470 THE DEPTIIS OF TUE SEA. [chap. x. 



cliffs we have the chalk in its very purest form, and 

 that in various parts of the world it assumes a very 

 different character, and contains carbonate of lime in 

 very different proportions. Mr. Prestwich instances 

 a bed 28 to 30 feet thick of the white chalk (Terrain 

 Senonien) of Touraine, in which carbonate of lime is 

 entirely absent. 



There can be no doubt whatever that we have 

 forming at the bottom of the present ocean, a vast 

 sheet of rock which very closely resembles chalk ; 

 and there can be as little doubt that the old chalk, 

 the cretaceous formation which in some parts of Eng- 

 land has been subjected to enormous denudation, and 

 which is overlaid by the beds of the tertiary series, 

 was produced in the same manner, and under closely 

 similar circumstances ; and not the chalk only, but 

 most probably all the great limestone formations. In 

 almost all of these the remains of foraminifera are 

 abundant, some of them apparently specifically iden- 

 tical with living forms ; and in a large number of 

 limestones of all ages Dr. Gtimbel has detected the 

 characteristic ' coccoliths.' 



Long before commencing the present investigation, 

 certain considerations had led me to regard it as 

 'highly probable that in the deeper parts of the At- 

 lantic a deposit, differing possibly from time to time 

 in composition but always of the same general cha- 

 racter, might have been accumulating continuously 

 from the cretaceous or even earlier periods to the 

 present day. This view I suggested in my first letter 

 to Dr. Carpenter urging the exploration of the sea- 

 bed ; and from the first it has had the cordial support 

 of my colleague, Avhose intimate acquaintance with 



