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 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, (i.) LOWER EOCENE. The base 

 of the Eocene series in Britain is constituted by about 90 feet 

 of light-coloured, sometimes argillaceous sands (Thanct 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.) 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 (Bagshot and BracklcsJiam 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 (Hcadon 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 EOCENE. If the Headon and Osborne beds of 



