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 sand, clays, and gravels, and attaining a very considerable thickness (Bag-shot and Bracklesham beds).  The superior portion of the Middle Eocene of Britain, on the other hand, consists of deposits which are almost exclusively fresh-water or brackish-water in origin (Headon and Osborne series).

The chief Continental formations of Middle Eocene age are the “Calcaire grossier” of the Paris basin, and the “Nummulitic Limestone” of the Alps.

(3.) UPPER EOCENE.—­If the Headon and Osborne beds of the Isle of Wight be placed in the Middle Eocene, the only British representatives of the Upper Eocene are the Bembridge beds.  These strata consist of limestones, clays, and marls, which have for the most part been deposited in fresh or brackish water.