THE EOCENE PERIOD. 287 
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 J/o//usca—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. 
J. EoceNE oF BRITAIN. (1.) LOWER EocEnr.—The base 
of the Eocene series in Britain is constituted by about go feet 
of light-coloured, sometimes argillaceous sands ( Zhanet 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,” consist- 
ing 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.) MippLe Eocenr.—The inferior portion of the Middle 
Eocene of Britain consists of marine beds, chiefly consisting 
of sand, clays, and gravels, and attaining a very considerable 
thickness (Bagshot and Bracklesham beds), The superior por- 
tion of the Middle Eocene of Britain, on the other hand, con- 
sists of deposits which are almost exclusively fresh-water or 
brackish-water in origin (/Zeadon and Osborne series). 
The chief Continental formations of Middle Eocene age are 
the ‘‘Calcaire grossier” of the Paris basin, and the ‘‘ Num- 
mulitic Limestone” of the Alps. 
(3.) Upper Eocrenr.—If the Headon and Osborne beds of 
