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, cla.s.sified and arranged for public inspection in our museums cannot be much short of that number.

[Ill.u.s.tration: Fig. 25.--Illaenus Barriensis.--Dudley, Walsall.]

Towards the close of the Upper Silurian sub-period, the argillaceous beds pa.s.s upwards into more sandy and sh.o.r.e-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 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 quant.i.ties. 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 numbers and the small size of the sh.e.l.ls, 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 ma.s.s 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]

[39] "Siluria," p. 148.

[Ill.u.s.tration: Fig. 26.--Halysites catenularius.]

[Ill.u.s.tration: Fig. 27.--Pterygotus bilobatus.]

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.

[Ill.u.s.tration: Fig. 28.--Plants of the Palaeozoic Epoch.--1 and 2, Algae; 3 and 4, Lycopods.]

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 ma.s.ses 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 Vendee; the other const.i.tuted 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 lat.i.tude, 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 lat.i.tude, 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 c.u.mberland, 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 sh.e.l.ls 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 sh.e.l.ls 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-ma.s.ses 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 c.u.mberland, 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 lat.i.tude, 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 sh.o.r.es, 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.'"

[Ill.u.s.tration: Fig. 29.--Ischadites Knigii. Upper Ludlow Rocks.]

NOTE.--For accurate representations of the typical fossils of the Palaeozoic strata of Britain, the reader may consult, with advantage, the carefully executed "Figures of Characteristic British Fossils," by W. H. Baily, F.G.S. (Van Voorst).

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 circ.u.mstances which marked the pa.s.sage 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]

[40] "On the Red Rocks of England," by A. C. Ramsay. _Quart. Jour.

Geol. Soc._, vol. xxvii., p. 243.

Mr. G.o.dwin 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 sh.e.l.ls helps to this conclusion, and the nearest living a.n.a.logues 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 sh.e.l.ls, 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, _Adiant.i.tes_ (Pecopteris) _Hibernicus_, and a fresh-water sh.e.l.l, _Anodon Jukesii_, together with the fish _Glyptolepis_.[41]

[41] "On the Red Rocks of England," by A. C. Ramsay. _Quart. Jour.

Geol. Soc._, vol. xxvii., p. 247.

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 algae 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 G.o.d.

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 sh.o.r.e we see armour-coated Fishes of strange forms. A group of plants (_Asterophyllites_) covers one of the islets, a.s.sociated 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.

[Ill.u.s.tration: IX.--Ideal Landscape of the Devonian Period.]

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.

[Ill.u.s.tration: Fig. 30.--Plants of the Devonian Epoch. 1. Algae. 2.

Zostera. 3. Psilophyton, natural size.]

In the woodcut (Fig. 30) we have represented three species of aquatic plants belonging to the Devonian period; they are--1, _Fucoids_ (or _Algae_); 2, _Zostera_; 3, _Psilophyton_. The Fucoid closely resembles its modern ally; but with the first indications of terrestrial vegetation we pa.s.s from the _Thallogens_, to which the _Algae_ 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 sh.o.r.es 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 s.h.a.ggy 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 sh.e.l.ly sand, covered by a deep-green swathe of _Zosterae_, 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.