294 PHYSIOGRAPHY. [chap. 



neighbourhood of Oxford, beds of the oolitic series are 

 exposed, which are so rich in fossil corals that they go by 

 the name of the Coral-rag. These corals resemble those 

 which are now forming reefs, and the coral-rag itself is 

 altogether similar to modern coral-limestone ; so that there 

 can be no reasonable doubt that the coral-rag is the product 

 of the reefs of a sea, which covered this region long before 

 the chalk began to be deposited. 



Putting all these facts together, it becomes clear that 

 the present condition of the basin of the Thames was 

 preceded by one in which the river flowed at a higher level 

 and the climate was much more extreme, if not much colder, 

 than it is at present, during which the quaternary deposits 

 were formed. Antecedent to this, was a period in which 

 the region, at present covered by the London clay, was a 

 great estuary, and the climate was much warmer than at 

 present. This was preceded by the period during which the 

 chalk was deposited, when the greater part, if not the whole, 

 of the Thames basin was far beneath the surface of the sea ; 

 and a similar condition appears to have obtained as far back 

 as positive evidence carries us. 



It further becomes certain, that the whole thickness of the 

 floor of the Thames basin, from the Bagshot sands to the 

 furthest point reached by the borer, is nothing but mud 

 which has been accumulated, by various agencies, at the 

 bottom of the sea, and which has subsequently been up- 

 heaved. Much of this mud represents the denudation of 

 the land surfaces, which were contemporaneous with these 

 deposits ; but still more is the work of animal Hfe. Upheaved 

 into dry land, the rain has worn and excavated its surface, 

 and accumulated into streams, which, gradually cutting 

 their way deeper and deeper, have at length, produced the 



