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.
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