The World Before the Deluge
The vertebrated animals are only represented byrare 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 Crus- Fig. 20. taceans. They consisted mainly of the Ogygia Guettardi. Natural size. 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 fomi of an oval buckler; the body is composed of a series of articulations, or
io8 THE WORLD BEFORE THE DELUGE.
Tings, 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
Fig. 21.— Lituites comu-arietis. Fig. 22. — Hemicosmites pyriformis. One-third natural size. One-third natural size.
occur; " and yet," says Hugh Miller, " how admirably do they exhibit the articulated type of being, and illustrate that unity of design which
beingspervadeswhichall Nature,have leftamidtheir itstracesendlessin thediversity!"'Silurian strataAmongis Nereitesother 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 comu-arietis {¥\g. 21), whose living representatives are the Nautilus and Cuttlefish of every sea. The genus Bellerophon (Figs. 54 and 56), with many others, represented the
LOWER SILURIAN.
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 Liiigulce 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 Bemiscos7?iites, 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 Bedarrieux. 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 Llandovery.—Hard sandstones, conglomerates, and flaggy shaly beds; thickness 600 to 1,000; fossils: Pentamerus lens.
Brachiopods; Lamellibranchs; Shelly sandstones, shales, and' slaty beds, with grits, con-
Caradoc or Bala - glomerates, and occasional Pteropods;Cysti-calcareous l)ands (Bala lime- deans; Graptolites Trilobites.; . stone) Dark-grey flagstones, occasion-
Llandeilo Flags ally calcareous sandstones, Trilobites (Fig. 36); with black slates, containing 1,000to Graptolites teropodsGraptolites;;largeHe- and ferruginous Lower Llandeilo Dark-grey
Tremadoc slates, sandy shales, and bluish 1,500 Cephalopods.
Slates - - - flags, with occasional beds ( Trilobites (Olenus, of pisolitic iron-ore - Black and dark shaly, grey 1 Conocoryphe, Par- and brown slaty flagstones 6,000 \ adoxides, Fig. 17);
Lingula Flags - and sandstones, with silice Brachiopods; ous grits and quartzites t Cystideans. Cambrian Group. Llanberis slates, with sandy ) Annelides.
Cambrian - strata \ ^' Harlech grits -./--- 6,000 Oldhamia. Laurentian Group.
Upper Lauren- j Stratified, highly- crystalline, tian - - - 1 and felspathic rocks - - -
Lower Lauren- j Gneiss, quartzite, hornblende tian - - - 1 and mica-schists... -
J 10 THE WORLD BEFORE THE DELUGE.
Upper Silurian Period.
Upper Silurian Group. t the ^ Lithological Characters. Thickness Fossils. Passage Beds, Tile-stones, and Downton sandstones, at, and ■) So Sea-Mollusca.weeds, Lingulse, base of the bone-bed Crustacea and Fish- Ludlow Rocks Micaceous,grey, sandy yellowishmudstone and. "^ Argillaceous (Ajonestry) lime Crinoids. stone Argillaceous Shale with im Mollusca of many pure limestones... 700 f Argillaceous or semi-crystal f Mollusca of many I line limestone... ^ genera. Argillaceous shales, in places Echinodermata;
Wenlock Rocks -{ slaty, J- 3000 \ genera.Actinozoa; Trilo- Woolhope Limestone and occasional bands of argil laceous nodules... I II Graptolites.bites. 50 Grey and yellowish sand stones (occasionally con j Pentamerus oblon- Llando- Upper gus, Rhynchon- Rocks. glomerates) with bands of 800 \ very limestone ella, Orthides, &c.
Among the fossils of this period may be remarked a number of Trilobites, which then attained their greatest development. Among others, CalyjneneBlume?ibachii{^\g. 23), some Cephalopoda^dccA Brachiopoda, among which last may be named Pentamerus Kfiightii, Orthis, &c., and some Corals, as JIaly sites catenidarius (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, Calyinene Blumen-backii, is represented in that form, coiled upon itself. (See also IIIcbhus Barriensis, Fig. 25.) Crustaceans of a very strange form, and in no respects resembling the Trilobites, have been met with in the Silurian rocks of England and America — the Pterygotus (Fig. 27) and the Eurypterus, (Fig.
Fig. 23.— Calymene Blumenbachii par- 24). They are supposed to have been the inhabi-tially rolled up. tants of fresh water. They were called " Seraphim " by the Scotch quarry men, from the winged form and feather-like ornamentation upon the thoracic appendage, the part most usually met with. Agassiz figured them in his work on the * Fossil
Fishes of the Old Red Sandstone,' but, subsequently recognising their crustacean character, removed them from the Class of Fishes, and placed them with the Poecilipod Crustacea. The Eurypterida and Fterygoti in England almost exclusively belong to the passage beds — the Downton sandstone and the Upper Ludlow rocks.
Among the marine plants which have been found in the rocks
Fi^. 24. — Eurjrpterus remipes. Natural sLze.
corresponding with this sub-period are some species of Algae, and others belonging to the Lycopodiaceae, which become still more abundant in the Old Red Sandstone and Carboniferous Periods.
Fig. 28 represents some examples of the impressions they have left
The seas were, evidently, abundantly inhabited at the end of the Upper Silurian period, for naturalists have examined nearly 1,500 species belonging to these beds, and the number of British species,
classified and arranged for public inspection in our museums cannot be much short of that number.
Towards the close of the Upper Silurian sub-period, the argillaceous beds pass upwards into more sandy and shore-like deposits, in which the most ancient known fossil Fishes occur, and then usher us into the first great ichthyic period of the Old Red Sandstone, or Devonian, so well marked by its fossil fishes in Britain, Russia, and North America. The so-called fish-bones have been the subject of considerable doubt. Between the Upper Ludlow rocks opposite Downton Castle and the next overlying stratum, there occurs a thin bed of soft earthy shale, and fine, soft, yellowish greenstone, immediately
Fig. 25. — Illaenus Barriensis. — Dudley, Walsall.
overlying the Ludlow rock: just below this a remarkable fish-deposit occurs, called the Ludlow bone-bed, because the bones of animals are found in this stratum in great quantities. Old Drayton treats these bones as a great marvel:— ** With strange and sundry tales Of all their wondrous things; and not the least in Wales, Of that prodigious spring (him neighbouring as he past), That little fishes' bones continually doth cast." POLYOLBION.
Above the yellow beds, or Downton sandstone, as they are called, organic remains are extensively diffused through the argillaceous strata, which have yielded fragments of fishes' bones (being the earliest trace yet found of vertebrate life), with seeds and land-plants, the latter clearly indicating the neighbourhood of land, and the poverty of
UPPER SILURIAN PERIOD.
numbers and the small size of the shells, a change of condition in the nature of the waters in which they lived. " It was the central part only," says Sir R. Murchison, "of this band, or a ginger-bread-coloured layer of a thickness of three or four inches, and dwindling away to a quarter of an inch, exhibiting, when my attention was first directed to it, a matted mass of bony fragments, for the most part of
\ Fig. 26.— Halysites catenularius. Fig. 27.— Pterygotus bilobattis.
small size and of very peculiar character. Some of the fragments of fish are of a mahogany hue, but others of so brilliant a black that when first discovered they conveyed the impression that the bed was a heap of broken beetles."*
The fragments thus discovered were, after examination on the spot, supposed to be those of fishes, but, upon further investigation, many of them were found to belong to Crustaceans. The ichthyic nature of some of them is, however, now well established.
"Siluria,»p. 148.
Silurian Rocks are found in France in the departments of La Manche, Calvados, and of the Sarthe, and in Languedoc the Silurian formation has occupied the attention of Messrs. Graff and Fournet, who have traced along the base of the Espinouse, the green, primordial chlorite-schists, surmounted by clay-slates, which become more and more pure as the distance from the masses of granite and gneiss increases, and the valley of the Jour is approached. Upon these last