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.