CHAP. XI.] HA.NTONIAN PERIOD. 233 



silting up of the estuary and an increase in the area of the 

 delta, but also of a gradual lowering of the temperature ; 

 till at length, in the time of the Hempstead Beds, the vege- 

 tation ceased to have any specially tropical characters, and 

 the former estuary was converted into a swampy tract 

 covered with a growth of the rushes and water-plants that 

 belong to temperate climates. 1 



With respect to the highest member of the Hempstead 

 group, Mr. Gardner remarks that the assemblage of marine 

 inollusca which it contains presents such a paucity of species, 

 and these so stunted in growth, as to suggest that the sea 

 which they inhabited had contracted to the dimensions of 

 a mere salt or brackish-water lake, without any communi- 

 cation with the open sea then lying over Belgium ; at the 

 same time, it is clear that at this period the depth of the 

 water in the Isle of Wight area was increased, not 

 diminished, and this must have been due either to sub- 

 sidence or to an influx of river-water into the lake. 



When we consider that the whole of the British Oligo- 

 cene series consists of shallow- water deposits, it is evident 

 that a certain amount of subsidence must have taken place 

 during the period in order to allow of the accumulation of 

 such a thickness of beds (600 feet). That such subsidence 

 took place is proved also by the Belgian succession, and by 

 the comparatively deep-water character of the Argile de 

 Boom, which is generally regarded as the equivalent of 

 our Hempstead Beds. It is probable, however, that this 

 subsidence was more or less localized to the basins of de- 

 position, and it may have been contemporaneous with and 

 complementary to a rise of the Wealden axis. 



The records of the Hampshire delta are here abruptly 

 broken off, and we have no means of ascertaining the last 

 phases of its history ; we can only guess from the analogy 



1 " Proc. Geol. Assoc.," vol. vi. p. 98. 



