Part 10

After the terrible tempests of the primitive period--after these great disturbances of the mineral kingdom--Nature would seem to have gathered herself together, in sublime silence, in order to proceed to the grand mystery of the creation of living beings.

During the primitive epoch the temperature of the earth was too high to admit the appearance of life on its surface. The darkness of thickest night shrouded this cradle of the world; the atmosphere probably was so charged with vapours of various kinds, that the sun’s rays were powerless to pierce its opacity. Upon this heated surface, and in this perpetual night, organic life could not manifest itself. No plant, no animal, then, could exist upon the silent earth. In the seas of this epoch, therefore, only unfossiliferous strata were deposited.

Nevertheless, our planet continued to be subjected to a gradual refrigeration on the one hand, and, on the other, continuous rains were purifying its atmosphere. From this time, then, the sun’s rays, being less obscured, could reach its surface, and, under their beneficent influence, life was not slow in disclosing itself. “Without light,” said the illustrious Lavoisier, “Nature was without life; it was dead and inanimate. A benevolent God, in bestowing light, has spread on the surface of the earth organisation, sentiment, and thought.” We begin, accordingly, to see upon the earth--the temperature of which was nearly that of our equatorial zone--a few plants and a few animals make their appearance. These first generations of life will be replaced by others of a higher organisation, until at the last stage of the creation, man, endowed with the supreme attribute which we call intelligence, will appear upon the earth. “The word _progress_, which we think peculiar to humanity, and even to modern times,” said Albert Gaudry, in a lecture on the animals of the ancient world, delivered in 1863, “was pronounced by the Deity on the day when he created the first living organism.”

Did plants precede animals? We know not; but such would appear to have been the order of creation. It is certain that in the sediment of the oldest seas, and in the vestiges which remain to us of the earliest ages of organic life on the globe, that is to say, in the argillaceous schists, we find both plants and animals of advanced organisation. But, on the other hand, during the greater part of the primary epoch--especially during the Carboniferous age--the plants are particularly numerous, and terrestrial animals scarcely show themselves; this would lead us to the conclusion that plants preceded animals. It may be remarked, besides, that from their cellular nature, and their looser tissues composed of elements readily affected by the air, the first plants could be easily destroyed without leaving any material vestiges; from which it may be concluded, that, in those primitive times, an immense number of plants existed, no traces of which now remain to us.

We have stated that, during the earlier ages of our globe, the waters covered a great part of its surface; and it is in them that we find the first appearance of life. When the waters had become sufficiently cool to allow of the existence of organised beings, creation was developed, and advanced with great energy; for it manifested itself by the appearance of numerous and very different species of animals and plants.

One of the most ancient groups of organic remains are the Brachiopoda, a group of Mollusca, particularly typified by the genus Lingula, a species of which still exist in the present seas; the Trilobites (Fig. 17), a family of Crustaceans, especially characteristic of this period; then come Productas, Terebratulæ, and Orthoceratites--other genera of Mollusca. The Corals, which appeared at an early period, seem to have lived in all ages, and survive to the present day.

Contemporaneously with these animals, plants of inferior organisation have left their impressions upon the schists; these are Algæ (aquatic plants, Fig. 28). As the continents enlarged, plants of a higher type made their appearance--the Equisetaceæ, 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 Red Sandstone, and Devonian, the Silurian, and the Cambrian.

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 _Oldhamia_, 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,[34] “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 assemblages. 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.

[34] Trans. Roy. Irish Acad., vol. xxiii., p. 556.

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 Merionethshire 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 the Old Red Sandstone, and that the presence of sun-cracks and rain-pittings in the Longmynd beds is a corroboration of this suggestion.[35]

[35] “On the Red Rocks of England,” by A. C. Ramsay. _Quart. Jour. Geol. Soc._, vol. xxvii., p. 250.

THE SILURIAN PERIOD.

The next period of the Primary Epoch is the _Silurian_, a system of rocks 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 Journal of the Geological Society_: “In every country,” he says,[36] “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 of physical groups, were hardly at all modified by any 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 _Devonian_. 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 value and meaning.” The period, then, for the purposes of scientific description, may be divided into three sub-periods--the Upper and Lower Silurian, and the Cambrian.

[36] _Quart. Jour. Geol. Soc_., vol. iii., p. 159.

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 Algæ, and frequented by various Mollusca and articulated animals. The earliest traces of vegetation belong to the _Thallogens_, flowerless plants of the class Algæ (Fig. 28), without leaves or stems, which are found among the Lower Silurian rocks. To these succeed other plants, according to Dr. Hooker, belonging to the Lycopodiaceæ (Fig. 28), the seeds of which are found sparingly in the Upper Ludlow beds. Among animals, 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 1,677 species, 224 of which are met with in Great Britain and Ireland, as we are taught by the “Thesaurus Siluricus.”[37] 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.

[37] “The Flora and Fauna of the Silurian Period,” by John T. Bigsby, M.A., F.G.S. 4to, 1868.

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.[38]

[38] Ibid, p. vi.

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.)