THE EOCENE PERIOD.
Before commencing the study of the subdivisions of the Kainozoic series, there are some general considerations to be noted. In the first place, there is in the Old World a complete and entire physical break between the rocks of the Mesozoic and Kainozoic periods. In no instance in Europe are Tertiary strata to be found resting conformably upon any Secondary rock. The Chalk has invariably suffered much erosion and denudation before the lowest Tertiary strata were deposited upon it. This is shown by the fact that the actually eroded surface of the Chalk can often be seen; or, failing this, that we can point to the presence of the chalk-flints in the Tertiary strata. This last, of course, affords unquestionable proof that the Chalk must have been subjected to enormous denudation prior to the formation of the Tertiary beds, all the chalk itself having been removed, and nothing left but the flints, while these are all rolled and rounded. In the continent of North America, on the other hand, the lowest Tertiary strata have been shown to graduate downwards conformably with the highest Cretaceous beds, it being a matter of difficulty to draw a precise line of demarcation between the two formations.
In the second place, there is a marked break in the life of the Mesozoic and Kainozoic periods. With the exception of a few Foraminifera, and one Brachiopod (the latter doubtful), no Cretaceous species is known to have survived the Cretaceous period; while several characteristic families, such as the Ammonitidoe, Belemnitidoe, and Hippuritidoe, died out entirely with the close of the Cretaceous rocks. In the Tertiary rocks, on the other hand, not only are all the animals and plants more or less like existing types, but we meet with a constantly-increasing number of living species as we pass from the bottom of the Kainozoic series to the top. Upon this last fact is founded the modern classification of the Kainozoic rocks, propounded by Sil Charles Lyell.
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 palaeontological 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.