The World Before the Deluge

Contemporaneously with these animals, plants of inferior organisation have left their impressions upon the schists; these are Algae (aquatic plants. Fig. 28). As the continents enlarged, plants of a higher type made their appearance — the Equisetaceie, herbaceous Ferns, and other plants. These we shall have occasion to specify when noticing the periods which constitute the Primary Epoch, and which consists of the following periods: the Carboniferous, the Old Hed Sandstone, and Devonian, the Silurian, and the Cambrian.

CAMBRIAN PERIOD. lOlI

Cambrian Period.

The researches of geologists have discovered but scanty traces of organic remains in the rocks which form the base of this system in England. Arenicolites, or worm-tracks and burrows, have been found in Shropshire, by Mr. Salter, to occur in countless numbers through a mile of thickness in the Longmynd rocks; and others were discovered by the late Dr. Kinahan in Wicklow. In Ireland, in the picturesque tract of Bray Head, on the south and east coasts of Dublin, we find, in slaty beds of the same age as the Longmynd rocks, a peculiar zoophyte, which has been named by Edward Forbes Old/lamia^ after its discoverer, Dr. Oldham, Superintendent of the Geological Survey of India. This fossil represents one of the earliest inhabitants of the ocean, which then covered the greater part of the British Isles. " In the hard, purplish, and schistose rocks of Bray Head," says Dr. Kinahan,* "as well as other parts of Ireland which are recognised as Cambrian rocks, markings of a very peculiar character are found. They occur in masses, and are recognised as hydrozoic animal assem-blages. They have regularity of form, abundant, but not universal, occurrence in beds, and permanence of character even when the beds are at a distance from each other, and dissimilar in chemical and physical character." In the course of his investigations, Dr. Kinahan discovered at least four species of Oldhamia, which he has described and figured. The Cambrian rocks consist of the Llanberis slates of Llanberis and Penrhyn in North Wales, which, with their associated sandy strata, attain a thickness of about 3,000 feet, and the Barmouth and Harlech Sandstones. In the Longmynd hills of Shropshire these last beds attain a thickness of 6,000 feet; and in some parts of Merioneth-shire they are of still greater thickness.

Neither in North Wales, nor in the Longmynd, do the Cambrian rocks afford any indications of life, except annelide-tracks and burrows. From this circumstance, together with general absence of Mollusca in these strata, and the sudden appearance of numerous shells and trilobites in the succeeding Lingula Flags, a change of conditions seems to have ensued at the close of the Cambrian period.

Believing that the red colour of rocks is frequently connected with their deposition in inland waters. Professor Ramsay conceives it to be possible, that the absence of marine mollusca in the Cambrian rocks may be due to the same cause that produced their absence in

• Trans. Roy. Irish Acad., vol. xxiii., p. 556.

the Old Red Sandstone, and that the presence of sun-cracks and rain-pittings in the Longmynd beds is a corroboration of this suggestion.* The Silurian Period.

The next period of the Primary Epoch is the Silurian, a system of Tocks universal in extent, overspreading the whole earth more or less completely, and covering up the rocks of older age. The term "Silurian" was given by the illustrious Murchison to the epoch which now occupies our attention, because the system of rocks formed by the marine sediments, during the period in question, form large tracts of country in Shropshire and Wales, a region formerly peopled by the Silures, a Celtic race who fought gloriously against the Romans, under Caractacus or Caradoc, the British king of those tracts. The reader may find the nomenclature strange, as applied to the vast range of rocks which it represents in all parts of the Old and New World, but it indicates, with sufficient exactness, the particular region in our own country in which the system typically prevails — reasons which led to the term being adopted, even at a time when its vast geographical extent was not suspected.

On this subject, and on the principles which have guided geologists in their classification of rocks. Professor Sedgwick remarks in one of his papers in the Quarterly Jourfial of the Geological Society: "In every country," he says,t " which is not made out by reference to a pre-existing type, our first labour is that of determining the physical groups, and establishing their relations by natural sections. The labour next in order is the determination of the fossils found in the successive physical groups; and, as a matter of fact, the natural groups of fossils are generally found to be nearly co-ordinate with the physical groups — each successive group resulting from certain conditions which have modified the distribution of organic types. In the third place comes the collective arrangement of the groups into systems, or groups of a higher order. The establishment of the Silurian system is an admirable example of this whole process. The groups called Caradoc, Wenlock, Ludlow, &c., were physical groups determined by good natural sections. The successive groups of fossils were determined by the sections; and the sections, as the representatives ofphysical groups, were hardly at all modified by any

• " On the Red Rocks of England," by A. C. Ramsay. Quart. Jour. GeoL Soc., vol. xxvii., p. 250, i Quari. Jour. GeoL Soc, vol iii., p. 159.

I

I

VIII.—Ideal Landscape of the Silurian Period.

THE SILURIAN PERIOD.

consideration of the fossils, for these two distinct views of the natural history of such groups led to co-ordinate results. Then followed the collective view of the whole series, and the establishment of a nomenclature. Not only the whole series (considered as a distinct system), but every subordinate group was defined by a geographical name, referring us to a local type within the limits of Siluria; in this respect adopting the principle of grouping and nomenclature applied by W. Smith to our secondary rocks. At the same time, the older slate rocks of Wales (inferior to the system of Siluria), were called Cambrian^ and soon afterwards the next great collective group of rocks (superior to the system of Siluria) was called Devotiian. In this way was established a perfect congruity of language. It was geographical in principle, and it represented the actual development of all our older rocks, which gave to it its true —Back of Asaphus value and meaning." The period, then, for caudatusStokes), with(Dudley,the eyes Mus.well the purposes of scientific description, may be preserved. (Buckland.) divided into three sub-periods — the Upper and Lower Silurian, and the Cambrian.

The characteristics of the Silurian period, of which we give an ideal view opposite (Plate VIII.), are supposed to have been shallow seas of great extent, with barren submarine reefs and isolated rocks rising here and there out of the water, covered with Algse, and frequented by various MoUusca and articulated animals. The earliest traces of vegetation belong to the Thaliogens, flowerless plants of the

Fig. 19.— rt, Side view of the left eye of the above, magnified. class Algae (Fig. 28), (BuckUnd.) b, Magnified view of a portion of the eye of without leaves or stems, Calymene macrophthalmus. (Hoeninghaus.) which are found among the Lower Silurian rocks. To these succeed other plants, according to Dr. Hooker, belonging to the Lycopodiaceoe (Fig. 28), the seeds of which are found sparingly in the Upper Ludlow beds. Among animals;

I06 THE WORLD BEFORE THE DELUGE.

the Orthoceratites led a predacious life in the Silurian seas. Their organisation indicates that they preyed upon other animals, pursuing them into the deepest abysses, and strangling them in the embrace of their long arms. The Trilobites, a remarkable group of Crustacea, possessing simple and reticulated compound eyes, also highly characterise this period (Figs. 17 to 20); presenting at one period or other of their existence 1,67.7 species, 224 of which are met with in Great Britain and Ireland, as we are taught by the " Thesaurus Siluricus."* Add to this a sun, struggling to penetrate the dense atmosphere of the primitive world, and yielding a dim and imperfect light to the first created beings as they left the hand of the Creator, organisms often rudimentary, but at other times sufficiently advanced to indicate a progress towards more perfect creations. Such is the picture which the artist has attempted to portray.

The elaborate and highly valuable " Thesaurus Siluricus " contains the names of 8,997 species of fossil remains, but it probably does not tell us of one-tenth part of the Silurian life still lying buried in rocks of that age in various parts of the world. A rich field is here offered to the geological explorer, f

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 sen, 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

• "The Flora and Fauna of the Silurian Teriod," by John T. Bigsby, M.A., F.G.S. 4to, 1868. f Ibid, p. vi.

LOWER SILURIAN.

book in which is written the history of the antiquities of Great Britain — in fine, of the worid.

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 Livgula. 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. Thege are coelenterata, articulata, and mollusca. They mark, however, only a very ephemeral passage over the globe, and soon disappear altogether.