Strata of Upper Eocene age occur in North America at Vicksburg, Mississippi, and are known as the Vicksburg series.  They consist of lignites, clays, marls, and limestones.  Freshwater deposits of Eocene age are also largely developed in parts of the Rocky Mountain region.  The most remarkable fossils of these beds are Mammals, of which a large number of species have been already determined.

LIFE OF THE EOCENE PERIOD.

The fossils of the Eocene deposits are so numerous that nothing more can be attempted here than to give a brief and general sketch of the life of the period, special attention being directed to some of the more prominent and interesting types, amongst which—­as throughout the Tertiary series—­the Mammals hold the first place.  It is not uncommon, indeed, to speak of the Tertiary period as a whole under the name of the “Age of Mammals,” a title at least as well deserved as that of “Age of Reptiles” applied to the Mesozoic, or “Age of Molluscs” applied to the Palaeozoic epoch.

As regards the plants of the Eocene, the chief point to be noticed is, that the conditions which had already set in with the commencement of the Upper Cretaceous, are here continued, and still further enforced.  The Cycads of the Secondary period, if they have not totally disappeared, are exceedingly rare; and the Conifers, losing the predominance which they enjoyed in the Mesozoic, are now relegated to a subordinate though well-defined place in the terrestrial vegetation.  The great majority of the Eocene plants are referable to the groups of the Angiospermous Exogens and the Monocotyledons; and the vegetation of the period, upon the whole, approximates closely to that now existing upon the earth.  The plants of the European Eocene are, however, in the main most closely allied to forms which are now characteristic of tropical or sub-tropical regions.  Thus, in the London Clay are found numerous fruits of Palms (Napdites, fig. 213), along with various other plants, most of which indicate a warm climate as prevailing in the south of England at the commencement of the Eocene period.  In the Eocene strata of North America occur numerous plants belonging to existing types—­such as Palms, Conifers, the Magnolia, Cinnamon, Fig.  Dog-wood, Maple, Hickory, Poplar, Plane, &c.  Taken as a whole, the Eocene flora of North America is nearly related to that of the Miocene strata of Europe, as well as to that now existing in the American area.  We conclude, therefore, that “the forests of the American Eocene resembled those of the European Miocene, and even of modern America” (Dana).

[Illustration:  Fig. 213.—­Napadites ellipticus, the fruit of a fossil Palm.  London Clay, Isle of Sheppey.]