Part 13

LOWER SILURIAN.

The Silurian rocks have been estimated by Sir Roderick Murchison to occupy, altogether, an area of about 7,600 square miles in England and Wales, 18,420 square miles in Scotland, and nearly 7,000 square miles in Ireland. Thus, as regards the British Isles, the Silurian rocks rise to the surface over nearly 33,000 square miles.

The Silurian rocks have been traced from Cumberland to the Land’s End, at the southern extremity of England. They lie at the base of the southern Highlands of Scotland, from the North Channel to the North Sea, and they range along the entire western coast of that country. In a westerly direction they extended to the sea, where the mountains of Wales--the Alps of the great chain--would stand out in bold relief, some of them facing the sea, others in detached groups; some clothed with a stunted vegetation, others naked and desolate; all of them wild and picturesque. But an interest surpassing all others belongs to these mountains. They are amongst the most ancient sedimentary rocks which exist on our globe, a page of the book in which is written the history of the antiquities of Great Britain--in fine, of the world.

In Shropshire and Wales three zones of Silurian life have been established. In rocks of three different ages Graptolites have left the trace of their existence. Another fossil characteristic of these ancient rocks is the Lingula. This shell is horny or slightly calcareous, which has probably been one cause of its preservation. The family to which the Lingula belongs is so abundant in the rocks of the Welsh mountains, that Sir R. Murchison has used it to designate a geological era. These Lingula-flags mark the beginning of the first Silurian strata.

In the Lower Llandovery beds, which mark the close of the period, other fossils present themselves, thus greatly augmenting the forms of life in the Lower Silurian rocks. These are cœlenterata, articulata, and mollusca. They mark, however, only a very ephemeral passage over the globe, and soon disappear altogether.

The vertebrated animals are only represented by rare Fishes, and it is only on reaching the Upper Ludlow rocks, and specially in those beds which pass upward into the Old Red Sandstone, that the remains have been found of fishes--the most ancient beings of their class.

The class of Crustaceans, of which the lobster, shrimp, and the crab of our days are the representatives, was that which predominated in this epoch of animal life. Their forms were most singular, and different from those of all existing Crustaceans. They consisted mainly of the Trilobites, a family which became entirely extinct at the close of the Carboniferous epoch, but in whose nicely-jointed shell the armourer of the middle ages might have found all his contrivances anticipated, with not a few besides which he has failed to discover. The head presents, in general, the form of an oval buckler; the body is composed of a series of articulations, or rings, as represented in Fig. 20; the anterior portion carrying the eyes, which in some are reticulated, like those of many insects (Figs. 18 and 19); the mouth was placed forward and beneath the head. Many of these Crustaceans could roll themselves into balls, like the wood-louse (Figs. 23 and 25). They swam on their backs.

During the middle and later Silurian ages, whole rocks were formed almost exclusively of their remains; during the Devonian period they seem to have gradually died out, almost disappearing in the Carboniferous age, and being only represented by one doubtful species in the Permian rocks of North America. The Trilobites are unique as a family, marking with certainty the rocks in which they occur; “and yet,” says Hugh Miller, “how admirably do they exhibit the articulated type of being, and illustrate that unity of design which pervades all Nature, amid its endless diversity!” Among other beings which have left their traces in the Silurian strata is Nereites Cambriensis, a species of annelide, whose articulations are very distinctly marked in the ancient rocks.

Besides the Trilobites, many orders of Mollusca were numerously represented in the Silurian seas. As Sir R. Murchison has observed, no zoological feature in the Upper Silurian rocks is more striking than the great increase and profusion of Cephalopods, many of them of great size, which appear in strata of the age immediately antecedent to the dawn of vertebrated life. Among the Cephalopods we have Gyroceras and Lituites cornu-arietis (Fig. 21), whose living representatives are the Nautilus and Cuttlefish of every sea. The genus Bellerophon (Figs. 54 and 56), with many others, represented the Gasteropods, and like the living carinaria sailed freely over the sea by means of its fleshy parts. The Gasteropods, with the Lamellibranchs, of which the Oyster is a living type, and the Brachiopods, whose congeners may still be detected in the Terebratula of our Highland lochs and bays, and the Lingulæ of the southern hemisphere, were all then represented. The Lamellibranchiata are without a head, and almost entirely destitute of power of locomotion. Among the Echinodermata we may cite the Hemiscosmites, of which H. pyriformis (Fig. 22) may be considered an example.

The rocks of the Lower Silurian age in France are found in Languedoc, in the environs of Neffiez and of Bédarrieux. They occupy, also, great part of Brittany. They occur in Bohemia, also in Spain, Russia, and in the New World. Limestones, sandstones, and schists (slates of Angers) form the chief part of this series. The Cambrian slates are largely represented in Canada and the United States.

LOWER SILURIAN GROUP.

Formation. Prevailing Rocks. Thickness. Fossils.

Lower { Hard sandstones, conglomerates, { 600 to } Pentamerus lens. Llandovery { and flaggy shaly beds { 1,000 }

{ Shelly sandstones, shales, and } { Brachiopods; { slaty beds, with grits, con- } { Lamellibranchs; Caradoc or { glomerates, and occasional } 12,000 { Pteropods; Bala { calcareous bands (Bala lime- } { Cystideans; { stone) } { Graptolites; { } { Trilobites.

{ Dark-grey flagstones, occasionally } Llandeilo { calcareous sandstones, } Flags { with black slates, containing } { Trilobites { Graptolites } 1,000 { (Fig. 36); { } to { Graptolites; } 1,500 { Heteropods; Lower { Dark-grey and ferruginous } { large Llandeilo { slates, sandy shales, and bluish } { Cephalopods. Tremadoc { flags, with occasional beds } Slates { of pisolitic iron-ore }

{ Trilobites { Black and dark shaly, grey } { (Olenus, { and brown slaty flagstones } { Conocoryphe, Lingula { and sandstones, with siliceous } 6,000 { Paradoxides, Flags { grits and quartzites } { Fig. 17); { } { Brachiopods; { } { Cystideans.

CAMBRIAN GROUP.

{ Llanberis slates, with sandy } 3,000 Annelides. Cambrian { strata } { Harlech grits 6,000 Oldhamia.

LAURENTIAN GROUP.

Upper { Stratified, highly-crystalline, } 12,060 Eozoon. Laurentian { and felspathic rocks }

Lower { Gneiss, quartzite, hornblende } 18,000 None. Laurentian { and mica-schists }

UPPER SILURIAN PERIOD.

UPPER SILURIAN GROUP. Lithological Characters. Thickness. Fossils.

{ Passage Beds, Tile-stones, and } { Sea-weeds, { Downton sandstones, at the } 80 { Lingulæ, { base of the bone-bed } { Mollusca. { { { Micaceous, yellowish and } 700 { Crustacea and Ludlow { grey, sandy mudstone } { Fish-remains. Rocks { { { Argillaceous (Aymestry) limestone } 50 { Crinoids. { { { Argillaceous Shale with impure } 1000 { Mollusca of { limestones } { many genera.

{ Argillaceous or semi-crystalline } { Mollusca of { limestone } { many genera. { } { { Argillaceous shales, in places } { Echinodermata; Wenlock { slaty } 3000 { Actinozoa; Rocks { } { Trilobites. { Woolhope Limestone and } { { occasional bands of argillaceous } { Graptolites. { nodules } {

Upper { Grey and yellowish sandstones } { Pentamerus Llandovery { (occasionally conglomerates) } 800 { oblongus, Rocks { with bands of } { Rhynchonella, { limestone } { Orthides, &c.

Among the fossils of this period may be remarked a number of Trilobites, which then attained their greatest development. Among others, Calymene Blumenbachii (Fig. 23), some Cephalopoda, and Brachiopoda, among which last may be named Pentamerus Knightii, Orthis, &c., and some Corals, as Halysites catenularius (Fig. 26), or the chain coral.

The Trilobites, we have already said, were able to coil themselves into a ball, like the wood-louse, doubtless as a means of defence. In Fig. 23, one of these creatures, Calymene Blumenbachii, is represented in that form, coiled upon itself. (See also Illænus Barriensis, Fig. 25.)