THE EOCENE PERIOD. 297 



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-colored, 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," 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 condition 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 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 (Headon 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 

 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 brack- 

 ish water. 



II. EOCENE BEDS OF THE PARIS BASIN. The Eocene 

 strata are very well developed in the neighborhood of Paris, 

 where they occupy a large area or basin scooped out of the 

 Chalk. The beds of this area are partly marine, partly fresh- 

 water in origin ; and the following table (after Sir Charles 

 Lyell) shows their subdivisions and their parallelism with the 

 English series: 



