THE CRETACEOUS EPOCH. 
345 
we have no satisfactory evidence to determine 
whetlier the chalk were deposited over tlie entire 
surface of the Wealden (as seems most probable), 
or whether the latter were undergoing elevation 
during the dej)osition of the chalk, and were but 
partially covered by the cretaceous stratii, yet there 
can be no doubt that the chalk originally very much 
exceeded its ])resent limits. It is true that gravel, 
and partially rolled flints, occur but rarely on the ' 
Wealden, the diluvial covering of the latter chiefly 
consisting of its own debris ; yet this fact may have 
resulted from the action of the sea during the ele- 
vation of the strata, or many other causes, and 
cannot be admitted as affording conclusive evidence 
that the Wealden was never wholly covered by the 
chalk. Our limits will not allow us to examine 
this interesting question in all its bearings, which 
will be fully elucidated in the 3d volume of iNIr. 
Ly ell’s “ Principles of Geology,” now in the press ; 
and we proceed to the consideration of the next 
geological era — that in which the older tertiary 
strata were deposited. 
The epochs we have already noticed are marked 
by immense mutations in the relative situation of 
the land and sea; yet these changes appear to 
have been effected in such manner as to have 
occasioned comparatively but little derangement in 
the strata, and to have been succeeded by periods 
the cretaceous strata of Europe, given by iNI. De la Beche, there are, 
of Reptiles, G or 8; Fishes, 10 or 12; Crustacea, 15 ; Mollusca, 225; 
Conchifera, 285; Aniiulata, 110; Racliaria, 90; Vegetable remains, 
20 species, IG of which are marine. 
