silurian rocks lower upper species limestone ludlow shales series abundant
GREAT BRITAIN. - In the typical area where Murchison's discoveries were first made Ile found the Silurian rocks divisible into two great and well-marked series, which he termed Lower and Upper. This classification has been found to hold good over a large part of the world. The subjoined table shows the present arrangement and nomenclature of the various subdivisions of the Silurian system.
A. Lower Siluricm.
Lingulella, Obolellct, Discina, Siphonotreta, Orthis), 8 lamellibranehs, 3 gasteropods, and 5 cephalopods have been found ; but the most abundant organisms are the graptolites, of which the Arenig rocks of St David's, in Pembrokeshire, have yielded 48 species, which belong to 20 genera, including Diciymograptits, Tetragraptus, graptus, Dendrograptus, and Callograptus.2 Altogether Llandeilo Flag Group. - Dark argillaceons flagstones, sandstones, and shales, some parts often calcareous. These beds were first described by Murchison as occurring at Llandeilo, in Carmarthenshire. They reappear on the coast of Pembrokeshire, and at Builth, in Radnorshire. Up to the present time they have yielded 227 species of fossils. Of these 13 are common to the Arenig below, 82 to the Caradoc or Bala above, while 145 are peculiar. The hydrozoa are still the most abundant forms, 94 species being here met with, no fewer than 81 of these being confined to Llandeilo rocks, and only 9 passing down into the Arenig group. Of crustacea 44 species have been obtained. These include the characteristic trilobites - Ampyx aaud3ts, Asap/us tyrannus, Barraluiea Cordwi, Calymene duplicata, C. Cambrensis, Cheirurus Sedywickii, Ogygia Buchii, Trinucleus concentricus, T. Lloydii. The brachiopods number 37 species, including the genera Orthis, Lepkena, Strophomena, Liugala, Siphonotreta. The lamellibranchs are represented by 6 species, the gasteropods by 10 (.3furchisonia, Cyclonema, Loxonema), the heteropods by 7 (Bellerophon), the pteropods by 2 (Comdaria, Theca), the cephalopods by S (Orthoceras, Cyrtoceras).
A remarkable feature in the history of the Llandeilo rocks in Britain was the outbreak of volcanic action abundantly in North Wales and in Cumberland. Vast piles of lava and ashes were thrown out, which even to this day remain in mass sufficient to form groups of important hills, as Cader Idris, Aran Mowddwy, the Arenigs, and the Moelwyns in Wales, and Helvellyn and Scaw Fell in Westmoreland and Cumberland.
A large suite of fossils has been obtained from this formation : - the sponges represented by Splaerospongia and other genera ; the graptolites by Diplograptus Craptolithus priodon, and C. Sedgu'ickii, &c. ; the corals by species of Ileliolites, Favosites, Monticulipora, Halysites, Petraia; the echinoderms by encrinites of the genera Cyathocrinus and Glyptocrinus, by cystideans of the genera Echinosplaerites and Sphceronites, and by star-fishes of the genera Palmaster and Slenaster; the annelides by Serpulites, Tentaculites, and numerous burrows and tracks ; the trilobites by many species of the genera Phacops, Cheirurus, Cybele, Lichas, Acidaspis, Calymene, Remopleurides, Asaphus, Ilkenus, A mpyx, and Trinucleus; the polyzoa by Fenestella, Clauconome, and Pti/odictpa : the brachiopods by Atrypa, Rhynchonella, Lepta'na, Orthis (many species), Strophomena, Discina, and Lingula ; the lamellibranchs by Modiolopsis, Palrearca, Pterinea, Ortlionota, and Ctenodonta; the gasteropods by Murchisonia, Pleurotomaria, liaphistonia, Cyclone:WU!, Euomphalus, Alaclurea, Holopea ; the pteropods by Cunularia, Theca, and Ecculiomphalvs; the heteropods by various species of Bellerophon; and the cephalopods by many species of Orlhoceras, with forms of Cyrtoceras and Lituites.
The Lower Silurian rocks, typically developed in Wales, extend over nearly the whole of Britain, though largely buried under more recent formations. They rise into the hilly tracts of Westmoreland and Cumberland, where they consist of the following subdivisions in descending order : - (Lower Llandovery not represented.) Coniston Limestone and Shale = Bala beds.
Volcanic series: tuffs and lavas Part of Bala, whole without any intermixture of of Llandeilo, and ordinary sedimentary strata ex- perhaps part of cept at the base, 12,000 ft Arenig formation.
Skiddaw Slates, 10,000 or 12,000 ft Arenig, with perhaps base not seen Tremadoe and Lingula Flags.
Apart from the massive intercalation of volcanic rocks these strata present considerable lithological and palaeontological differences from the typical subdivisions in Wales. The Skiddaw slates are black or dark grey argillaceous, and in some beds sandy rocks, often much cleaved though seldom yielding workable slates, sometimes soft and black like Carboniferous shale. As a rule they are singularly unfossiliferous, but in some of their less cleaved and altered portions they have yielded about 40 species of graptolites (chiefly of the genera Didymograptus, Diplograptus, Dichoyroptu.s, Tetragraptus, Phyllographes, and Climacograptus) Liagula brens, traces of annelides, a few trilobites (.-Eglina, Apostles, Avrphus, &e.), some phyllopods (Caryocaris), and remains of plants (Buthotrephis, &c.). In many places the slates have been metamorphosed, passing into chiastolite-slate, mica-schist, andalusite-sehist, &c., with protrusions of granite, syenite, and other crystalline rocks. Towards the close of the long period represented by the Skiddaw slates, volcanic action manifested itself, first by intermittent showers of ashes and streams of lava which were interstratified with the ordinary marine sediment, and then by a more powerful and continuous series of explosions, whereby a huge volcanic mountain or group of cones was piled up above the sea-level. The length of time occupied by this volcanic episode in Cumbrian geology may be inferred from the fact that all the Llandeilo and nearly all the Bala beds are absent here. The volcanic island slowly sank into a sea where Bala organisms flourished. Among these we find such familiar Bala species as Fanosiles fibrosa, Heliolitcs interslinctus, Cybele verrucosa, Lepa a ciut serieca, orls Actonicc, O. biforata, 0. caligramma, 0. elegantula, 0. poreata, and Strophomena rhomboidalis. These organisms and their associates gathered on the submerged flanks of the sinking volcano into a bed of limestone - the Coniston limestone - which can still be traced for many miles through the Westmoreland hills, as the Bala limestone which it represents can be followed through the volcanic tracts of North Wales, The Coniston limestone is covered by certain flags and grits which from their organic remains are referred to the Upper Silurian series.
In the South of Scotland, according to the detailed researches of the Geological Survey, the Lower Silurian formations are represented by the subjoined groups of strata in descending order: Smdstones and conglomerates, C iryan = Llandovery.
Conglomerates, grits, shales, and len-) titular bands of limestone, Peebles- I = Caradoc or Bala.
shire, Dumfriesshire, S.W. Ayr- j shire, sometimes 2000 ft.
Carsphairn group, coarse pebbly grits with band. of fine conglomerate, = Llandeilo (14 000 ft.) Queensberry group, massive greywackes and grits, with occasional conglomerate bands and some shales, 4500 ft. Lower or Moffat Black Shale group, 200-400 ft. Ardwell group, brown flags, greywackes, and shales, sometimes purplish and red ; base not seen .. J As a whole these strata are singularly barren of organic remains. Most of the fossils which the Llandeilo groups contain lie in the bands of dark anthracitic shale which have been traced across nearly the whole breadth of the country. These shales are crowded with graptolites of recognizable Llantleilo forms, Climacograptus teretiuscubus, Diplograptus pristis, and Graptolithus sagitlarius being particularly abundant. Crustacea are exceedingly rare, but two phyllopods, Discinocaris Ilroutniana and Pcllocharis aptychoides, occur ; while from Dumfriesshire two obscure trilobites are referred doubtfully to Encrinurus and Phacops. The vast thickness of sandy, gritty, and shaly unfossiliferous strata is the distinguishing feature of the Lower Silurian series in the south of Scotland. The Caradoc or Bala group lies unconformably upon the upper parts of the Llandeilo rocks. It contains in the eastern districts some calcareous conglomerates which here and there swell out into local masses of limestone. In the south-west of Ayrshire the limestones attain considerable dimensions. In these calcareous bands numerous Caradoe species have been found, among them Cheirurus gelasinosus, Encrinurus punetatus, with species of ///ccnus and ..4sapkus, Orthis calligramma, 0. confinis, Leptcena scricea, Haclurca, and such corals as Ilcliolites, Fcvositcs, Omphyma, and Strephodcs. In the south-west of Ayrshire certain shales and sandstones full of Caradoc fossils are overlaid with sandstones, shales, and conglomerates containing Pentamorus oblongus, A trypa hemispherica, Mcristella angustifrons, Lichas laxatus, Petraia clongata, Nidulitcs fames, and numerous other fossils which indicate the horizon of the Llandovery rocks.
The Highlands of Scotland consist mainly of crystalline rocks - gneiss, mica-schist, chlorite-schist, clay-slate, quartz-rock, schistose flagstone, and many others, often much invaded by granite and other intrusive masses. It was at one time supposed that these rocks all belonged to the so-called primary or primitive series, older than any of the fossiliferous systems. But the discovery by Mr C. W. Peach, already referred to, that recognizable fossils occur in the limestone of Harness in Sutherlandshire, led Murchison to infer that the whole of the overlying gneissose and schistose masses are really metamorphosed Lower Silurian rocks - a generalization which has been completely confirmed by subsequent investigation. At the base of this great series of rocks masses of white quartz-rock are found lying with a marked uneonforinability upon the red sandstones described in a previous page. These quartzose beds are merely hardened and somewhat metamorphosed sandstones ; they still show their original false-bedding, and the casts of sea-weeds and worm-burrows. They contain a band of limestone which in Assynt swells out to a thickness of 1000 feet or more, and can be traced almost continuously from the Kyles of Skye to the north coast of Sutherlandshire. Over these strata, in perfect conformable sequence, and with a complete lithological gradation, conic quartzose flagstones dipping like the rocks below at gentle angles to the southeast. They become more schistose and crumpled as they are traced. upwards, until, after a thickness of several thousand feet has been passed over, they begin to undulate in steep folds and pass into the ordinary schistose rocks which cover so much of the Highlands. The gradation from the comparatively unaltered lower quartz-rocks and limestones on the west to the intensely crumpled crystalline upper schists and flagstones on the east can be followed step by step in numerous fine natural sections from the north_of Sutherland to the Kyles of Skye. The proof is thus complete that a vast mass of schists and other crystalline rocks overlies fossiliferous limestones in the Scottish Highlands. It therefore becomes of the utmost importance to determine the geological horizon of the fossils in the limestones. This was done by the late Mr Salter, who declared his conviction that they were unequivocally Lower Silurian, and bore a most remarkable resemblance to a group of fossils- from the Lower Silurian rocks of North America. Five of the species lie regarded as identical with known American forms (Orthoceras arcuoliralum, IIall ; Orthis striatula, Emmons ; Ophileta compacta, Salt. ; Murchisonia gracilis, Hall ; AL belliciucta, Hall), 4 as representative, 3 doubtful, and 1 new genus, found also in Canada. " That this truly North American assemblage," he remarks, " should he found in the extreme north of Scotland on the same parallel as the Canadian, - that species of Macturca and Raphistoma, resembling those of the St Lawrence basin, and Orthoccrata, bearing large siphuncles like those of North America, Scandinavia, and Russia, should occur in Scotland and yet be scarcely known further south, is at least suggestive of a geographical distribution - perhaps even of climatal conditions - not very unlike that of more modern times."1 From this pal ontological decision it follows that the overlying schistose series• of the Scottish Highlands is a mass of metamorphosed Silurian strata. Examined in detail they show very unequal and sporadic metamorphism. Some portions are scarcely more changed than the ordinary greywackes and shales of unaltered districts. False-bedding, pebble-beds, and other common features of sedimentation occur abundantly throughout the whole vast series of schists. Here and there the metamorphism has become extreme, the rocks passing into coarsely crystalline schists full of garnets, with bands of hornblende-rock, actinolite-schist, and other metamorphic products, and passing even into granitic gneiss and true granite. No more convincing proof could be obtained that vast masses of schist do not necessarily belong to an azoic period of the earth's history, but may have been produced by the alteration of previously existing sediments.
It is not necessary to believe that the sediments so altered were in all cases mere ordinary marine sand and mud. The white quartz-rocks were no doubt at one time pure white siliceous sand, the rounded grains of which can still be readily detected in them. The quartzose flagstones were stratified sand with thin partings of clay or mud. The clay slates were evidently thick accumulations of mud. But the rocks containing a marked percentage of magnesia, such as chlorite-slate, actinolite-schist, ho•nblende-rock, &c., may have resulted from the alteration of volcanic sediments and submarine lavas. The evidence from Cumberland and Wales proves how voluminous and long continued were the volcanic eruptions of the Lower Silurian period in Britain. The abundant diffusion of volcanic detritus over the present sea-bottom is now well known. The " Challenger" researches have also shown us that, besides the glauconite previously known to be deposited from sea-water in the chambers of foraminifera and other dead organisms on the ocean-bottom, true magnesian silicates are now in the process of elimination from sea-water in some of the abysses of the ocean. It is quite possible therefore that some of the rocks of the metamorphic series rich in magnesian silicates may have arisen from the alteration of volcanic tuffs or submarine lavas, and that others may owe their distinctive composition to original chemical precipitation, as ably contended by Sterry Hunt, though their present crystalline structure must he regarded as a part of the general metamorphism by which the whole of the Lower Silurian rocks of the Highlands have been affected.
In the south-east of Ireland, grey, greenish, and purple grits, and grey and dark shales, lie uncomformably upon the Cambrian rocks, and contain a few fossils of Llandeilo age. They present interstratified beds of tuff and felsitic lavas indicating contemporaneous volcanic action. In the north-east of the island a broad belt of Lower Silurian rocks runs from the coast of Down into the heart of Roscommon and Longford. This belt is evidently a prolongation of that in the southern uplands of Scotland. It is marked by the occurrence of similar dark anthracitic shales crowded with graptolites. The richest fossiliferous localities among the Irish Lower Silurian ro.:ks are found at the Chair of Kildare, Portrane near Dublin, Pomeroy in Tyrone, and Lisbellan in Fermanagh, where small protrusions of the older rocks rise as oases among the surrounding later formations. Portlock brought the northern and western localities to light, and Murchison pointed out that, while a number of the trilobites (Trinucleus, Phaeops, Calymote, and //knus), as well as the simple plaited Orthichc, Lephcnce, and Strophoutencr, some spiral shells, and many Orthoeerata, are specifically identical with those from the typical Caradoc and Bala beds of Shropshire and Wales, yet they are associated with peculiar forms, first discovered in Ireland, and very rare elsewhere in the British Islands. Among these distinctive fossils lie cites the trilobites, Bemopleurides, liarpes, Amphion, and Bronteus, with the smooth forms of Asaphus (lsotelus), which, though abundant in Ireland and America, seldom occur in Wales or England, and never on the Continent.' In the north and west of Ireland a large area of surface is occupied by crystalline rocks - gneiss, schists, quartz•rocks, limestone, granite, Ste. - which are manifestly a continuation of those of the Highlands of Scotland. They run south-westward parallel with the belt of unaltered Lower Silurian rocks from which, in sonic places, as in county Tyrone, they are only a few miles distant. The district of Pomeroy, so rich in Silurian fossils, promises to afford the greatest light on the interesting but difficult problem of the metamorphism of the Lower Silurian rocks of the Scottish Highlands and the north-west of Ireland. It will be seen from the evidence furnished by the sections in West Mayo (p. 337) that the metamorphism must have taken place prior to the deposition of the Upper Silurian formations of the west of Ireland.
B. Upper Silurian.
The formations which in the British Islands are classed as Upper Silurian occur in two very distinct types. So great indeed is the contrast between these types that it is only by a comparison of organic remains that the whole can he grouped together as the deposits of one great geological period. In the original region described by Murchison, and from which his type of the system was taken, the strata are comparatively flat, soft, unaltered, consisting mainly of somewhat incoherent sandy mud with occasional bands of limestone. But as these rocks are followed into North Wales, they are found to swell out into a vast series of grits and shales so like portions of the hard altered Lower Silurian rocks that, save for the evidence of fossils, they would naturally be grouped as part of that more ancient series. In 'Westmoreland and Cumberland, and still further north in the border counties of Scotland, also in the south-west of Ireland, it is the North Welsh type which prevails, so that in Britain the general lithological characters and minute palaeontological subdivisions ascertained in the typical Silurian district are almost confined to that limited region, while over the rest of the British area for thousands of square miles the hard sandy and shaly type of North Wales is prevalent.
Taking first the Silurian tract of thesouth-west of England, and the east and south of Wales, we find a decided uncoil-formability separating the Lower from the Upper Silurian formations. In some places the latter are found passing across the edges of the former, group after group, till they come to lie directly upon the Cambrian rocks. Indeed, in one district between the Longmynd and Wenlock edge, the base of the Upper Silurian rocks is found within a few miles to pass frum the Caradoc group across to the Lower Cambrian rocks. It is evident, therefore, that in the Welsh region very great disturbance and extensive denudation preceded the commencement of the deposition of the Upper Silurian rocks. As Professor Ramsay has pointed out, the area of Wales, previously covered by a wide though shallow sea, was ridged up into a series of islands, round the margin of which the conglomerates at the base of the Upper Silurian series began to be laid down. This took place during a time of submergence, for these conglomeratic and sandy strata are found creeping up the slopes and even capping sonic of the heights, as at Bogmine, where they reach a height of 1150 feet above the sea." The subsidence probably continued during the whole of the interval occupied by the deposition of the -Upper Silurian strata, which thus were piled to a depth of from 3000 to 5000 feet over the disturbed and denudecl platform of Lower Silurian rocks.
Arranged in tabular form, the subdivisions of the Upper Silurian rocks of Wales and the adjoining counties of England are in descending order as follows: - Base of Old Red Sandstone.
r Luw Rock.
Aymestr ydlo Limestone.
Lower Ludlow Rock.
Wenlock or Dudley LimeWoolhope or Barr Lime- North Wales.
t_ stone and Shale I. Upper Lland- Tarannon Shale.
overy group... ( May Hill Sandstones.
Lower Llandovery Rocks.
I. Upper Llandovery Group. - (a.) Hay Hill Sancldones. - The position of these rocks as the true bass of the Upper Silurian formations was first shown in 1853 by Sedgwick, who named thesis the May Hill Sandstones from the locality in Gloucestershire where they are so well displayed. Appearing on the coast of Pembrokeshire at Marloes Bay, they range across South Wales until they are overlapped by the Old Red Sandstone. They emerge again in Carmarthenshire, and trend north-eastward as a narrow strip at the base of the Upper Silurian series, from a few feet to 1000 feet or more in thickness, as far as the Lougmynd, where as a marked conglomerate wrapping round that ancient Cambrian ridge they disappear. In the course of this long tract they pass successively and uncouforma,bly ovet Lower Llandoyery, Caradoc, Llandeilo, and Cambrian rocks. They consist of yellow and brown ferruginous sandstones, often full of shells, which are apt to weather out and leave casts. Their lower parts are commonly conglomeratic, the pebbles being largely derived from older parts of the Silurian formations. Here and there, where the organic remains become extraordinarily abundant, the strata pass into a kind of sandy limestone, known as the " Pentamerus limestone," from the numbers of this brachiopod contained in it. The species of fossils found in the May Hill Sandstones number about 230.
Among these arc some traces of fucoids; sponges (Mona, Isehadites); the widely diffused Or«ptolithus priorlun; a number of corals (Petraia, Zleliolites, Favosites, Halysites, Syringopora, a. few erinoids; sonic annelides, particularly the Tentaeulitcs av gl ions, which is abundant ; a member of trilobites, of which Phaeops Stoixsii, P. Weaveri, Enerinnrus punctatus, and Calymu cue Blumenbachii arc common ; numerous brachiopods, as A trypa leemispherica, A. relieularis, Pentamervs oblongus, .S7ricklundinia lirala (S. has also occurs), Leptccna transversalis, Orthis ealligramma, 0. argon-tide, 0. reverse, Strophomena compressa, peeten, and Bingula parallela ; lamellibranchs of the mytiloid genera Orthonola, lllytilus, and lifodiolopsis, with forms of Pterinea, Clenodonta, and Lyrodesma; gasteropods, particularly the genera Hurcleisonia , Pleurotomaria, Cyclonema, Holopella; and cephalopods, chiefly O•thecerata, with some forms of Actinoeeras aunt Phragmoceras, and ties old species Lituites eorn-u-arietis.
(b.) Tarannon Shale. - Above the Upper Llandovery beds comes a very persistent zone of fine, smooth, light grey or blue slates, which has been traced down the whole length of Wales from the mouth of the Conway into Carmarthen-shire. These rocks, termed the " paste-rock " by Sedgwiek, have an extreme thickness of 1000 to 1500 feet, Barren in organic remains, their chief interest lies in the fact that the persistence of so thick a band of rock between what were supposed to be continuous and conformable formations should have been unrecognized until it was proved by the detailed mapping of the Geological Survey.
It is a characteristic of the older Paleozoic limestones to occur in a very lenticular form, swelling in some places to a great thickness and rapidly dying out, to reappear again perhaps some miles away with increased proportions. This local character is well exhibited by the Woolhope limestone. Where it dies out, the shales underneath and intercalated with it join on continuously to the overlying ‘Venlock shale, and no line for the Woolhope group can then be satisfactorily drawn. The same discontinuity is strikingly traceable in the Weulock limestone to be immediately referred to.
TFem luck Shale. - Thisis a group of grey and black fine shales, traceable from the banks of the Severn near Coal-brook Dale across Radnorshire to near Carmarthen - a distance of about 90 miles. The same strata reappear in the protrusions of Upper Silurian rock which rise out of the Old Red Sandstone plains of Gloucestershire, Herefordshire, and Monmouthshire. In the Malvern Hills they were estimated by Professor Phillips to reach a thickness of 640 feet, but towards the north they thicken out to 1000 or even 1-100 feet. On the whole the fossils are identical with those of the overlying limestone. The corals, however, so abundant in that rock are here comparatively rare. The brachiopods (of the genera Leptama, Orthis, St•ophomena, Atrypa, and Rhynehonella) are generally of small sizeOrthis biloba, 0. hybrida, and the large flat 0. rustica, being characteristic. Of the higher mollusca thin-shelled forms of Orthoeeras are specially abundant. Among the trilobites, Eacrin ?was pun ctatus, E. variolar is, Calymene Blomenbachii, C. tubwrecelosa, Phacops eaudatos, and P. longicaudatos are common. The Graptolithas priocion, so frequent among the Bala beds of the Lower Silurian series, also occurs in the Wenlock shale. liraptolithos Rentingii is here a characteristic species.
Wenlock Limestone is a thick-bedded, sometimes flaggy; usually more or less concretionary limestone, grey or pale pink, often highly crystalline, occurring in some places as a single massive bed, in others as two or more strata separated by grey shales, the whole forming a thickness of rock ranging from 100 to 300 feet. As its name denotes, this stratum is typically developed along Wenlock Edge in Shropshire, where it runs as a prominent ridge for fully 20 miles, also between Aymestry and Ludlow. It likewise appears at the detached areas of Upper Silurian strata above referred to, being specially well seen near Dudley (whence it is often spoken of as the Dudley limestone), Woolhope, Malvern, May Hill, and Usk in Monmouthshire.
A distinguishing characteristic of the Wenlock limestone is the abundance and variety of its corals, of which 53 species have been described. The rock seems indeed to have been formed in part by massive sheets and bunches of coral. Among characteristic species are Banishes catcnularia, Hcliolites interstinctus, II. tubulatus, Alceolites labechei, Favosites aspera., P. fibrosa, F. Gotlelauulitu, Comitcs juniperinus, Sgringopora faseicatlaris, and Omphyma turblization. The crinoids arc also specially abundant, and are often beautifully preserved : Periechocrinus moniliforanis is one of the most frequent species; others arc Crotalocrimes rugosus, Cyathocrinus goniodactylits, and .711arsupiocrinus ccelatus; with several eystideans, as Pscudocrivites guadrifaseiatus. The crustaceans include numerous trilobites, among which we miss some of the persistent Lower Silurian genera, such as Asaphus, Ofiggia, and TriYtudeus, none of which ascend into the Wen-lock group. The most abundant trilobite is the long-lived Calymenc Bluntenbaeltii, which ranges from the Llandeilo flags up to near the top of the Upper Silurian formations. It occurs abundantly at Dudley, where it received the name of the " Dudley Locust." Other common forms are Encrinurics punctatus, E. variolaris, Phacops emulates, P. Downingim, P. Stokesii, 8717,7mq-us Barricnsis, llama°. notes delphinocephalus, and Cheintras binzucronatus. The brachiopods continue to be abundant ; among typical species may be noted Atrypa rcticularis, Meristella tumicla, Spirifcr elevates, S.
Rhyuchonclla borealis (very common), R. euneata, Orthis elegantula, 0. rust lea, Strophomena rhomboidalis, and Pcntaincrus galcatus. The lamellibranchs are not well represented; but several species of Ptcrinea are abundant, with Graminysia cingulata, and some species of Mocliolopsis and Ctenodonta. The gasteropods are most characteristically marked by 8 or 9 species of Euomphalus, 3 or more of Hurdeisonia, with species of Plcurotomaria, Acroculia, and Cycloncnia. The cephalopods are confined to few genera, Lituites, Actinoceras, Crloccras, Orthoceras, and Phragmoccras ; of these the orthoceratites are by far the most abundant both in species and individuals. Orthoceras annulatum is the most common form. The pteropods appear in the beautiful and very abundant Conularia Somecrbgi, and the heteropods in the common and characteristic Bellerophon Wenlockensis.
(a.) Lower Ludlow Bosh - This is a group of soft dark-grey to pale greenish-brown or olive sandy shales, often with calcareous concretions. Much of the rock, however, presents so little fissile structure as to get the name of mudstone, weathering out into concretions which fall to angular fragments as the rock crumbles down. It becomes more sandy and flaggy towards the top. From the softness of the shales this zone of rock has been extensively denuded, and the Wenlock limestone rises up boldly from under it.
An abundant suite of fossils has been yielded by those shales. No fewer than 18 species of star-fishes, belonging to 0 genera, have been described (Protastcr, like the brittle-stars of the British seas, Palcrocoma, Palastcrina). A few graptolites occur, particularly the persistent Grapto/ithas priodon (common), G. colones, and G. Flemingii. A few of the Wenlock corals survive in the Lower Ludlow rook, but the conditions of deposit were evidently unfavourable for their growth. The trilobites are less numerous than in older beds ; they include the venerable Calymcue Blumcnbachii, Phacops caudatus, and its still longer-tailed variety P. longicam-lotus ; also Aciclaspis Wrig7mdi, Homalonotus delphinocephalus, and Cyphaspis inegalops. But other forms of crustacean life occur in some number. As the trilobites begin to wane numerous phyllopods appear, the genus Ccratiocaris being represented by 10 or more species. Large eurypterids now make their entrance upon geological history - Eurgptcrus, Ptarygotus, and Hemiaspy. Though brachiopods are not scarce, hardly any seem to be peculiar to the Lower Ludlow rock, the Lingula Iota, which Murchison suggested might be peculiar, having been obtained From what is supposed to be representative of this group of strata in IrVestmoreland. Bhyiwhonclla tyn7ivWn, Spirifcr exporratus, Stronho?nenc, suglypha, Atrypa rcticularis, and Chonetes 7ithii1)1(1 are not infrequoit. Among the more frequently reclining species of lamellibrawls the following may be named - Card/oha interrupta, C. striata, Orlhonota rigid«, 0. scmisulcata, and a number of species of Pterinca. The orthoeeratites are numerous, as Orthoccras Ladcusc, 0. subundulatum, also species of Phragmoceras and Lituites. The numbers of these straight and curved cephalopods form one of the distinguishing features of the zone. At one locality, near Leintwardine in Shropshire, which has been prolific in Lower Ludlow fossils, particularly in sta•-fishes and eurypterid crustaceans, a fragment of the fish. Pleraspis was discovered in 1859. This is the earliest trace of vertebrate life yet detected. It is interesting to note that the Pteraspis does not stand low in the scale of organiza- ' tion, but has affinities with our modern sturgeon.
(b.) Aymestry Limestone is a dark grey somewhat earthy concretionary limestone in beds from 1 to 5 feet thick, Where at its thickest it forms a conspicuous feature, rising above the soft and denuded Lower Ludlow shales and, owing to the easily removable nature of some fuller's earth on which it lies, it has here and there been dislocated by large landslips. It is still more inconstant than the Wen lock limestone. Though well developed at Aymestry it soon dies away into bands of calcareous nodules, which finally disappear, and the lower and upper divisions of the Ludlow group then come together. The most characteristic fossil is the Pentaneerus linightii; other common forms are Rhynehonella Linqula Lesoisii, Strophomena euglypha, Bellerophon dilatvt us, Pterinea Soicerbyi, with many of the same shells, corals, and trilobites found in the Wenlock limestone. Indeed, as -Murchison has pointed out, except in the less number of species and the occurrence of some of the shells more characteristic of the Upper Ludlow zone, there is not much pakeontologieal distinction between the two limestones.( (e.) Upper Ludlow Bock. - In the original Silurian district described by Murchison, the Aymestry limestone is covered by a calcareous shay band full of Rhynehonella navicula, sometimes 30 or 40 feet thick. This layer is succeeded by grey sanely shale or mudstone, often weathering into concretions, as in the Lower Ludlow zone, and assuming externally the same rusty-brown or greyish olive-green hue. Its harder beds are quarried for building stone ; but the general character of the deposit, like that of the argillaceous portions of the Upper Silurian formations as a whole in the typical district of Siluria, is soft, incoherent, and crumbling, easily decomposing once more into the original mud, and presenting in this respect a contrast to the hard fissile and often slaty shales of the Lower Silurian series. Many of the sandstone beds are crowded with ripple-marks, rill-marks, and annelid-trails, indicative of the shallow littoral waters in which they were deposited. One of the uppermost sandstones is termed the " Fucoid Bed," from the number of its cylindrical sea-weed-like stems. It likewise contains numerous inverted pyramidal bodies, which are believed to be casts of the cavities made in the muddy sand by the rotatory movement of crinoids rooted and half-buried in the micaceous mud.2 At the top of the Upper Ludlow rock near the town of Ludlow, a brown layer occurs from a quarter of an inch to 3 or 4 inches in thickness, full of fragments of fish, Pterygotus, and shells. This layer, termed the " Ludlow Bone-bed," is the oldest from which any considerable number of vertebrate remains has been obtained. In spite of its insignificant thickness it has been detected at numerous localities from Ludlow as far as Pyrton passage, at the mouth of the Severn - a distance of 45 miles from north to south, and from Kington to Ledbury and Malvern - a distance of nearly 30 miles from west to east ; so that it probably covers an area (now largely buried under Old Red Sandstone) not less than 1000 square miles in extent, yet it appears never to exceed and usually to fall short of a thickness of 1 foot. Fish remains, however, are not confined to this horizon. They have been detected in strata above the original bone-bed at Ludlow, together with some minute globular bodies believed to be the sporangia of a lycopod. These, with some other plant remains from the same district, are the earliest traces of land vegetation yet found. The higher parts of the Ludlow rock consist of fine yellow sandstone and harder grits known as the Downton sandstone. Originally the whole of these flaggy upper parts of the Ludlow group were called " Tilestones " by Murchison, and being often red in colour were included by him as the base of the Old lied Sandstone, into which they gradually and conformably ascend. Undoubtedly they show the gradual change of physical conditions which took place at the close of the Silurian period in the west of England, and brought in the deposits of the Old Red Saud-stone. But as their organic contents are still unequivocally those of the Ludlow group, they are now classed as the uppermost zone of the Silurian system.
A considerable suite of organic remains has been obtained from the Upper Ludlow rock, which on the whole are the same. as those in the zones underneath. Vegetable remains, some of which seem to be fueoids, but most of which are probably terrestrial and lyeopodiaecous, abound in the Downton sandstone and passage-buds into the Old Red Sandstone. Corals, as might be supposed from the muddy character of the deposit, seldom occur, though Murchison mentions that the encrusting form Alccolitcs fibres-us may not infrequently be found enveloping shells, Cyclancsna eamiui and .2111trchisonia corolla being, as their names imply, its favourite habitats. Sonic annehides (Scrpulites longispin.US, Cornaliles scipularins, Tentoculites tennis, and Trachyderma coriacca) are not uncommon. The ernstacea are represented chiefly by small ostracods (Berichia Kltedeni, Lepertlitia marginata, Emtoneis tubmsa), and by species cf. CeroticLh'cipocaris, Earyptcrus, Hemiaspis, Plerygolus, and Sty/onunts ; the trilobites having still further waned, though Ilonalonotus Kniglstii, Encrisricrus punclatits, Pliacops Downiarlitc, and a few others still occur, and even the persistent Calynienc Blanton. bachli may occasionally be found. Of the brachiopods the most abundant forms are Rhysichonella aucala, Chonctes stria kilo, _Disci o a rugata, and Livula Corsica. The most characteristic lamellibranchs are Orthonota amygdalisia, Goalophora cymbafi,rmis, Ptcrinca lincata, P. retrojlexa; some of the commonest gasteropods are Alurchisonia corallii, Plalyschisina hclicitcs, and Holopclla obsoleta. The orthoceratites arc specifically identical with those of the Lower Ludlow rock, and are sometimes of large size, (Whore/as bullatum being specially abundant. The fish remains consist of bones, teeth, shagreen-like reales, plates, and fin-spines. They include some plagiostomous (placoid) forms (.77,c/odus, shagrcenscales, 097)7tagodies, skin, Ortchus, spines) and some ostracosteans (Cephalasins, Auchcnaspis, and Ptcraspis).
In the typical Silurian region of Shropshire and the adjacent counties, nothing can be more decided than the lithologieal evidence for the gradual disappearance of the Silurian sea, with its crowds of graptolites, trilobites, and brachiopods, and for the gradual introduction of those geographical conditions which brought about the deposit of the Old Red Sandstone. The fine grey and olive-coloured muds, with their occasional zones of limestone, are succeeded by bright red clays, sandstones, cornstones, and conglomerates. The evidence from fossils is equally explicit. Up to the top of the Ludlow rocks the abundant Silurian fauna continues in hardly diminished numbers. But as soon as the red strata begin the organic remains rapidly die out, until at last only the fish and the large eurypterid crustaceans continue to occur.
Turning how from the interesting and extremely important though limited area in which the original type of the Upper Silurian rocks is developed, we observe that whether we pass northwards or south-westwards the soft mndstones and thick limestones give way to hard slates, grits, and flagstones, among which it is scarcely possible sometimes even to discriminate what represents the Wenlock from what may be the equivalent of the Ludlow group. It is in Denbighshire and the adjacent counties that this change becomes most marked. The Tarannon shale above described passes into that region of North Wales, where it forms the base of the Upper Silurian formations. It is covered by a series of grits or sandstones which in some places are at least 3000 feet thick. These are covered by and pass laterally into hard shales, which are believed to represent harts of the true Wenlock group, perhaps even some portien of the Ludlow rocks. It is evident, however, that in spite of the wide extent over which these Silurian rocks of North Wales are spread, and the great thickness which they attain, they do not present an adequate stratigraphical equivalent for the complete succession in the original Silurian district. Instead of passing up conformably into the base of the Old Red Sandstone, as at Ludlow, they are covered by that formation unconformably. In fact they have been upturned, crumpled, faulted, and cleaved before the deposition of those portions of the Old lied Sandstone which lie upon them. These great physical changes took place in Denbighshire when, so far as the evidence goes, there was entire quiescence in the Shropshire district ; yet the distance between the two areas was not more than about 60 miles. These subterranean movements were doubtless the precursors of those more widely extended upheavals which converted the floor of the Silurian sea into a series of isolated basins, in which the Old Red Sandstone was laid down.
In Westmoreland and Cumberland a vast mass of hard slates, grits, and flags was identified by Sedgwick as of Upper Silurian age. These form the varied ranges of hills in the southern part of the lake district from near Shap to Duddon mouth. The following are the local subdivisions with the conjectural equivalents in Siluria.1 Muggy beds, with lamelli- (1)Tilestones.
Hay Fell and branchs abundant Kirkby Moor Massive greenish and greysandFlags stones, with bands of fossils, ,____. i truer Lud( low.
//o/opc//ct abundant Calcareous beds, with Rhyncho- _ Aymestry nella nay/mire abundant ( Limestone.
Bannisdale Sandstone and shale, with star-Lower LudSlates fish - low.
Dark blue flags and grits of_ lipperWengreat thickness i lock.
e Flags and greywacke (Ortho-1 ccras subundulatam, 0. augulatunt, Graptolithats Flemingii, G. colones, Ceratiocarts .1turchisoni), upwards L° = i of 4000 feet ... lock.
Dark grey coarse flags (CarConiston Flagsdida interrupta, Orthoceras ( subuudulatum), 1000 feet I j Coniston Limestone (Lower Silurian) - Caradoe or Bala.
In the northern part of the Lake district a great anticlinal fold takes place. The Skiddaw slates arch over and are succeeded by the base of the volcanic series above described. But before more than a small portion of that series has appeared the whole Silurian area is overlapped unconformably by the Carboniferous Limestone series. It is necessary to cross the broad plains of Cumberland and the south of Damfriesshire before Silurian rocks are again met with. In this intervening tract a synclinal fold must lie, for along the southern base of the uplands of the south of Scotland a belt of Upper Silurian rocks, dipping on the whole to the south-east, can be traced from the heart of the Cheviot Hills to the headlands of Wigtownshire. These rocks must reach a thickness of several thousand feet, but their top is nowhere seen. They repose on some of the older parts of the Llandeilo series, with so close a coincidence of dip and strike that no decided unconformability has yet been traced between them. They consist essentially of shales, with a considerable proportion of grey wacke bands towards the base. At different horizons they contain lenticular bands of a calcareous pebbly grit. But their most characteristic feature, and one which at once distinguishes them locally from die adjoining Lower Silurian rocks, is the occurrence of a nearly black, highly fissile shale, composed of layers in most cases as thin as ordinary writing paper and usually crowded with graptolites. These peculiar bands occur throughout the whole series of rocks from bottom to top. They are sometimes so thin that 20 or 30 seams or ribs, each finely fissile, may be seen intercalated within the space of an inch of the ordinary shale or greywacke. Occasionally they form zones SO to 100 feet thick, consisting entirely of finely leaved graptolitic shales. As a whole these Upper Silurian strata resemble lithologically the corresponding series in Westmoreland, though here and there they assume the character of mudst-ones not unlike those of Shropshire. The abundant fossils in them are simple graptolites (Graptolithus ,Sfedgzoickii' Becki, G. Flemingii, G. colon us, 0. Griestonensis, Betiolites Geinitzianus, Orthoceratites come next in point of numbers (Orthoceras annulatunt, O. teuuicinetunt, die.). In some of the shales crustacean fragments are numerous. They include large pieces of the carapace of Dictyocaris, with remains of Pterygotus and Ceratiocaris. The pebbly grits contain Petraia and crinoid sterns. In the south of Kirkcudbright certain limestones and conglomerates intercalated among these shales have yielded a more varied fauna, having ou the whole a decidedly Wenlock character. It includes Favosites, Catenipora, Beyrichia tuberculata, Phacops caudatus, Meristellct, Leptcena sericea, Atrypa reticularis, Strophomena imbrex, Murchisonia, Orthoceras tensicinctum, &e. • It is impossible in the south of Scotland to separate the Upper Silurian rocks into Wenlock and Ludlow groups. On the whole these rocks seem to be representative mainly of the older half of the Upper Silurian formations. They are covered unconformably by Lower Old Red Sandstone and later formations. In the counties of Edinburgh and Lanark, however, the base of the Lower Old lied Sandstone is found to graduate downward into a thick series of brown, olive, and grey shales, sandstones, and grits, containing undoubted Ludlow fossils. It is deserving of remark also that the peculiar litholoeical type so characteristic of the strata in the original Silurian area reappears in the centre of Scotland, many of the concretionary brown shales and olive-coloured mudstones being undistingnishable from those in the typical sections at Ludlow. Some of these beds are crowded with fossils. Among the most characteristic are Leptcena transversalis, Orthonota amygdalina, Platyschisma helicites, Beyrichict Klcedeni, Orthoceras Maclareni, with many crustaceans of the genera Ceratiocaris, Eurypterus, Pterygotus, Slimonia, and Stylonurus. In the Pentland IIills these strata are estimated to attain a thickness of 3500 to 4000 feet, but their base is nowhere reached; in Lanarkshire they are at least as thick. Their lower portions may represent some of the higher parts of the Wenlock group.
Ireland furnishes some interesting evidence regarding the geographical changes in the west of Europe between the close of the Lower Silurian and the beginning of the Upper Silurian period. It has already been pointed out that the metamorphosed Lower Silurian rocks of the Scottish Highlands are prolonged into the north of Ireland, whence they range south-westwards to Galway Bay. In the picturesque tract between Loch Mask and the mouth of Killary harbour these metamorphosed rocks are unconformably overlaid by masses of sandstones, conglomerates, and shales more than 7000 feet thick, and containing Llandovery and Wenlock fossils with a mixture of Caracloc forms. In the midst .
of the greatly metamorphosed Lower Silurian platform, portions are to be found still little altered and full of fossils. The overlyino. Upper Silurian strata have not been metamorphosed, but contain pebbles of the altered rocks on the upturned edges of which they lie. [t is evident therefore, as Mr Hull has remarked, that the metamorphism must have occurred between the close of the Lower and the commencement of the Upper Silurian perio1.1 In connexion with this question it should be remarked that abundant volcanic activity accompanied the deposit of these Upper Silurian rocks in the west of Ireland, successive sheets of lava (eurite) and beds of tuff forming conspicuous bands mu nix the stratified rocks, and reaching a collective thickness of 800 feet and upwards. Between Brandon Head and Dingle Bay a thick mass of strata on the coast, must, from the comparatively few fossils obtained from it, be held to represent Upper Silurian formations.