formations, while all the _Mollusca_ belong
to existing species, most of the _Mammals_ belong to extinct
species. In the Recent period, the quadrupeds, as well as the
shells, belong to living species.

The above, with some modifications, was the original classification
proposed by Sir Charles Lyell for the Tertiary rocks, and now
universally accepted. More recent researches, it is true, have
somewhat altered the proportions of existing species to extinct,
as stated above. The general principle, however, of an increase
in the number of living species, still holds good; and this is as
yet the only satisfactory basis upon which it has been proposed
to arrange the Tertiary deposits.

EOCENE FORMATION.

The Eocene rocks are the lowest of the Tertiary series, and comprise
all those Tertiary deposits in which there is only a small proportion
of existing _Mollusca_--from three and a half to five per cent.
The Eocene rocks occur in several basins in Britain, France,
the Netherlands, and other parts of Europe, and in the United
States. The subdivisions which have been established are extremely
numerous, and it is often impossible to parallel those of one
basin with those of another. It will be sufficient, therefore,
to accept the division of the Eocene formation into three great
groups--Lower, Middle, and Upper Eocene--and to consider some of
the more important beds comprised under these heads in Europe
and in North America.

I. EOCENE OF BRITAIN. (1.) LOWER EOCENE.--The base of the Eocene
series in Britain is constituted by about 90 feet of light-coloured,
sometimes argillaceous sands (_Thanet Sands_), which are of marine
origin. Above these, or forming the base of the formation where these
are wanting, come mottled clays and sands with lignite (_Woolwich
and Reading series_), which are estuarine or fluvio-marine in
origin. The highest member of the Lower Eocene of Britain is the
"London Clay," consisting of a great mass of dark-brown or blue
clay, sometimes with sandy beds, or with layers of "septaria,"
the whole attaining a thickness of from 200 to as much as 500
feet. The London Clay is a purely marine deposit, containing
many marine fossils, with the remains of terrestrial animals and
plants; all of which indicate a high temperature of the sea and
tropical or sub-tropical conditions of the land.

(2.) MIDDLE EOCENE.--The inferior portion of the Middle Eocene
of Britain consists of marine beds, chiefly consisting of