The Ancient Life History of the Earth / A Comprehensive Outline of the Principles and Leading Facts of Palæontological Science

by Henry Alleyne Nicholson

The absence in strata of Tertiary age of the chambered Cephalopods, the Belemnites, the Hippurites, the Inocerami, and the diversified types of Reptiles which form such conspicuous features in the Cretaceous fauna, render the palæontological break between the Chalk and the Eocene one far too serious to be overlooked. At the same time, it is to be remembered that the evidence afforded by the explorations carried out of late years as to the animal life of the deep sea, renders it certain that the extinction of marine forms of life at the close of the Cretaceous period was far less extensive than had been previously assumed. It is tolerably certain, in fact, that we may look upon some of the inhabitants of the depths of our existing oceans as the direct, if modified, descendants of animals which were in existence when the Chalk was deposited.

It follows from the general want of conformity between the Cretaceous and Tertiary rocks, and still more from the great difference in life, that the Cretaceous and Tertiary periods are separated, in the Old World at any rate, by an enormous lapse of unrepresented time. How long this interval may have been, we have no means of judging exactly, but it very possibly was as long as the whole Kainozoic epoch itself. Some day we shall doubtless find, at some part of the earth's surface, marine strata which were deposited during this period, and which will contain fossils intermediate in character between the organic remains which respectively characterise the Secondary and Tertiary periods. At present, we have only slight traces of such deposits—as, for instance, the Maestricht beds, the Faxöe Limestone, and the Pisolitic Limestone of France.

CLASSIFICATION OF THE TERTIARY ROCKS.—The classification of the Tertiary rocks is a matter of unusual difficulty, in consequence of their occurring in disconnected basins, forming a series of detached areas, which hold no relations of superposition to one another. The order, therefore, of the Tertiaries in point of time, can only be determined by an appeal to fossils; and in such determination Sir Charles Lyell proposed to take as the basis of classification the proportion of living or existing species of Mollusca which occurs in each stratum or group of strata. Acting upon this principle, Sir Charles Lyell divides the Tertiary series into four groups:—

I. The Eocene formation (Gr. eos, dawn; kainos, new), containing the smallest proportion of existing species, and being, therefore, the oldest division. In this classification, only the Mollusca are taken into account; and it was found that of these about three and a half per cent were identical with existing species.

II. The Miocene formation (Gr. meion, less; kainos, new), with more recent species than the Eocene, but less than the succeeding formation, and less than one-half the total number in the formation. As before, only the Mollusca are taken into account, and about 17 per cent of these agree with existing species.

III. The Pliocene formation (Gr. pleion, more; kainos, new), with generally more than half the species of shells identical with existing species—the proportion of these varying from 35 to 50 per cent in the lower beds of this division, up to 90 or 95 per cent in its higher portion.

IV. The Post-Tertiary Formations, in which all the shells belong to existing species. This, in turn, is divided into two minor groups—the Post-Pliocene and Recent Formations. In the Post-Pliocene formations, while all the Mollusca belong to existing species, most of the Mammals belong to extinct species. In the Recent period, the quadrupeds, as well as the shells, belong to living species.

The above, with some modifications, was the original classification proposed by Sir Charles Lyell for the Tertiary rocks, and now universally accepted. More recent researches, it is true, have somewhat altered the proportions of existing species to extinct, as stated above. The general principle, however, of an increase in the number of living species, still holds good; and this is as yet the only satisfactory basis upon which it has been proposed to arrange the Tertiary deposits.

EOCENE FORMATION.

The Eocene rocks are the lowest of the Tertiary series, and comprise all those Tertiary deposits in which there is only a small proportion of existing Mollusca—from three and a half to five per cent. The Eocene rocks occur in several basins in Britain, France, the Netherlands, and other parts of Europe, and in the United States. The subdivisions which have been established are extremely numerous, and it is often impossible to parallel those of one basin with those of another. It will be sufficient, therefore, to accept the division of the Eocene formation into three great groups—Lower, Middle, and Upper Eocene—and to consider some of the more important beds comprised under these heads in Europe and in North America.