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 Faxoee 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.