Before proceeding farther, I will enumerate the geological epochs in their succession, confining myself, however, to such as are perfectly well established, without alluding to those of which the limits are less definitely determined, and which are still subject to doubts and discussions among geologists.  As I do not propose to make here any treatise of Geology, but simply to place before my readers some pictures of the old world, with the animals and plants that inhabited it at various times, I shall avoid, as far as possible, all debatable ground, and confine myself to those parts of my subject which are best known, and can therefore be more clearly presented.

First, we have the Azoic period, devoid of life, as its name signifies,—­namely, the earliest stratified deposits upon the heated film forming the first solid surface of the earth, in which no trace of living thing has ever been found.  Next comes the Silurian period, when the crust of the earth had thickened and cooled sufficiently to render the existence of animals and plants upon it possible, and when the atmospheric conditions necessary to their maintenance were already established.  Many of the names given to these periods are by no means significant of their character, but are merely the result of accident:  as, for instance, that of Silurian, given by Sir Roderick Murchison to this set of beds, because he first studied them in that part of Wales occupied by the ancient tribe of the Silures.  The next period, the Devonian, was for a similar reason named after the county of Devonshire, in England, where it was first investigated.  Upon this follows the Carboniferous period, with the immense deposits of coal from which it derives its name.  Then comes the Permian period, named, again, from local circumstances, the first investigation of its deposits having taken place in the province of Permia, in Russia.  Next in succession we have the Triassic period, so called from the trio of rocks, the red sandstone, Muschel Kalk, (shell-limestone.) and Keuper, (clay,) most frequently combined in its formations; the Jurassic, so amply illustrated in the chain of the Jura, where geologists first found the clue to its history; and the Cretaceous period, to which the chalk cliffs of England and all the extensive chalk deposits belong.  Upon these follow the so-called Tertiary formations, divided into three periods, all of which have received most characteristic names.  In this epoch of the world’s history we see the first approach to a condition of things resembling that now prevailing, and Sir Charles Lyell has most fitly named its three divisions, the “Eocene,” or the dawn, the “Miocene,” meaning the continuance and increase of that light, and lastly, the “Pliocene,” signifying its fulness and completion.  Above these deposits comes what has been called in science the present period,—­the modern times of the geologist,—­that period to which man himself belongs, and since the beginning of which, though its duration be counted by hundreds of thousands of years, there has been no alteration in the general configuration of the earth, consequently no important modification of its climatic conditions, and no change in the animals and plants inhabiting it.