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.

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.

Upper Silurian Period.

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.)

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. 24). They are supposed to have been the inhabitants of fresh water. They were called “Seraphim” by the Scotch quarrymen, 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 Pœcilipod Crustacea. The Eurypteridæ and Pterygoti 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 corresponding with this sub-period are some species of Algæ, and others belonging to the Lycopodiaceæ, 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.

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 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 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.”[39]

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 the Silurian system rests, sinking towards the plain under Secondary and Tertiary formations. In Great Britain, Silurian strata are found enormously developed in the West and South Highlands of Scotland, on the western slopes of the Pennine chain and the mountains of Wales, and in the adjoining counties of Shropshire—their most typical region—and Worcestershire. In Spain; in Germany (on the banks of the Rhine); in Bohemia—where, also, they are largely developed, especially in the neighbourhood of Prague—in Sweden, where they compose the entire island of Gothland; in Norway; in Russia, especially in the Ural Mountains; and in America, in the neighbourhood of New York, and half way across the continent—in all these countries they are more or less developed.

We may add, as a general characteristic of the Silurian system as a whole, that of all formations it is the most disturbed. In the countries where it prevails, it only appears as fragments which have escaped destruction amid the numerous changes that have affected it during the earlier ages of the world. The beds, originally horizontal, are turned up, contorted, folded over, and sometimes become even vertical, as in the slates of Angers, Llanberis, and Ireleth. D’Orbigny found the Silurian beds with their fossils in the American Andes, at the height of 16,000 feet above the level of the sea. What vast upheavals must have been necessary to elevate these fossils to such a height!

In the Silurian period the sea still occupied the earth almost entirely; it covered the greater part of Europe: all the area comprised between Spain and the Ural was under water. In France only two islands had emerged from the primordial ocean. One of them was formed of the granitic rocks of what are now Brittany and La Vendée; the other constituted the great central plateau, and consisted of the same rocks. The northern parts of Norway, Sweden, and of Russian Lapland formed a vast continental surface. In America the emerged lands were more extensive. In North America an island extended over eighteen degrees of latitude, in the part now called New Britain. In South America, in the Pacific, Chili formed one elongated island. Upon the Atlantic, a portion of Brazil, to the extent of twenty degrees of latitude, was raised above water. Finally, in the equatorial regions, Guiana formed a later island in the vast ocean which still covered most other parts of the New World.

There is, perhaps, no scene of greater geological confusion than that presented by the western flanks of the Pennine chain. A line drawn longitudinally from about three degrees west of Greenwich, would include on its western side Cross Fell, in Cumberland, and the greater part of the Silurian rocks belonging to the Cambrian system, in which the Cambrian and Lower Silurian rocks are now well determined; while the upper series are so metamorphosed by eruptive granite and the effects of denudation, as to be scarcely recognisable. “With the rare exception of a seaweed and a zoophyte,” says the author of ‘Siluria,’ “not a trace of a fossil has been detected in the thousands of feet of strata, with interpolated igneous matter, which intervene between the slates of Skiddaw and the Coniston limestone, with its overlying flags; at that zone only do we begin to find anything like a fauna: here, judging from its fossils, we find representations of the Caradoc and Bala rocks.” This much-disturbed district Professor Sedgwick, after several years devoted to its study, has attempted to reconstruct, the following being a brief summary of his arguments. The region consists of:—

I. Beds of mudstone and sandstone, deposited in an ancient sea, apparently without the calcareous matter necessary to the existence of shells and corals, and with numerous traces of organic forms of Silurian age—these were the elements of the Skiddaw slates.

II. Plutonic rocks were, for many ages, poured out among the aqueous sedimentary deposits; the beds were broken up and re-cemented—plutonic silt and other finely comminuted matter were deposited along with the igneous rocks: the process was again and again repeated, till a deep sea was filled up with a formation many thousands of feet thick by the materials forming the middle Cambrian rocks.

III. A period of comparative repose followed. Beds of shells and bands of coral were formed upon the more ancient rocks, interrupted with beds of sand and mud; processes many times repeated: and thus, in a long succession of ages, were the deposits of the upper series completed.

IV. Towards the end of the period, mountain-masses and eruptive rocks were pushed up through the older deposits. After many revolutions, all the divisions of the slate-series were upheaved and contorted by movements which did not affect the newer formations.

V. The conglomerates of the Old Red Sandstone were now spread out by the beating of an ancient surf, continued through many ages, against the upheaved and broken slates.

VI. Another period of comparative repose followed: the coral-reefs of the mountain limestone, and the whole carboniferous series, were formed, but not without any oscillations between the land and sea-levels.

VII. An age of disruption and violence succeeded, marked by the discordant position of the rocks, and by the conglomerate of the New Red Sandstone. At the beginning of this period the great north and south “Craven fault,” which rent off the eastern calcareous mountains from the old slates, was formed. Soon afterwards the disruption of the great “Pennine fault,” which ranges from the foot of Stanmore to the coast of North Cumberland, occurred, lifting up the terrace of Cross Fell above the plain of the Eden. About the same time some of the north and south fissures, which now form the valleys leading into Morecambe Bay, may have been formed.

VIII. The more tranquil period of the New Red Sandstone now dawns, but here our facts fail us on the skirts of the Lake Mountains.

IX. Thousands of ages rolled away during the Secondary and Tertiary periods, in which we can trace no movement. But the powers of Nature are never still: during this age of apparent repose many a fissure may have started into an open chasm, many a valley been scooped out upon the lines of “fault.”

X. Close to the historic times we have evidence of new disruptions and violence, and of vast changes of level between land and sea. Ancient valleys probably opened out anew or extended, and fresh ones formed in the changes of the oceanic level. Cracks among the strata may now have become open fissures, vertical escarpments formed by unequal elevations along the lines of fault; and subsidence may have given rise to many of the tarns and lakes of the district.

Such is the picture which one of our most eminent geologists gives as the probable process by which this region has attained its present appearance, after he had devoted years of study and observation to its peculiarities; and his description of one spot applies in its general scope to the whole district. At the close of the Silurian period our island was probably an archipelago, ranging over ten degrees of latitude, like many of the island groups now found in the great Pacific Ocean; the old gneissic hills of the western coast of Scotland, culminating in the granite range of Ben Nevis, and stretching to the southern Grampians, forming the nucleus of one island group; the south Highlands of Scotland, ranging from the Lammermoor hills, another; the Pennine chain and the Malvern hills, the third, and most easterly group; the Shropshire and Welsh mountains, a fourth; and Devon and Cornwall stretching far to the south and west. The basis of the calculation being, that every spot of this island lying now at a lower elevation than 800 feet above the sea, was under water at the close of the Silurian period, except in those instances where depression by subsidence has since occurred.

There is, however, another element to be considered, which cannot be better stated than in the picturesque language of M. Esquiros, an eminent French writer, who has given much attention to British geology. “The Silurian mountains,” he says, “ruins in themselves, contain other ruins. In the bosom of the Longmynd rocks, geologists discover conglomerates of rounded stones which bear no resemblance to any rocks now near them. These stones consequently prove the existence of rocks more ancient still; they are fragments of other mountains, of other shores, perhaps even of continents, broken up, destroyed, and crumbled by earlier seas. There is, then, little hope of one discovering the origin of life on the globe, since this page of the Genesis of the facts has been torn. For some years geologists loved to rest their eyes, in this long night of ages, upon an ideal limit beyond which plants and animals would begin to appear. Now, this line of demarcation between the rocks which are without vestiges of organised beings, and those which contain fossils, is nearly effaced among the surrounding ruins. On the horizon of the primitive world we see vaguely indicated a series of other worlds which have altogether disappeared; perhaps it is necessary to resign ourselves to the fact that the dawn of life is lost in this silent epoch, where age succeeds age, till they are clothed in the garb of eternity. The river of creation is like the Nile, which, as Bossuet says, hides its head—a figure of speech which time has falsified—but the endless speculations opened up by these and similar considerations led Lyell to say, ‘Here I am almost prepared to believe in the ancient existence of the Atlantis of Plato.’”

OLD RED SANDSTONE AND DEVONIAN PERIOD.

Another great period in the Earth’s history opens on us—the Devonian or “Old Red Sandstone,” so called, because the formation is very clearly displayed over a great extent of country in the county of Devon. The name was first proposed by Murchison and Sedgwick, in 1837, for these strata, which had previously been referred to the “transition” or Silurian series.

The circumstances which marked the passage of the uppermost Silurian rocks into Old Red Sandstone seem to have been:—First, a shallowing of the sea, followed by a gradual alteration in the physical geography of the district, so that the area became changed into a series of mingled fresh and brackish lagoons, which, finally, by continued terrestrial changes, were converted into a great fresh-water lake; or, if we take the whole of Britain and lands beyond, into a series of lakes.[40]

Mr. Godwin Austen has, also, stated his opinion that the Old Red Sandstone, as distinct from the Devonian rocks, was of lacustrine origin.

The absence of marine shells helps to this conclusion, and the nearest living analogues of some of the fishes are found in the fresh water of Africa and North America. Even the occurrence in the Devonian rocks of Devonshire and Russia of some Old Red Sandstone fishes along with marine shells, merely proves that some of them were fitted to live in either fresh or salt water, like various existing fishes. At the present day animals that are commonly supposed to be essentially marine, are occasionally found inhabiting fresh water, as is the case in some of the lakes of Sweden, where it is said marine crustacea are found. Mr. Alexander Murray also states that in the inland fresh-water lakes of Newfoundland seals are common, living there without even visiting the sea. And the same is the case in Lake Baikal, in Central Asia.

The red colour of the Old Red Sandstone of England and Scotland, and the total absence of fossils, except in the very uppermost beds, are considered by Professor Ramsay to indicate that the strata were deposited in inland waters. These fossils are terrestrial ferns, Adiantites (Pecopteris) Hibernicus, and a fresh-water shell, Anodon Jukesii, together with the fish Glyptolepis.[41]

The rocks deposited during the Devonian period exhibit some species of animals and plants of a much more complex organisation than those which had previously made their appearance. We have seen, during the Silurian epoch, organisms appearing of very simple type; namely, zoophytes, articulated and molluscous animals, with algæ and lycopods, among plants. We shall see, as the globe grows older, that organisation becomes more complex. Vertebrated animals, represented by numerous Fishes, succeed Zoophytes, Trilobites, and Molluscs. Soon afterwards Reptiles appear, then Birds and Mammals; until the time comes when man, His supreme and last work, issues from the hands of the Creator, to be king of all the earth—man, who has for the sign of his superiority, intelligence—that celestial gift, the emanation from God.

Vast inland seas, or lakes covered with a few islets, form the ideal of the Old Red Sandstone period. Upon the rocks of these islets the mollusca and articulata of the period exhibit themselves, as represented on the opposite page (Plate IX.). Stranded on the shore we see armour-coated Fishes of strange forms. A group of plants (Asterophyllites) covers one of the islets, associated with plants nearly herbaceous, resembling mosses, though the true mosses did not appear till a much later period. Encrinites and Lituites occupy the rocks in the foreground of the left hand.

The vegetation is still simple in its development, for forest-trees seem altogether wanting. The Asterophyllites, with tall and slender stems, rise singly to a considerable height. Cryptogams, of which our mushrooms convey some idea, would form the chief part of this primitive vegetation; but in consequence of the softness of their tissues, their want of consistence, and the absence of much woody fibre, these earlier plants have come down to us only in a fragmentary state.

The plants belonging to the Devonian period differ much from the vegetation of the present day. They resembled both mosses and lycopods, which are flowerless cryptogamic plants of a low organisation. The Lycopods are herbaceous plants, playing only a secondary part in the vegetation of the globe; but in the earlier ages of organic creation they were the predominant forms in the vegetable kingdom, both as to individual size and the number and variety of their species.

In the woodcut (Fig. 30) we have represented three species of aquatic plants belonging to the Devonian period; they are—1, Fucoids (or Algæ); 2, Zostera; 3, Psilophyton. The Fucoid closely resembles its modern ally; but with the first indications of terrestrial vegetation we pass from the Thallogens, to which the Algæ belong (plants of simple organisation, without flower or stem), to the Acrogens, which throw out their leaves and branches at the extremity, and bear in the axils of their leaves minute circular cases, which form the receptacles of their spore-like seeds. “If we stand,” says Hugh Miller, “on the outer edge of one of those iron-bound shores of the Western Highlands, where rock and skerries are crowned with sea-weeds; the long cylindrical lines of chorda-filum, many feet in length, lying aslant in the tideway; long shaggy bunches of Fucus serratus and F. nodosus drooping from the sides of the rock; the flat ledges bristling with the stiff cartilaginous many-cleft fronds of at least two species of Chondrus; now, in the thickly-spread Fucoids of this Highland scene we have a not very improbable representation of the Thallogenous vegetation. If we add to this rocky tract, so rich in Fucoids, a submarine meadow of pale shelly sand, covered by a deep-green swathe of Zosteræ, with jointed root and slim flowers, unfurnished with petals, it would be more representative still.”

Let us now take a glance at the animals belonging to this period.

The class of Fishes seem to have held the first rank and importance in the Old Red Sandstone fauna; but their structure was very different from that of existing fishes: they were provided with a sort of cuirass, and from the nature of the scales were called Ganoid fishes. Numerous fragments of these curious fishes are now found in geological collections; they are of strange forms, some being completely covered with a cuirass of many pieces, and others furnished with wing-like pectoral fins, as in Pterichthys.

Let any one picture to himself the surprise he would feel should he, on taking his first lesson in geology, and on first breaking a stone—a pebble, for instance, exhibiting every external sign of a water-worn surface—find, to appropriate Archdeacon Paley’s illustration, a watch, or any other delicate piece of mechanism, in its centre. Now, this, thirty years ago, is exactly the kind of surprise that Hugh Miller experienced in the sandstone quarry opened in a lofty wall of cliff overhanging the northern shore of the Moray Frith. He had picked up a nodular mass of blue Lias-limestone, which he laid open by a stroke of the hammer, when, behold! an exquisitely shaped Ammonite was displayed before him. It is not surprising that henceforth the half-mason, half-sailor, and poet, became a geologist. He sought for information, and found it; he found that the rocks among which he laboured swarmed with the relics of a former age. He pursued his investigations, and found, while working in this zone of strata all around the coast, that a certain class of fossils abounded; but that in a higher zone these familiar forms disappeared, and others made their appearance.

He read and learned that in other lands—lands of more recent formation—strange forms of animal life had been discovered; forms which in their turn had disappeared, to be succeeded by others, more in accordance with beings now living. He came to know that he was surrounded, in his native mountains, by the sedimentary deposits of other ages; he became alive to the fact that these grand mountain ranges had been built up grain by grain in the bed of the ocean, and the mountains had been subsequently raised to their present level by the upheaval of one part of its bed, or by the subsidence of another. The young geologist now ceased to wonder that each bed, or series of beds, should contain in its bosom records of its own epoch; it seemed to him as if it had been the object of the Creator to furnish the inquirer with records of His wisdom and power, which could not be misinterpreted.

Among the Fishes of Old Red Sandstone, the Coccosteus (Fig. 31, No. 1) was only partially cased in a defensive armour; the upper part of the body down to the fins was defended by scales. Pterichthys (No. 2), a strange form, with a very small head, furnished with two powerful paddles, or arms, like wings, and a mouth placed far behind the nose, was entirely covered with scales. The Cephalaspis (No. 3), which has a considerable outward resemblance to some fishes of the present time, was nevertheless mail-clad, only on the anterior part of the body.

Other fishes were provided with no such cuirass, properly so called, but were protected by strong resisting scales, enveloping the whole body. Such were the Acanthodes (1), the Climatius (2), and the Diplacanthus (3), represented in Fig. 32.

Among the organic beings of the Devonian rocks we find worm-like animals, such as the Annelides, protected by an external shell, and which at the present day are probably represented by the Serpulæ. Among Crustaceans the Trilobites are still somewhat numerous, especially in the middle rocks of the period. We also find there many different groups of Mollusca, of which the Brachiopoda form more than one-half. We may say of this period that it is the reign of Brachiopoda; in it they assumed extraordinary forms, and the number of their species was very great. Among the most curious we may instance the enormous Stringocephalus Burtini, Davidsonia Verneuilli, Uncites gryphus, and Calceola Sandalina, shells of singular and fantastic shape, differing entirely from all known forms. Amongst the most characteristic of these Mollusca, Atrypa reticularis (Fig. 33) holds the first rank, with Spirifera concentrica, Leptæna Murchisoni, and Productus subaculeatus. Among the Cephalopoda we have Clymenia Sedgwickii (Fig. 34), including the Goniatites, illustrating the Ammonites, which so distinctly characterise the Secondary epoch, but which were only foreshadowed in the Devonian period.

Among the Radiata of this epoch, the order Crinoidea are abundantly represented. We give as an example Cupressocrinus crassus (Fig. 35). The Encrinites, under which name the whole of these animals are sometimes included, lived attached to rocky places and in deep water, as they now do in the Caribbean sea.

The Old Red Sandstone rocks are composed of schists, sandstone, and limestones. The line of demarcation between the Silurian rocks and those which succeed them may be followed, in many places, by the eye; but, on a closer examination, the exact limits of the two systems become more difficult to fix. The beds of the one system pass into the other by a gradual passage, for Nature rarely admits of violent contrasts, and shows few sudden transitions. By-and-by, however, the change becomes very decided, and the contrast between the dark grey masses at the base and the superincumbent yellow and red rocks become sufficiently striking. In fact, the uppermost beds of the Silurian rocks are the passage-beds of the overlying system, consisting of flagstones, occasionally reddish, and called in some districts “tile-stones.” Over these lie the Old Red Sandstone conglomerate, the Caithness flags, and the great superincumbent mass which forms the upper portion of the system. Though less abrupt than the eruptive and Silurian mountains, the Old Red Sandstone scenery is, nevertheless, distinguished by its imposing outline, assuming bold and lofty escarpments in the Vans of Brecon, in Grongar Hill, near Caermarthen, and in the Black Mountain of Monmouthshire, in the centre of a landscape which, wood, rock, and river combine to render perfect. But it is in the north of Scotland where this rock assumes its grandest aspect, wrapping its mantle round the loftiest mountains, and rising out of the sea in rugged and fantastic masses, as far north as the Orkneys. In Devon and Cornwall, where the rocks are of a calcareous, and sometimes schistose or slaty character, they are sufficiently extensive to have given a name to the series, which is recognised all over the world.