XVII.] GEOLOGY OF THE THAMES BASIN. 297 



that elephants and rhinoceroses, bears and hysenas, roamed 

 through its forests ; and that the hippopotamus wallowed in 

 the streams of what was, in all probability, a river of much 

 larger dimensions than the present Thames. Arguments of 

 similar cogency have led to the conclusion, that the solid 

 floor of the Thames basin, throughout the thirteen hundred 

 feet of thickness which have been directly explored, owes its 

 origin to agents of denudation and reconstruction, such as 

 are at work at the present day; and testifies to the general 

 uniformity of nature, throughout a period which must be 

 counted by hundreds of thousands of years. 



Looking at the record of the past history of the Thames 

 basin, as it now lies before us, it would appear to indicate an 

 uninterrupted progress from marine to terrestrial conditions 

 — as if the bottom of the ancient sea had been gradually 

 upheaved and converted into dry land, after the deposit of 

 the Tertiary strata. But, it must be recollected, that the 

 ordinary stratified deposits accumulate only under water. 

 A dry land surface leaves no indication of its existence, 

 except so far as it may support fluviatile, or lacustrine, 

 deposits ; or be overgrown by a vegetation, thick and strong 

 enough not to be swept away in the next period of sub- 

 mergence. Thus it is possible, and indeed probable, that 

 the ancient rocks which lie beneath the chalk and gault, were 

 upheaved and remained as dry land, for an immense period 

 after their formation, and were submerged, and became part 

 of the floor of the ocean, only at the end of the Secondary 

 period. The chalk may have been a dry land surface for ages 

 before the formation of the London clay estuary ; and the 

 greater part of the London clay itself, with its superjacent 

 Bagshot beds, has probably been dry land, ever since the 

 latter were formed. 



There is no reason to believe that any one of this vast 



