There is a sort of transition formation which is classed with the Cretaceous System, and termed “Maestricht,” after the town in Belgium. It appears that this is an upper chalk layer, an intermediary between the Secondary and Tertiary, and here on the banks of the Meuse we find the Mosasaurus, the “lizard of the Meuse,” of whose remains we give specimens in the illustration. This transition chalk—as we may call it to distinguish it—must have been laid down at a later period than the flinty chalk, and we find it in many places. It serves therefore as a fitting introduction to the Tertiary Period of Geological time.
The Tertiary Period.
We now enter upon a period when the animal creation attained its greatest development, the “Age of Mammals”; for they were then the kings of creation. The Tertiary Period is divided into three stages, viz.—
The Eocene, or the Dawning of the now existing creation.
The Miocene, or the Middle, or “minority” of existing creation.
The Pliocene, or the Recent, or still more developed period.
We will glance at them in that order, which Sir C. Lyell introduced.
Fig. 680.—Skull of the Dinotherium.