fauna, the bracliiopods are represented by 2 species (Orthis and Orbicula), the pteropods by 5

Countries. (Theca), and the echinoderms by 5 cystideans.

. Soandinavia -In Norway the vast masses of Archæan
gnaias (Tellemark) are overlaid by schists, red sandstones,
and conglomerates. These are termed the Sparagmite
formation, and have hitherto proved barren of fossila 1. Bohemia....

40 They are covered, however, by beds containing Dictyonema

2 Spain Norvegicum, which may represent the primordial zone of


96 Barranda In Sweden the sparagmite formation has been 4. Eng. | Meneyian.

land Harlech, part

33 1 4 4 7 more productive of organic remains. It is there represented

1 2 58 by a sandy zone not more than 50 or 60 feet thick-a poor

ū. Newfoundland ..

6. New Brunswick equivalent for the great mass of strata in the Cambrian

7. New York

5 system of Wales. It was, originally termed the Regio 8. Braintree (Massachusetts) i l'ucoidarum by Angelin, from the fucoids alone found in it. In more recent years, however, its list of organic re

172 1 104.14 2 28 5 8 2-246 mains has been considerably increased; 12 species of plants, chiefly facoids, but including some (Eophyton) of higher grade, 9 species of annelides, 4 brachiopods, a pteropod, a

SILURIAN. bryozan, a coral, a crinoid, and a sponge have been The important system of rocks next to be described was obtained Above the strata containing these organisms first investigated by the late Sir R. Murchison in Wales and comes a zone which has yielded 77 species of primordial the bordering counties of England. He found it to be chartrilobites, including the genera Agnostus (19 species), Cono-acteristically developed over the tract once inhabited by the coryphe (13), Olenus (21), Paradoxides (9).

Silures, an ancient British tribe, and he thence close the NORTH AMERICA.-Rocks corresponding in position and name of Silurian as & cozwenient designation. It there in the general character of their organic contents with the passes down conformably into the Tremadoc slates at the Cambrian formations of Britain have been recognized in top of the Cambrian series, and is covered conformably by different parts of the United States and Canada. They the base of the Old Red Sandstone. appear in Newfoundland, whence, ranging by Nova Scotia GREAT BRITAIN.-In the typical area where Murchison's and New Brunswick, they enter Canada, the northern parts discoveries were first made he found the Silurian rocks of New York, Vermont, and eastern Massachusetts. They divisible into two great and well-marked series, which he rise again along the Appalachian ridge, in Wisconsin, termed Lower and Upper. This classification has been found Minnesota, Missouri, Arkansas, Texas, and Geurgia West to hold good over a large part of the world. The subjoined wards from the great valley of the Mississippi, where they table shows the present arrangement and nomenclature of have been found in many places, they reappear from under the various subdivisions of the Silurian system. the Secondary and younger Palæozoic rocks of the Rocky

Feet. Mountains. They have been divided by American geo R Upper Silurian.

7. Ludlow group

1,950 6. Wenlock group.

1,600 logists into two formations—(1) Acadian, a mass (2000

6. Upper Llandovery group

1,500 feet) of grey and dark shales and some sandstones; and (2)

4. Lower Llandovery group.

1,000 Potsdain (or Georgian), which attains in Newfoundland á

3. Bala and Caradoc group.........." A. Lower Silurian.


2. Llandeilo group...... depth of $600 feet, but thins away westward and southward

........... 2,500 1. Arenig or Štiper Stone group ........

4,000 till in the valley of the St Lawrence, where it was stadied by Logan and his associates of the Geological Survey of

Approximate average thickness – 18,550 Cauana, it is only from 300 to 600 feet thick.

A. Lower Silurian.
Among the organie remains of the North American
Cambrian rocks fucoid casts appear in many of the sand-

1. Arenig or Stiper Stone Group. These rocks consist stones, but no traces of higher vegetation. The Acadian of dark slates, shales, flags, and bands of sandstone, formation has yielded primordial trilobites of the genera They are abundantly developed in the Arenig mountain, Paradoxides, Conocoryphe, Agnostus, and some others; where, as originally described by Sedgwick, they contain brachiopods of the genera Lingutella, Discina, Obolella, and masses of associated porphyry. Throughout that district Orthis; and several kinds of annelide-tracks. The Potsdam they have boen deposited at a time when streams of lava rocks contain a few sponges, the earliest forms of graptolite, and showers of volcanic ashes were thrown out in great somo brachiopods, including, besides the genera in the quantity from submarine vents. They contain an abundAcadian beds, Obdus, Camarella, and Orthisina ; some

ant suite of organic remains (184 species), of which ouły pteropods (Hyolites or Theca); two species of Orthoceras ; 13. species are common to the Tremadoc beds below. annelide tracks; trilobites of the genera Conocoryphe, Trilobites occur of the genera Eglina, Agnostus, Ampya, Agrostus, Dikelocephalus, Olenellus, Ptychaspis, Charioce Barrandea, Calymene, Cheirutus, Mlænopsis, Illanus, phalus, Aglaspis, and Illænurus.

Ogygia, Phacops, and Trinucleus. Thres species of ptero M. Barrande has called attention to the remarkable uni- pods (Conularia, Theca), 18 species of brachiopods (Ling. formity of character in the organic remains of his primordial ula, Lingulella, Obolella, Discina, Siphonotreta, Orthis), 8 zone over the continents of Europe and America. He pub lamellibranchs, 3 gasteropods, and 5 cephalopods have lished in 1871 the subjoined table, to show how close is the been found, but the most abundant organisms are the parallelism between the proportions in which the different graptolites, of which the Arenig rocks of St David's, in classes of the animal kingdom are represented.

Pembrokeshire, have yielded 48 species, which belong to

20 genera, including Didymograptus, Tetragraptus, Dipla Trilobites, Prague, 1871, p. 196. Since the publication of this graptus, Dendrograptus, and Callograptus.? · Altogether table the progress of research has increased the number of species from some localities; but the general facies of the primordial fauna has not ? Hicks, Criart. Journ. Geol. Soc., saxi. 167: Hopkinson and Lap. been materially affected thereby.

worth, ibid., p. 635.

[ocr errors]
[ocr errors]

78 species of hydrozoa liave been obtained from the by Atrypa, Rhynchonella, Leptæna, Orthis (many species), British Arenig rocks, but none from any older strata. Strophomena, Discina, and Lingula; the lamellibranchs This sudden and great developnient of these organisms by Alodiolopsis, Mytilus, Palæarca, Plerinea, Orthonotul

, gives a distinctive aspect to the Arenig rocks. It con- and Ctenodonta; the gasteropods by Murchisoniu, Pleurotinues in the overlying Llandeilo group, so that the tomaria, Raphistoma, Cyclonema, Enomphalus, Maclurea,

. graptolites form in Britain a convenient character by Holopea ; the pteropods by Conularia, Theca, and Ecculiwhich to mark off the Cambrian from the Lower Silurian omphalus; the heteropods by various species of Bellerophon; fauna.

and the cephalopods by muny species of Orthoceras, with 2. Llandeilo Flag Group.-Dark argillaceous flagstones, forms of Cyrtoceras and Lituites. sandstones, and shales, some parts often calcareous. These 4. Lower Llandovery Group. - In North Wales' the Bala beds were first described by Murchison as occurring at beds about 5 miles S.E. of Bala Lake begin to be covered Llandeilo, in Carmarthenshire. They reappear on the with grey grits, which gradually expand southwards until coast of Pembrokeshire, and at Builth, in Radnorshire. they attain a thickness of 1000 feet in South Wales. These Up to the present time they have yielded 227 species of overlying rocks are well displayed near the town of Llanfossils. * Of these 13 are commou to the Arenig below, 82 dovery, where they contain some conglomerate bands, and to the Caradoc or Bala above, while 145 are peculiar. | where Mr Aveline detected an unconformability between The hydrozou are still the most abundant forms, 94 | them and the Bala group below them, so that the subterspecies being here met with, no fewer than 81 of these ranean movements had already begun, which in Wales being confined to Llandeilo rocks, and only 9 passing marked the close of the Lower Silurian period. Else. down into the Arenig group. Of crustacea 44 species have where they seem to graduate downwards conformably into been obtained. These include the characteristic trilobites that group. They cover a considerable breadth of country - Ampyx nudus, Asaphus tyrannus, Barrandea Cordai, in Cardigan and Carmarthensbire, owing to the numerous Calymene duplicata, C. Cambrensis, Cheirurus Sedgwickii, undulations into which they have been thrown. Their Ogygia Buchii, Trinucleus concentricus, T. Lloydiš

. The chief interest lies in the transition which they present brachiopods number 37 species, including the genera between the fanpa of the Lower and Upper Silurian forOrthis, Leptæna, Strophomena, Lingula, Siphonotreta. mations. They have yielded in all about 128 species of The lamellibranchs are represented by 6 species, the fossils, whereof 11 are peculiar, 93 are common to the gasteropods by 10 (Murchisonia, Cyclonema, Loxonema), Bala group below, and 83 pass up into Upper Llandovery the beteropods by 7 (Bellerophon), the pteropods by 2 rocks above. Some of the peculiar fossils are Nidulités (Conularia, Theca), the cephalopods by 8 (Orthoceras, favus, Meristella crassa, M. angustifrons, and Murchisonia Cyrtoceras).

angulata. Among the forms which come up from the A remarkable feature in the history of the Llandeilo Bala-group and disappear here are the corals Heliolites rocks in Britain was the outbreak of volcanic action interstinctus, Petraia subduplicatu, and Favosites aspera ; abundantly in North Wales and in Cumberland. Vast the trilobites Lichas laxatus and íllænus Bowmanni ; the piles of lava and ashes were thrown out, which even to brachiopods Orthis Actonice and O. insularis ; the gasterothis day remain in mass sufficient to form groups of pods NȚurchisonia gyrogonia and Cylonema crebristria; important hills, as Cader Idris, Aran Mowddwy, the and the cepbalopod Orthoceras tenuicinctum. But many Arenigs, and the Moelwyns in Wales, and Helvellyn and of the Lower Silurian forms continue on into the Upper Scaw Fell in Westmoreland and Cumberland.

Llandovery beds. From the abundance of the peculiar 3. Caradoc or Bala Group.—Under this name are brachiopods termed Pentamerus in the Lower, but still placed the thick yellowish and grey sandstones of Caer more in the Upper Llandovery rocks, these strata were Caradoc in Shropshire, and the grey and dark slates,. grits, formerly grouped together under the name of "Pentamerus and sandstones round Bala in Merionethshire. In the beds." Though the same species are found in both divi. Shropsbire area some of the rocks aro so shelly as to sions, Pentamerus oblongus is chiefly characteristic of the become strongly calcareous. In the Bala district the upper group and comparatively infrequent in the lower, strata contain two limestones separated by a sandy and while Stricklandinia (Pentamerus) lens abounds in the slaty group of rocks 1400 feet thick. The lower or Bala lower but appears more sparingly in the upper. limestone (25 feet thick) has been traced as a variable The Lower Silurian rocks, typically developed in Wales, extend band over a large area in North Wales. It is usually over nearly the whole of Britain, though largely buried under more identified with the Coniston limestone of the Westmore recent formations. They rise into the hilly tracts of Westmoreland land region. The upper or Hirnant limestone (10 fect) is 1 in descending order :

and Cumberland, where they consist of the following subdivisions more local Bands of voleanic tuff and large beds of (Lower Llandovery not represeuted.) various felsitic lavas occur among the Bala beds, and Coniston Limestone and Shale

Bala beds. prove the contemporaneous ejection of volcanic products.

Volcanic series: tuffs and lavas Part of Bala, whole

without any intermixture of These attain a thickness of several thousand feet in the

of Llandeilo, and

ordinary sedimentary strata ex- perhaps part of Snowdon region.

cept at the base, 12,000 ft. ...... Arenig formation. A large suite of fossils has been obtained from this

Skiddaw Slates, 10,000 or 12,000 ft.

Arenig, with perhaps

Tremadoc and formation :-the sponges represented by Sphærospongia

Lingula Flags. and other genera; the graptolites by Diplograptus pristis,

Apart from the massive intercalation of volcanic rocks these Graptolithics priodon, and G. Sedgwickii

, &c. ; the corals

present considerable lithological and palæontological differby species of Heliolites, Favosites, Monticulipora, Huly- ences from the typical subdivisions in Wales. The Skiddaw slates sites, Petraia; the echinoderms by encri nites of the genera are black or dark-grey argillaceous, and in some beds“ sandy rocks, Cyathocrinus and Glyptocrinus, by cysti deans of the genera

often much cleaved though seldom yielding workable slates, somes

times soft and black like Carboniferous shale. As a rule they are Echinosphærites and Sphæronites, and by star-fishes of the singularly unfossiliferous, but in some of their less cleaved and genera Palæaster and Stenaster; the annolides by Serpulites, altered portions they have yielded about 40 species of graptolites Tentaculites, and numerous burrows and tracks; the trilo- (chiefly of the genera Didymograptus, Diplograptus, Dichograptus, bites by many species of the genera Phacops, Cheirurus, Tetragraptus, Phyllograptus, and Climacograptus) Lingulă brevis,

traces of annelides, a few trilobites (Æglina, Agnostus, Asaphus, Cybele, Lichas, Acidaspis, Calymene, Remopleurides, Asa

&c.), some phyllopods (Caryocaris), and remains of plants (Buthophus, Illænus, Ampyć, and Trinucleus ; the polyzoa by trephis, &c.). In many places the slates have been metamorphosed, Fenestella, Glauconome, and Ptilodictya ; the brachiopods / passing into chiastolite-slate, mica-schist, andalusite-schist, &cia

[ocr errors]

base not seen



« EelmineJätka »