an 
+e 
470 THE DEPTHS OF THE 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. Giimbel 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, whose intimate acquaintance with 
