Part 7 (2/2)
PRIMARY EPOCH.
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 ill.u.s.trious Lavoisier, ”Nature was without life; it was dead and inanimate. A benevolent G.o.d, 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 p.r.o.nounced 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, Terebratulae, and Orthocerat.i.tes--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.
[Ill.u.s.tration: Fig. 17.--Paradoxides Bohemicus--Bohemia.]
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 Equisetaceae, herbaceous Ferns, and other plants. These we shall have occasion to specify when noticing the periods which const.i.tute 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 Shrops.h.i.+re, 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 ma.s.ses, and are recognised as hydrozoic animal a.s.semblages. 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 a.s.sociated sandy strata, attain a thickness of about 3,000 feet, and the Barmouth and Harlech Sandstones. In the Longmynd hills of Shrops.h.i.+re these last beds attain a thickness of 6,000 feet; and in some parts of Merioneths.h.i.+re 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 circ.u.mstance, together with general absence of Mollusca in these strata, and the sudden appearance of numerous sh.e.l.ls 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 ill.u.s.trious 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 Shrops.h.i.+re 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 cla.s.sification 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 establis.h.i.+ng 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.
[Ill.u.s.tration: VIII.--Ideal Landscape of the Silurian Period.]
<script>