XVII.] GEOLOGY OF THE THAMES BASIN. 291 



Salisbury Plain ; while, along the south-eastern coast, it 

 forms those dazzling white cliffs which gained for this island 

 the old name of Albion. The chalk, which, beneath London, 

 is 600 or 700 feet thick, is chiefly composed of carbonate of 

 lime, in some parts interspersed with beds of flints, which 

 are nearly pure silex. There cannot be the least doubt 

 that the chalk represents the mud of an ancient sea-bottom, 

 for multitudes of remains of animals have been obtained 

 from it, most of which belong to such groups as are ex- 

 clusively marine at the present day. The area once occu- 

 pied by the chalk was therefore, at one time, covered 

 by sea. And it may be further concluded that it was at 

 some distance from any extensive land, inasmuch as the 

 chalk contains no such mixture of clay and sand, as would 

 be derived from denudation. But there is another reason 

 for believing that the chalk ocean was pretty deep; that 

 is to say, over 100 fathoms. If a slice of chalk is cut, 

 ground thin, and mounted in Canada balsam, so as to 

 become transparent, shells of Forainiiiifera may almost 

 always be detected in it; and, sometimes, they abound. The 

 commonest form is a Globigerina, indistinguishable from 

 that which constitutes the bulk of the Atlantic ooze. 

 Moreover, coccoliths and coccospheres are plentiful in the 

 chalk : which thus differs from the ooze, chiefly, in the 

 greater proportion of granular particles, without any de- 

 finite shape, to recognizable organic remains; and, in the 

 entire absence of those siliceous shells and skeletons, which 

 are so constant in the ooze (Figs. 88 and 89).^ 



The former difference presents no difficulty. The chalk 



^ Fig. 88 represents a section of chalk from Little Hampton, on the 

 coast of Sussex, while Fig. 89, which is placed by its side for compari- 

 son, represents a sample of Atlantic ooze, taken by Captain Dayman 

 from a depth of 2, 250 fathoms. Both figures are magnified to the same 

 extent. 



