75 



Very few of the high level Gravels contain organic remains, 

 and the general fauna of these Estuarine deposits shews a large 

 percentage of those now living. There is one remarkable shell, 

 the Cyrena (Corbicella) fiuminalis, which is now not met with in 

 European rivers or lakes, but is found in the Nile deposits, 

 and parts of Asia. 



Sir Charles Ltell says : — " It may be contended that when 

 Cyrena [Corbicella) fiuminalis abounded in the Thames, the 

 Hippopotamus may have been suited to the same climate, just as 

 the same moUusk and the living hippopotamus co-exist in the 

 Nile. We may doubtless imagine that dm-ing the countless 

 centuries which may have passed away since the Glacial epoch, 

 there have been oscillations of temperature, in the course of 

 which certain members of a more Southern fauna migrated 

 northwards, and then retreated again when a succession of less 

 genial seasons prevailed ; while other migrations in an opposite 

 direction took place whenever there was a change from a 

 warmer to a colder climate."'* 



Formation of Gravel. — ^' In all rivers," says Mr. Peestwich, 

 ** subject to floods and carrying down much sediment, (as for 

 example the Severn in its lower course,) three forms of sediment 

 will be deposited. 



1st. — Coarse gravel and shingle in the more direct channels 

 through which the waters flow with the greatest velocity. 



2nd. — Sand and fine gravel in those portions of the more direct 

 channel, where the velocity of the stream is checked from any 

 cause ; and 



3rd. — Fine silt and sediment in those parts where the flood- 

 waters out of the direct channel remain for a time in a state 

 of comparative repose ; such places are the lee-side of the 

 hills, lateral valleys and plains, and any local depressions or 

 hollows. None or little would accumulate in the main channel, 

 as the scour of the retiring waters would there prevent its 

 deposition. 



1. — That certain beds of gravel at various levels follow the 



' Antiquity of Man," p. 130. 



