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with protrusions of granite, syenite, and other crystalline roc!:s.

1

thc base of this ollut series of rocks masses of white quartz-rock Towards the close of the long period represented by the Skiddaw are found lying with a marked unconformability upon the red slates, volcanic action manifested itself

, first by intermittent i sandstones described in a previous page. These quartzose beds are showers of ashes and streams of lava which were interstratified with merely hardened and sofnewhat metamorphosed sandstones; they the ordinary marine sediment, and then by a more powerful and still show their original false- bedding, and the casts of sea-weeds continuous series of explosions, whereby a huge volcanic mountain and worm-burrows. They contain a band of limestone which in Op group ni rones was piled up above the sea-level. The length of Assynt gwells out to a thickness of 1000 feet or more, and can be time occupied by this volcanic episode in Cumbrian geology may be traced almost continuously from the Kyles of Skye to the north inferred from the fact that all the Llandeilo and nearly all the coast of Sutherlandshire. Over these strata, in perfect conformable Bala beds are absent here. The volcanic island slowly sank into a sequence, and with a complete lithological gradation, come quartzose sea where Bala organisms flourished. Among these we find such | flagstones dipping like the rocks below at gentle angles to the south: familier Bala species as Favosites fibrosa, Heliolites interstinctus, east. They become more schistose and crumpled as they are traced Cübc!c verrucosa, Leptæna sericca, Orthis Actoniæ, 0. biforata, O. upwards, until, after a thickness of several thousand feet has been aligramma, 0. elegantula, O. porcata, and Strophomena rhom. \ passed over, they begin to undulate in steep folds and pass into the boidalis. These organisms and their associates gathered on the ordinary schistose rocks .which cover so much of the Highlands. submerged flanks of the sinking volcano into a bed of limestone- The gradation from the comparatively unaltered lower quartz-lunko the Coniston limestone-which can still be traced for many miles and limestones on the west to the intensely crumpled crystalline through the Westmoreland hills, as the Bala limestone which it Ie- upper schists and flagstones on the east can be followed step by presents can be followed

through the volcanic tracts of North Wales. 'step in nunerous fine natural sections from the north of Satherland The Coniston limestone is covered by certain flags and grits which to the Kyles of Skye. The proof is thus complete that a vast mass from their organic remains are referred to the Upper Silurian series. of schists and other crystalline rocks oyerlies fossiliferous limestones

In the South of Scotland, according to the detailed researches of in the Scottish Highlands. It therefore becomes of the utmost the Geological Survey, the Lower Silurian forinations are represented importance to determine the geological horizon of the fossils in the by the subjoined groups of strata in descending order :

limestones. This was done by the late Mr Salter, who declared his

conviction that they were unequivocally Lower Silurian, and bore Sandstones and conglomerates, Girvan } – Llandovery.

a most remarkable resemblance to a group of fossils from the Lower

Silurian rocks of North America. Five of the species he regarded Conglomerates, grits, shales, and len

as identical with known American forms (Orthoceras arcuoliratum, ticular bands of limestone, Peebles. shire, Dumfriesshire, & W. Ayr.

Caradoe or Bala. Hall; Orthis striatula, Emmons ; Ophilela compacta, Salt ; Mur.

chisonia gracilis, Hall; M. bellicincta, Hall), 4 as representative, s shire, sometimes 2000 ft.

doubtful, and 1 new genus, found also in Canada “That this Carsplairn group, coarse pebbly grits

truly North American assemblage," he remarks, " should be found and greywacke, 1200 ft.......

in the extreme north of Scotland on the same parallel as the Upper Black Shale, with graptolites,

Canadian,--that species of Maclurea and Raphistoma, resembling 550 ft.

those of the St Lawrence basin, and Orthocerata, bearing large Lowther grour, olive, grey, and blue shales, and sandstones, 4000 ft.

siphuncles like those of North America, Scandinavia, and Kussia,

should occur in Scotland and yet be scarcely known further south, Dalveen group, greywacke and shale, with band of fine conglomerate,

is at least suggestive of a geographical distribution-perhaps even

of climatal conditions-not very unlike that of more modern 3500 ft. ........

Llandeilo (14 000 ft.) times."? From this palæontological decision it follows that the Queensberry group, massive grey;

overlying schistose series of the Scottish Highlands is a mass of wackes and grits, with occasional

metamorphosed Silurian strata. Examined in detail they slow conglomerate bands and

very unequal and sporadic metamorphism. Some portions are shales, 4500 ft. Lower or Moffat Black Shalo group,

scarcely more changed than the ordinary greywackes and shales of

unaltered districts. False-ledding, pebble-beds, and other common 200-400 ft. Ardwell group, brown. flags, grey

features of sedimentation occur abundantly throughout the whole

vast series of schists. Here and there the metamorphism has bewackes, and shales, sometimes purplish and red; base not seen .......

come extreme, the rocks passing into coarsely crystalline schists

full of garnets, with bands of hornblende-rock, actinolite-schist, As a whole these strata are singularly barren of organic remains. and other metamorphic products, and passing even into granitic Most of the fossils which the Llandeilo groups contain lie in the gneiss and true granite. No more convincing proof could be ob bands of dark anthracitic shale which have been traced across tained that vast masses of schist do not necessarily belong to an nearly the whole breadth of the country. These shales are crowded azoic period of the earth's history, but may have been produced by with graptolites of recognizable Llandeilo forms, Climacograptus the alteration of previously existing sedimento. teretiusculus, Diplograptus pristis, and Graptolithus sagittarius It is not necessary to believe that the sediments so altered were being particularly abundant. Crustacea are exceedingly rare, but in all cases mere ordinary marine sand and mud. The white two phyllopods, Discinocaris Browniana and Peltocharis aptychoides, quartz-rocks were no doubt at one time pure white siliceous sand, occur; while from Dumfriesshire two obscure trilobites are referred the rounded grains of which can still be readily detected in them. doubtfully to Encrinurus and Phacops. The vast thickness of sandy, The quartzose flagstones were stratified sand with thin partings of gritty, and shaly unfossiliferous strata is the distinguishing feature clay or mud. The clay slates were evidently thick accumulations of the Lower Silurian series in the south of Scotland. The Caradoc of mud. But the rocks containing a marked percentage of magor Bala group lies unconformably upon the upper parts of the nesia, such as chlorite-slate, actinolite-schist, hornblende-rock, &c., Llandeilo rocks. It contains in the eastern districts some cal. may have resulted from the alteration of volcanic sediments and careous conglomerates which here and there swell out into local submarine lavas. The evidence

from Cumberland and Wales proves masses of limestone. In the south-west of Ayrshire the limestones how voluminous and long continued were the volcanic eruptions of attain considerable dimensions. In these calcareous bande numerous the Lower Silurian period in Britain. The abundant diffusion of Caradoc species have been found, among them Cheirurus gelasinosus, volcanic detritus over the present sea-bottom is now well known. Encrinurus punctatus, with species of Nlænus and Asaphus, Orthis The “Challenger” researches have also shown us that, besides the calligramma, O. confinis, Leptona scricca, Maclurca, and such glauconite previously known to be deposited from sea-water in the corals as Heliolites, Favosites, Oinphyma, and Strephodes. In the chambers of foraminifera and other dead organisms on the oceansouth-west of Ayrshire certain sbales and sandstones full of Caradoc bottom, true magnesian silicates are now in the process of eliminafossils are overlaid with sandstones, shales, and conglomerates con- tion from sea-water in some of the abysses of the ocean. It is taining Pentamerus oblongus, Atrypa hemispherica, Meristella quite possible thorefore that some of the rocks of the metamorphic angustifrons, Lichas lazatus, Petraia elongata, Nidulites favus, series rich in magnesian silicates may have arisen from the alteraand numerous other fossils which indicate the horizon of the tion of volcanic tuffs or submarine lavas, and that others may owe Llandovery rocks.

their distinctive composition to original chemical precipitation, The Highlands of Scotland consist mainly of crystalline rocks- as ably contended by Sterry Hunt, though their present crystalline gneiss, mica-schist, chlorite-schist, clay-slate, quartz-rock, schistose structure must be regarded as a part of the general metamorphism Aagstone, and many others, often much invaded by granite and by which the whole of the Lower Silurian rocks of the Highlands other intrusive masses. It was at one time supposed that these have been affected. rocks all belonged to the so-called primary or primitive series, older In the south-east of Ireland, grey, greenish, and purple grits, than any of the fossiliferous systems. But the discovery by Mr C. and grey and dark shales, lie uncomformably upon the Cambrian W. Peach, already referred to, that recognizable fossils occur in the rocks, and contain a few fossils of Llandeilo age. They present limestone of Durness in Sutherlandshire, led Murchison to infer interstratified beds of tuff and felsitic lavas indicating contemthat the whole of the overlying gneissose and schistose masses are poraneous volcanic action. In the north-east of the island a broad really inetamorphosed Lower Silurian rocks—a generalization which has been completely confirmed by subsequent investigation. · At

i Quart. Jour. Grol. Soc., XX. 881.

some

belt of Lower Silurian rocks rans from the coast of Down into the | Silurian series began to be laid down. This took place heart of Roscommon and Longford. This belt is evidently a pro during a time of submergence, for these conglomeratic and longation of that in the southern uplands of Scotland. It is marked by the occurrence of similar dark anthracitic sliales sandy strata are found creeping up the slopes and even crowded with graptolites. The richest fossiliferous localities among capping some of the heights, as at Bogmine, where they the Irish Lower Silurian rocks are found at the Chair of Kildare, reach a height of 1150 feet above the sea.2 The subsidence Portrane near Dublin, Pomeroy in Tyrono, and Lisbellan in Fer probably continued during the whole of the interval occamanugh, where mall pratrusions of the older rocks rise as oases among the surrounding later formations Portlock brought the pied by the deposition of the Upper Silurian strata, which northern and tvestern localities to light, and Murchison pointed out thus were piled to a depth of from 3000 to 5000 feet oper that, while a number of the trilobites (Frinucleus, Phacops, Cały the disturbed and donuded platform of Lower Silurian rockzi mens, and Illanus), as well as the simple plaited Orthide, Leptænd, Arranged in tabular form, the sgbdivisions of the Upper and Strophornende, some spiral shells, and many Orthocerata, are specifically identical with those from the typical Caradoc and Bala Silurian rocks of Wales and the adjoining counties of beds of Shropshire and Wales, yet they are associated with peculiar England are in descending order as follows:forms, first discovered in Ireland, and very rare elsewhere in the

Base of Old Red Sandstone. British Islands Among these distinctive fossile he cites the

Tilestones. trilobites, Remopleurides, Harpes, Amphion, and Brontous, with the smooth forms of Asaphus (Tsotetus), which, though_abundant in %. Ludlow group

Upper Ludlow Rock. Ireland and America, seldom oecur in Wales or England, and

Aymestry Limestone.

Lower Ludlow Rock. never on the Continent. 1

Wenlock or Dudley LimeIn the north and west of Ireland a large area of wrface is oceu.

stone pied by crystalline rocks-gneiss, schists, quartz-rocks, limestone, 2.. Wenlock group Wenlock Shale

Denbighshira

Grits of granite, &c.—which are manifestly a continuation of those of the

Woolhope or Barr Lime. North Wales Highlands of Scotland. They run south-westward parallel with

stone and Shale the belt of unaltered Lower Silurian rocks from which, in some

1. Upper Lland Tarannon Shale. places, as in county Tyrono, thoy are only a few miles distant. l'he district of Pomeroy, so rich in Silurian fossils, promises to

overy group... ) May Hill Sandstones

Lower Llandovery Rocks. afford the greatest light on the interesting but difficult problem of the metamorphism of the Lower Silurian rocks of the Scottish 1. Upper Llandowery Group.-(a.) May Hill Sandstones. Highlands and the north-west of Ireland. It will be seen from the evidence fornished by the sections in West Mayo (r. 387) that the Silurian formations was first shown in 1853 by Sedgwick,

-The position of these rocks as the true base of the Upper metamorphism must have taken place prior to the deposition of who named them the May Hill Sandstones from the locality the Upper Silarian formations of the west of Ireland.

in Gloucestershire where they are so well displayed. B. Upper Silurian.

Appearing on the coast of Pembrokeshire at Marloes Bay, The formations which in the British Islands are classed they range across South Wales until they are overlappend as Upper Silurian occur in two very distinct types. So by the Old Red Sandstone. They emerge again in Carmangreat indeed is the contrast between these types that it thenshire, and trend north-eastward as a narrow strip at the is only by a comparison of organie remains that the whole base of the Upper Silorian series, from a few feet to 1000 can be grouped together as the deposits of one great feet or more in thickness, as far as the Longmynd, whero geological period. In the original region described by as a marked conglomerate wrapping round that ancient Murchison, and from which bis type of the system was taken, Cambrian ridge they disappear. In the course of this the strata are comparatively flat, soft, unaltered, consisting long tract they pass successively and unconformably over mainly of somewhat incoherent sandy mud with occasional Lower Llandovery, Caradoc, 'Llandeilo, and Cambrian bands of limestone. But as these rocks are followed rocks. They consist of yellow and brown ferruginous sandInto North Wales, they are found to swell out into a vast stones, often full of shells, which are apt to weather out series of grits and shales so like portions of the hard altered and leave casts. Their lower parts are commonly conLower Silurian rocks that, save for the evidence of fossils, glomeratic, the pebbles being largely derived from older they would naturally be grouped as part of that more parts of the Silarian formations. Here and there, where ancient series. In Westmoreland and Cumberland, and still the organie remains become extraordinarily abandant, the further north in the border counties of Scotland, also in the strata pass into a kind of sandy limestone, known as the south-west of Ireland, it is the North Welsh type which “ Pentamerus limestone," from the numbers of this prevails, so that in Britain the general lithological charac- brachiopod contained in it. The species of fossils found in ters and minute palæontological subdivisions ascertained the May Hill Sandstones number about 230. in the typical Silurian district are almost confined to that Among these are some traces of fucoids; sponges (Cliona, limited region, while over the rest of the British area for 1schadites); the widely diffused Graptolithus priodon; a number of thousands of square miles the hard sandy and shaly type few crinoids ; some annelides, particularly the Tentaculites anglicks,

corals (Petraia, Heliolites, Favosites, Halysites, Syringopora, &c.); a of North Wales is prevalent.

which is abundant; a number of trilobites, of which Phacops Taking first the Silurian tract of the south-west of England, Stokesii, P. Weaveri, Encrinurus punctatus, and Calymene Blum.n. and the east and south of Wales, we find a decided uncon.

bachii are common; numerous brachiopods, as Atrypa hemispherica,

A. reticularis, Pentamerus oblongus, Stricklandinia lirata (S. lens formability separating the lower from the Upper Silurian 4. reticularis, Pentamerus

also occurs), Leptona transversalis, Orthis calligramma, 0. eleganformations. In some places the latter are found passing tula, 0. reversa, Strophomena compressa, s. pecten, and Lingia across the edges of the former, group after group. till they parallela; lamellibranchs of the mytiloid genera Orthonota, Mytilus

, come to lie directly upon the Calabrian rocks. Indeed, in and Modiolopsis, with forms of Pterinea, Clenodonta, and Lyra one district between the Longmynd and Weulock edge, the tomaria, Cyclonema, Holopella; and cephalopods, chiefly Ortho

desmn; gasteropods, particularly the genera Murchisonia, Pleurobase of the Upper Silurian rocks is found within a few cerata, with some forms of Actinoceras and Phragmoceras, and the niles to pass from the Caradoc group across to the Lower old species Lituites cornu-arietis. Cambrian rocks. It is evident, therefore, that in the Welsh (6.) Tarannon Shale.-Above the Upper Llandovery bedo region very great disturbance and extensive denudation comes a very persistent zone of fine, smooth, light grey or preceded the commencement of the deposition of the Upper blue slates, which has been traced down the whole length Silurian rocks. As Professor Ramsay has pointed out, the of Wales from the mouth of the Conway into Carmarthenarea ot Wales, previously covered by a wide though shallow shire. These rocks, termed the "paste-rock" by Sedgwick, sea, was ridged up into a series of islands, ruand the margin have an extreme thickness of 1000 to 1500 feet

. Barren of which the conglomerates at the base of the Upper in organic remains, their chief interest lies in the fact that · Siluria, p. 174.

? Physical Geology of Britain, p. 91.

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the persistence of so thick a band of rock between what A distinguishing characteristic of the Wenlock limestone is were supposed to be continuousand conformable formations the abundance and variety of its corals, of which 53 species should have been anrecognized until it was proved by the formed in part by massive sheets and buuches of coral. Among

hare been described. The rock seems indeed to have been detailed mapping of the Geological Survey.

characteristic species are Halysites catenularia, Heliolitcs inter 2. Wenlock Group.—(2) Woolhope Limestone. In the stinctus, 2. tubulatu, Alveoliles Labechein Favosites aspera, F. original typical Upper Silurian tract of Shropshire and the fibrosa, R. Gochlandica, Cænites juniperinus Syringopora fuscicuadjacent counties, the Upper Llandovery rocks are overlaid Laris, and Omphyma turbinatum. The erinoids are also specially

abundant, and are often beautifully preserved: Periechocrinus by a local group of grey shales containing nodular lime- moniliforinis is one of the most frequent species; others are Crotalostone which here and there swells out into beds having an crinus rugosus, Cyathocrinus geniodactylizat, and Marsupiocrinus aggregate thickness of 30 or 40 feet. These strata are well cælatus; with several cystideans, as Pseudocrinites quadrifasciatus. displayed in the picturesque valley of Woolhope in Here- The crustaceans include numerous trilobites, among which we miss fordshire, which lies upon a worn quaquaversal dome of Ogygia, and Trinuclcus, none of which

ascend into the Wen

sone of the persistent Lower Silarian genere, such as Asaphus, Upper Silurian strata rising in the midst of the surrounding lock group. The most abundant trilobite is the long-lived Caly. Oid, Red Sandstone. They are seen likewise to the north- mene Blumenbachii, which ranges from the Llandeilo flags up to near west at Presteign, Naslı Scar, and Old Radnor in Radnor the top of the Upper Silnrian formations It occurs abundantly at shire, and to the east and south in the Malveru Hills (where common forms are Encrinurus punctatus, E. variolaris, Phacops

Dudley, where it received the name of the “Dudley Locust.” Oiker they include a great thickness of shale below the limestone), caudatus, P. Downingia, P. Stokesii, Bumastus Barriensis, Homaloand May Hill in Gloucestershire. These strata have yielded nodus delphinocephalw, and Choirurus bimucronatus. The brachio many characteristically Upper Silurian fossils, among which pods, continue to be abundant; among typical species may be may be mentioned Bæmastus Barriensis, Homalonotus del- plicatellus, Rhynchonella borealis (very common), R cuncata,, R.

noted Alrypa reticularis, Meristella tumida, Spirifer elevatus, s. phinocephalus, Phacops caudatus, Atrypa reticularis, Orthis Wilsoni, Orthis elegantula, O. rustica, Strophomena rhomboidalis

, calligramma, Strophomena imbrex, Rkynchonella borealis, and Pentamerus galeatus. The lamellibravehs are not well repre R. Wilsoni, Eromphalus sculptus, Orthoceras annulatum, sented; but several species of Pterinca are abundant, with Gram. It is a characteristic of the older Palæozoic limestones to The gasteropods are most characteristically marked by 8 or?

mysia cingulata, and some species of Modiolopsis and Ctenodorta. occur in a very lenticular form, swelling in some places species of Euomphalus, 3 or more of Durchisonia, with species of to & great thickness and rapidly dying out, to reappear Pleurotomaria, Acroculia, and Cyclonerna. The cephalopods are again perhaps some miles away with increased proportions and Phragmoceras ; of these the orthoceratites are by far the most

confined to few genera, Lituites, Actinoceras, Cycloceras, Orthoceras, This local character is well exhibited by the Woolhope abundant both in species and individuals. Orthoceras annulatum limestone. Where it dies out, the shales underneath and is the most common form. The pteropods appear in the beautiful intercalated with it join on continuously to the overlying and very abundant Conularia Sowerbiji, and the heteropods in the Wenlock sbale, and no line for the Woolhope group can

common and characteristic Bellerophon Wenlockensis then be satisfactorily drawn. The same discontinuity is 3. Ludlow Group. —This series of strata consists essenstrikingly traceable in the Wenlock limestone to be immedi- tially of shales, with occasionally a calcareons band in ately referred to.

the middle. It graduates downward into the Wenlock (6.) Wenlock Shalė.—This is a group of grey and black fine group, so that when the Wenlock limestone disappears shales, traceable from the banks of the Severn near Coal the Wenlock and Ludlow sbales form one continuoup brook Dale across Radnorshire to near Carmarthen—a die argillaceous formation. It is in this united form that tance of about 90 miles. The same strata reappear in the the two groups stretch to the south-west through Brecon protrusions of Upper Silurian rock which rise out of the and Carmarthen. The Ludlow rocks are typically seen Old Red Sandstone plains of Gloucestershire, Herefordshire, between Ludlow and Aymestry. They appear likewise at and Monmoutlishire. In the Malvern Hills they were the detached Silurian areas from Dudley to the mouth of estimated by Professor Phillips to reach a thickness of 640 the Severn. They were grouped by Murchison into three feet, but towards the north they thicken out to 1000 or even 1400 feet. On the whole the fossils are identical (a.) Lower Ludlow Rock.— This is a group of soft dark

a with those of the overlying limestone. The corals, however, grey to pale greenish-brown or olive sandy shales, often with 80 abundant in that rock are here comparatively rare. The calcareous concretions. Much of the rock, however, presents brachiopods (of the genera Leptæna, Orthis, Strophomena, so little fissile structure as to get the name of mudstone, Atrypa, and Rhynchonella) are generally of small size weathering out into concretions which fall to angular frag. Orthis biloba, 0. hybrida, and the large flat O. rustica, being ments as the rock crumbles down. It becomes more sandy characteristic. Of the liigher mollusca thin-shelled forms and flaggy towards the top. From the softness of the shales of Orthoceras are specially abundant. Among the trilobites, this zone of rock has been extensively denuded, and the Encrinurus punctatus, E. variolaris, Calymene Blumenbachii, Wenlock limestone rises up boldly from under it. C. tuberculosa, Phacops caudatus, and P. longicaudatus are

An abundant suite of fossils has been yielded by these shales. common. The Graptolithus priodon, so frequent among No fewer than 18 species of star-fishes, belonging to 6 genera, havo the Bala beds of the Lower Silurian series, also occurs

been described (Protasler, like the brittle-stars of the British seas, in the Wenlock shale. Graptolithus Flemingii is here a

Palæocoma, Palasterina). A few graptolites occur, particularly

the persistent Graplolilhus priodon (corumon), G. colun us, and G. characteristic species.

Flemingii. A few of the Wenlock corals survive in the Lower (c.) Wenlock Limestone is a thick-bedded, sometimes Ludlow rock, but the conditions, of deposit were evidently unflaggy, usually more or less concretionary limestone, grey favourable for their growth. The trilobites are less numerous thau or pale pink, often highly crystalline, occurring in some

in older beds; they include the venerable Calumene Blumenbachii,

Phacops caudatus, and its still longer-tailed variety P. longi. places as a single massive bed, in others as two or more

caudatus ; also Acidaspis Brightii Homalonotus delphinocephalus, strata separated by grey shales, the whole forming a thick and Cyphaspis megalops. But other forms of crustacean life occur ness of rock ranging from 100 to 300 feet. As its name in some number. As the trilobites begin to wane numerous denotes, this stratum is typically developed along Wenlock phyllopods appear, the genus Ceratiocaris being represented by 10 Edge in Shropshire, where it runs as a prominent ridge for geological history - Eurypterus, Pterygotus, and Hemiaspis.

or more species. Large eurypterids now make their entrance upon fully 20 miles, also between Aymestry and Ludlow. It like Though brachiopods are not scarce, hardly any seem to be peculiar wise appears at the detached areas of Upper Silurian strata to the Lower Ludlow rock, the Lingula lata,'which Murchison above referred to, being specially well seen near Dudley suggested might be peculiar, having been obtained from what is (whence it is often spoken of as the Dudley limestone), laud. Rhynchonella Wilsoni, Spirijer exporrectus, Strophomena

supposed to be representative of this group of strata in WestmoreWoolhope, Malvern, May Hill, and Usk in Monmouthshire. euglypha, Atrypa reticularis, and Chonctes minima are not infre

zones.

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quent. Among the more frequently recurring species of lamelli. strata above the original bone-bed at Ludlow, wogether with Oranchs the following may be named-Cardiola interrupta, C.

some minute globular bodies believed to be the sporangia of striata, Orthonota rigida, O. sumisulcata, and a number of species of Perinea. The orthoceratites are numerous, as Orthoceras Ludense,

a lycopod. These, with some other plant remains from the O. subundulatum, also species of Phragroceras and Lituites. The same district, are the earliest traces of land vegetation yet numbers of these straight and curved cephalopods form one of the found. The higher parts of the Ludlow rock consist of distinguishing features of the zone. At one locality, near Leint. fine, yellow sandstone and harder grits known as the wardiue in Shropshire, which has been prolific in Lower Ludlow Downton sandstone. Originally the whole of these flaggy fossils, particularly in star-fishes and eurypterid crustaceans, : & fragment of the fish Pteraspis was discovered in 1859. This is the upper parts of the Ludlow group were called “Tilestones earliest trace of vertebrate life yet detected. It is interesting to by Murchison, and being often red in colour were included note that the Pteraspis does not stand low in the scale of organiza- by him ns the base of the Old Red Sandstone, into which tion, but has affinities with our modern sturgeon.

(6.) Aymestry Limestone is a dark grey somewhat earthy they gradually and conformably, ascend. Undoubtedly concretionary limestone in beds from 1 to 5 feet thick, they show the gradual change of physical conditions which Where at its thickest it forms a conspicuous feature, rising took place at the close of the Silurian period in the west of above the soft and denuded Lower Ludlow shales and, England, and brought in the deposits of the

Old Red Sandowing to the easily removable nature of some fuller's earth stone. But as their organic contents are still unequivocally on which it lies, it has here and there been dislocated by those of the Ludlow group, they are now classed as the large landslips. It is still more inconstant than the Wen uppermost zone of the Silurian system.

A considerable suite of organic remains has been obtained lock limestone. Though well developed at Aymestry it soon

from the Upper Ludlow rock, which on the whole are the same dies away into bands of calcareous nodules, which finally as those in the zones underneath. Vegetable remains, some of disappear, and the lower and upper divisions of the Ludlow which seem to be fucoids, but most of which are probably ter. group then come together. The most characteristic fossil restrial and lycopodiaceous, abound in the Downton sand

stono and passage-beds into the Old Red Sandstone.

l'orals, is the Pentamerus Knightii; other common forms are

as might be supposed from the muddy character of the de Rhynchonella Wilsoni, Lingula Lewisii, Strophomena posit, seldon occur, though Murchison mentions that the en. euglypha, Bellerophon dilatatus, Pterinea Sowerbyi, with crusting form Alveolites fibrosus may not infrequently be found many of the same shells, corals, and trilobites found in the enveloping, shells, Cyclonema corallii and Murchisonia corallii

being, as their names imply, its favourite habitats. Some anne. Wenlock limestone. Indeed, as Murchison has pointed out, lides" (Serpulites longispinus, Cornulites serpularius, Tentaculites except in the less number of species and the occurrence of tenuis, and Trachyderma coriacca) are not uncommon,

The crus. some of the shells more characteristic of the Upper Ludlow tacea are represented chiefly by small ostracods (Beyrichia Klodeni, zone, there is not much palæontological distinction between Leperditia marginata, Entomis tuberosa), and by species of Ceratic the two limestones.1

caris, Dictyocaris, Eurypterus, Hemiaspis, Pterygotus, and Stylo.

nurus; the trilobites having still further waned, though Homor (c.) Upper Ludlow Rock. - In the original Silurian district lonotus Knightii

, Encrinurus punctatus, Phacops Downingiæ, and described by Murchison, the Aymestry

limestone is covered a few others still

a few others still occur, and even the persistent Calymene Blumenby a calcareous shelly band full of Rhynchonella navicula, bachii may occasionally be found of the brachiopods the most sometimes 30 or 40 feet thick. This layer is succeeded by rugala, and Lingula cornea.

abundant forms are Rhynchonclla nucula, Chonctcs striatella, Discina

The most characteristic lamelli. grey sandy shale or mudstone, often weathering into con- branchs are Orthonota amygdalina, Goniophora cymbaforáis, cretions as in the Lower Ludlow zone, and assuming ex

Ptcrinea lincata, P. retroflexa; some of the commonest gasteropod, ternally the same rusty-brown or greyish olive-green hue.

are Murchisonia corallii, Platyschisma helicites, and Holopella Its harder beds are quarried for building stone; but the the Lower Ludlow rock, and are sometimes of large size, Orthcceras

obsoleta. The orthoceratites are specifically identical with those of general character of the deposit, like that of the argillaceous bullatum being specially abundant. The fish remains consist of portions of the Upper Silurian formations as a whole in the bones, teeth, shagreen-like scales, plates, and fin-spines. They typical district of Siluria, is soft, incoherent, and crumb- include some plagiostomous (placoid) forms (Thclodus, shagreen. ling, easily decomposing once more into the original mud, scales, Sphagodus, skin, Onchus, spines) and some ostracosteaus

(Cephalaspis, Auchenaspis, and Pteraspis). and presenting in this respect a contrast to the hard fissile and often slaty shales of the Lower Silurian series. Many adjacent counties, nothing can be more decided than the

Iu the typical Silurian region of Shropshire and the of the sandstone beds are crowded with ripple-marks, rill

. lithological evidence for the gradual disappearance of the marks, and annelid-trails, indicative of the shallow littoral Silurian sea, with its crowds of graptolites, trilobites, and waters in which they were deposited. One of the upper: brachiopods, and for the gradual introduction of those most sandstones is termed the “ Fucoid Bed,” from the geographical conditions which brought about the deposit of number of its cylindrical sea-weed-like stens. It likewise contains numerous inverted pyramidal bodies, which mudę, with their occasional zones of limestone, are suc

the Old Red Sandstone. The fine grey and olive-coloured are believed to be casts of the cavities made in the muddy ceeded by bright red clays, sandstones, corpstones, and con sand by the rotatory movement of crinoids rooted and half- glomerates. The evidence from fossils is equally explicit. buried in the micaceous mud.? At the top of the Upper Up to the top of the Ludlow rocks the abundant Silurian Ludlow rock near the town of Ludlow, a brown layer fauna continues in hardly diminished numbers. But as occurs from a quarter of an inch to 3 or 4 inches in thickness, full of fragments of fish, Pterygołus, and shells. This

soon as the red strata begin the orgnnic remains rapidly die layer, termed the “Ludlow Bone-bed,” is the oldest from out, until at last only the fish and the large eurypterid Cruis

taceans continue to ocour, which any considerable number of vertebrate remains has been obtained. In spite of its insignificant thickness it has

Turning now from the interesting and extremely importbeen detected at numerous localities from Ludlow as far as

ant though limited area in which the original type of the Pyrton passage, at the mouth of the Severn—a distance of Upper Silurian rocks is developed, we observe that whether 45 miles from north to south, and from Kington to Ledbury and thick limestones give way to hard slates, grits, and

we pass northwards or south-westwards the soft mudstones and Malvern -a distance of nearly 30 miles from west to east; so that it probably covers an area (now largely buried flagstones, among which it is scarcely possible sometimes under Old Red Sandstone) not less than 1000 square miles what may be the equivalent of the Ludlow group. It is

even to discriminate what represents the Wenlock from in extent, yet it appears never to exceed and usually to fall in Denbighshire and the adjacent counties that this change short of a thickness of 1 foot. Fish remains, however, are not confined to this horizon. They have been detected in scribed passes into that region of North Wales, where it forng

becomes most marked. The Tarannon shale above deSiluria, p. 130. * Siluria, p. 133.

the base of tie Upper Silurian formations. It is covered

stones with bands of sil; } - { Upper Lud. Strophomena imbrex

, Murcirisonia, Orthoceras tenuicinctum,

........

by a series of grits or sandstones which in some places are considerable proportio:1 of greywacke bands towards the at least 3000 feet thick. These are covered by and pass base. At different horizons they contain leuticular bands laterally into hard shales, which are believed to represent of a calcareous pebbly grit. But their most characteristic parts of the true Wenlock group, perhaps even some por- feature, and one which at once distinguishes them locally tion of the Ludlow rocks. It is evident, however, that in from the adjoining Lower Silurian rocks, is the occurrence spite of the wide extent over which these Silarian rocks of of a nearly black, highly fissile shale, composed of layers in North Wales are spread, and the great thickness which most cases as thin as ordinary writing paper and usually, they attain, they do not present an adequate stratigraphical crowded with graptolites. These peculiar bands occur equivalent for the complete succession in the original throughout the whole series of rocks from bottom to top. Silarian district. Instead of passing up conformably into They are sometimes so thin that 20 or 30 seams or ribs, each the base of the Old Red Sandstone, as at Ludlow, they are finely fissile, may be seen intercalated within the space of covered by that formation unconformably. In fact they an inch of the ordinary shale or greywacke. Occasionally have been upturned, crumpled, faulted, and cleaved before they form zones 80 to 100 feet thick, consisting entirely of the deposition of those portions of the Old Red Sandstone finely leaved graptolitic shales. As a whole these Upper which lie upon them. These great physical changes took Silurian strata resemble lithologically the correspondplace in Denbighshire when, so far as the evidence goes, ing series in Westmoreland, though here and there they there was entire quiescence in the Shropshire district; yet assume the character of mudstonez not unlike those of the distance between the two areas was not more than Shropshire. The abundant fossils in them are simple about 60 miles. These subterranean movements were graptolites (Graptolithus Sedgwickii, G. Becki, G. Flemingii, doubtless the precursors of those more widely extended G. colonus, G. Griestorensis, Retiolites Geinitrianus, &c.). upheavals which converted the floor of the Silurian sea into Orthoceratites come next in point of numbers (Orthoceras a series of isolated basins, in which the Old Red Sandstone annulatum, 0. tenuicinctum, &c.). In some of the shales was laid down.

crustacean fragments are numerous. They include large In Westmoreland and Cumberland a vast mass of hard pieces of the carapace of Dictyocaris, with remains ut slates, grits, and flags was identified by Sedgwick as of Pterygotus and Ceratiocaris. The pebbly grits contain Upper Silurian age. These form the varied ranges of bills Petraia and crinoid stems. In the south of Kirkcudbright in the southern part of the lake district from near Shap to certain limestones and conglomerates intercalated among Duddon mouth. The following are the local subdivisions these shales have yielded a more varied faunn, having on with the conjectural equivalents in Siluria.

the whole a decidedly Wenlock character. It includes Flaggy beds, with lamelli- } - (1) Tilestones. caudatus, Meristella, Leptæna sericea, Atrypa reticularis,

Favosites, Catenipora, Beyrichiu tuberculata, Phacops Hay Fell and branchs abundant.

= ?
Kirkby Moor Massive greenish and grey sand.
Flags
, fossils,

&c.
Holopella abundant
Calcareous beds, with Rhyncho- Aymestry

It is impossible in the south of Scotlaud to separate the nella navicula abundant Limestone. Upper Silurian rocks into Wenlock and Ludlow groups. Binnisdale Sandstone and shale, with star- Lower Lud. On the whole these rocks seem to be representative mainly Slates fish

low. of the older half of the Upper Silurian formations. They Dark blue flags and grits of Upper Wen.

Tock. great thickness

are covered unconformably by Lower Old Red Sandistone ( Flags and greywacke (Ortho

and later forinations. In the counties of Edinburgh and ceras subundulatum, O. an.

Lanark, however, the base of the Lower Old Red Sandstone Coniston Grits gulatuin, Graptolithus Fle

is found to graduate downward into a thick series of brown, mingii, G. colonus, Ceratio. cario áfurchisoni), upwards - { LowerWen- olive, and grey, shales, sandstones, and grits, containing of 4000 feet

lock undoubted Ludlow fossils. It is deserving of remark also Dark grey coarse flags (Car.

that the peculiar lithological type so characteristic of the diola interrupta, Orthoceras

strata in the original Silurian area reappears in the centre subundulatum), 1000 feet ...)

of Scotland, many of the concretionary brown shales and Conistón Limestone (Lower Silurian)...

Caradoc or
{
Bala.

olive-coloured madstones being undistinguishable from In the northern part of the Lake district a great anticlinal beds are crowded with fussils. Among the most character.

those in the typical sections at Ludlow. Some of these fold takes place. The Skiddaw slates arch over and are

istic are Leptæna transversalis, Orthonota amygdalina, succeeded by the base of the volcanic series above described. Platyschisma helicites, Beyrichia Klodeni, Orthoceras But before more than a small portion of that series has | Maclareni, with many crustaceans of the genera Ceratiocaris, appeared the whole Silurian area is overlapped unconform. Eurypterus, Pterygotus, Slimonia, and Stylonurus. In the ably by the Carboniferous Limestone series. It is necessary Pentland Hills these strata are estimated to attain a thickto cross the broad plains of Cumberland and the south of

ness of 3500 to 4000 feet, but their base is nowhere Damfriesshire before Silurian rocks are again met with. In reached; in Lanarkshire they are at least as thick. Their this intervening tract a synclinal fold must lio, for along the lower portions may represent some of the higher parts of southern base of the uplands of the south of Scotland a

the Wenlock group. belt of Upper Silurian rocks, dipping on the whole to the

Ireland furnishes some interesting evidence regarding the south-east, can be traced from the heart of the Cheviot Hills geographical changes in the west of Europe between the to the headlands of Wigtownshire. These rocks must reach close of the Lower Silurian ard the beginning of the Urper a thickness of several thousand feet, but their top is Silurian period. It has already been pointed out that the nowhere seen. They repose on some of the older parts of metamorphosed Lower Silurian rocks of the Scottish Highthe Llandeilo series, with so close a coincidence of dip and lands are prolonged into the north of Ireland, whence they strike that no decided unconformability has yet been traced range south-westwards to Galway Bay. In the picturesque between them. They consist essentially of shales, with a tract between Loch Mask and the mouth of Killary barbour

these metamorphosed rocks are unconformably overlaid by * The arrangement and thicknesses here given are those in the

masses of sandstones, conglomerates, and shales more than Kondal district as mapped by Mr Aveline and Mr Hughes in the course of the Geological Survey (Sheet 98, S.E., Erplanation, pp. 6-13, 7000 feet thick, and containing Llandovery and Wenlock 1972).

a

Soniston Flags {

fossils with a mixture of Cararoc forms. In the midst

10–14

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