Ch. XVI.] EOCENE STRATA IN" FRANCE. 297 



assume more and more of a marine character ; while in an opposite, 

 or south-western direction, they become, as near Chelsea and other 

 places, more freshwater, and contain Unio, Paludina, and layers of 

 lignite, so that the land drained by the ancient river seems clearly to 

 have been to the south-west of the present site of the metropolis. 



Before the minds of geologists had become familiar with the theory 

 of the gradual sinking of the land, and its conversion into sea at dif- 

 ferent periods, and the consequent change from shallow to deep water, 

 the freshwater and littoral character of this inferior group appeared 

 strange and anomalous. After passing through hundreds of feet of 

 London clay, proved by its fossils to have been deposited in deep salt 

 water, we arrive at beds of fluviatile origin, and in the same underly- 

 ing formation masses of shingle, attaining at Blackheath, near London, 

 a thickness of 50 feet, indicate the proximity of land, where the flints 

 . of the chalk were rolled into sand and pebbles, and spread continu- 

 ously over wide spaces. Such shingle always appears at the bottom 

 of the series, whether in the Isle of Wight, or in the Hampshire or 

 London basins. It may be asked why they did not constitute simple 

 narrow littoral zones, such as we might look for on an ancient sea- 

 shore. In reply Mr. Prestwich has suggested that such zones of 

 shingle may have been slowly formed on a large scale at the period 

 of the Thanet sands (C. 3, p. 281), and while the land was sinking 

 the well-rolled pebbles may have been dispersed simultaneously over 

 considerable areas, and exposed during gradual submergence to the 

 action of the waves of the sea, aided occasionally by tidal currents 

 and river floods. 



Thanet sands (C. 3, p. 281). — The mottled or plastic clay of the 

 Isle of Wight and Hampshire is often seen in actual contact with the 

 chalk, constituting in such places the lowest member of the British 

 Eocene series. But at other points another formation of marine 

 origin, characterized by a somewhat different assemblage of organic 

 remains, has been shown by Mr. Prestwich to intervene between the 

 chalk and the Woolwich series. For these beds he has proposed the 

 name of " Thanet Sands," because they are well seen in the Isle of 

 Thanet, in the northern part of Kent, and on the seacoast between 

 Heme Bay and the Reculvers, where they consist of sands with a 

 few concretionary masses of sandstone, and contain among other 

 fossils Pholadomya cuneata, Cyprina Morrisii, Corbula longirostris, 

 Scalaria Bowerbankii, &c. The greatest thickness of these beds is 

 about 90 feet. 



GENERAL TABLE OF FRENCH EOCENE STRATA. 



UPPER EOCENE. 

 French subdivisions. English equivalents. 



A. 1. Gypseous series of Montmartre. 1. Bembridge series, p. 281. 



A. 2. Calcaire silicieux, or Travertin 2. Osborne and Headon series, p. 284. 

 Inferieur. 



