548 HISTORICAL PALAEONTOLOGY. 



minor groups the Post-Pliocene and Recent Formations. In 

 the Post-Pliocene 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 classi- 

 fication 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. LOWER EOCENE. The base of the Eocene series in 

 Britain is constituted by about 90 feet of light-coloured, some- 

 times 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 thick- 

 ness 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. 



II. MIDDLE EOCENE. The inferior portion of the Middle 

 Eocene of Britain consists of marine beds, chiefly consisting 



