160 GEOLOGY OF THE ISLE OF WIGHT. 



desiccated, are from 200 to 300 feet in thickness ; 

 and as the lithological character of the beds denotes 

 a quiet and uniform deposit for a considerable 

 time, and the fossils indicate an unaltered con- 

 dition of animal life throughout the same period, 

 a tranquil and gradual subsidence of the sea- 

 bottom must have taken place. The many hun- 

 dred feet of sedimentary strata which were super- 

 imposed on the Bognor series, denote a subsequent 

 subsidence to a great depth, to allow of the accu- 

 mulation of the London clays, and sands. This 

 epoch was followed by a gradual transition to an 

 estuarine and lacustrine condition, as shown by 

 the layers of fluvio-marine and freshwater shells. 

 But the freshwater lake, or estuary, was subjected 

 to occasional irruptions of the sea ; hence the bids 

 of oysters, and interspersion of a few marine shells. 

 A barrier was at length raised to the influx of sea- 

 water, and the sediments became exclusively of a 

 iluviatilc and lacustrine character ; this is proved 

 by the prevalence of freshwater plants, crustaceans, 

 and mollusks ; and the presence of bones of ter- 

 restrial reptiles, and of mammalia.* 



Organic remains. — Alum Bay and Headon 



* This interpretation of the phenomena, is a concise expression of the 

 generalization of Mr. Prestwich, whose admirable Memoir should be carefully 

 studied by those who would enter fully into all the bearings of this infc 

 inquiry. See " Journal of the Gtolog. Sot ii ty," No. 7. 



