ally,
this formation exhibits the interesting point that it graduates
downwards insensibly and conformably into the Cretaceous, whilst
it is succeeded _uncomformably_ by strata of Middle Eocene age.
Lithologically, the series consists principally of sands and
clays, with beds of lignite and coal, and its organic remains
show that it is principally of fresh-water origin with a partial
intermixture of marine beds. These marine strata of the "Lignitic
formation" are of special interest, as showing such a commingling
of Cretaceous and Tertiary types of life, that it is impossible
to draw any rigid line in this region between the Mesozoic and
Kainozoic systems. Thus the marine beds of the Lignitic series
contain such characteristic Cretaceous forms as _Inoceramus_
and _Ammonites_, along with a great number of Univalves of a
distinctly Tertiary type (Cones, Cowries, &c.) Upon the whole,
therefore, we must regard this series of deposits as affording a
kind of transition between the Cretaceous and the Eocene, holding
in some respects a position which may be compared with that held
by the Purbeck beds in Britain as regards the Jurassic and
Cretaceous.

The Middle Eocene of the United States is represented by the
_Claiborne_ and _Jackson_ beds. The _Claiborne series_ is extensively
developed at Claiborne, Alabama, and consists of sands, clays,
lignites, marls, and impure limestones, containing marine fossils
along with numerous plant-remains. The _Jackson series_ is
represented by lignitic clays and marls which occur at Jackson,
Mississippi. Amongst the more remarkable fossils of this series
are the teeth and bones of Cetaceans of the genus _Zeuglodon_.

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, t