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.
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