Let us now briefly explain the very simple but satisfactory basis on which this threefold division of the Tertiary rocks rests, and then proceed to a brief explanation of each. The reader will already have noted a statement on a preceding page, to which we shall be pardoned, if, a second time, we ask attention to it. In imparting elementary instruction on geology we have always found our classes more or less puzzled by the tertiary system, on account of its nomenclature and minute subdivisions, and we have learnt from experience the importance of presenting this statement over and over again. We have remarked that in the _secondary rocks_—that is, up to the end of the chalk system—there are _no_ organic remains found precisely similar to any species existing at the present day; but when we come to the tertiary rocks, although we find many strangers, we find also a good many organic remains of the same kind and character as the shells, that are now found on our shores. In one part of the Tertiary the number of fossils that belongs to existing genera is many, in another more, in another more still; and upon this simple idea of positive, comparative, and superlative, the present division is based. Taking the _percentage_ principle as a guide, Sir Charles Lyell and a distinguished French geologist, M. Deshayes, have ascertained that in the lowest beds of this system there were only 3½ per cent. of fossil shells similar to existing species, and this, for “the sake of clearness and brevity,” says Sir Charles, “was called the _Eocene_ period, or the period of the dawn, the dawn of our modern era so far as its testaceous fauna are concerned.” Rising higher in the examination of these rocks, certain strata were found containing 18 per cent. of fossil shells, similar to shells found now; and to this was given the name of _Miocene_, the puzzling name already spoken of, because it means _less_ recent, whereas it is in reality _more_ recent, and is to be understood in relation to the series _below_, and not to the series _above_. A step higher up in this system revealed deposits of a coralline and craggy character, in which 41 per cent. of fossil shells like those of the present era were found; and to this the name of _Pliocene_, or more recent still, was given; and latterly, in Sicily chiefly, a series of strata has been discovered, referable to the Tertiary, in which 95 per cent. of recent species of shells have been found, and to this series the name of Post-Pliocene, or _Pleistocene_, has been given. Before our description of each of these divisions, let us add, that “the organic _remains of the system_ constitute its most important and interesting feature. The fossils of earlier periods presented little analogy, often no resemblance, to existing plants and animals; here, however, the similitude is frequently so complete, that the naturalist can scarcely point out a distinction between them and living races. Geology thus