chap, vi.] GEOGRAPHICAL AND GEOLOGICAL CHANGES. 
93 
abundance of ammonites, and other cephalopods, in the chalk, 
is another indication that the water in which they lived was not 
very deep, since Dr. S. P. Woodward thinks that these organ- 
isms were limited to a depth of about thirty fathoms. 
The best example of the modern formation of chalk is 
perhaps to be found on the coasts of sub-tropical North 
America, as described in the following passage : — 
“ The observations of Pourtales show that the steep banks of 
Bahama are covered with soft white lime mud. The lime- 
bottom, which consists almost entirely of Polythalamia, covers 
in greater depths the entire channel of Florida. This formation 
extends without interruption over the whole bed of the Gulf- 
stream in the Gulf of Mexico, and is continued along the 
Atlantic coast of America. The commonest genera met with 
in this deposit are Globigerina, Rotalia cultrata, in large num- 
bers, several Textilariae, Marginulinse, &c. Beside these, small 
free corals, Alcyonidse, Ophiurse, Mollusca, Crustacea, small 
fishes, &c., are found living in these depths. The whole sea- 
bottom appears to be covered with a vast deposit of white 
chalk still in formation.” 1 
There is yet another consideration which seems to have been 
altogether overlooked by those who suppose that a deep and 
open island-studded ocean occupied the place of Europe in Cre- 
taceous times. No fact is more certain than the considerable 
break, indicative of a great lapse of time, intervening between 
the Cretaceous and Tertiary formations. A few deposits of 
intermediate age have indeed been found, but these have been 
generally allocated either with the Chalk or the Eocene, leaving 
the gap almost as pronounced as before. Now, what does this 
gap mean ? It implies that when the deposition of the various 
Cretaceous beds of Europe came to an end they were raised 
above the sea-level and subject to extensive denudation, and 
that for a long but unknown period no extensive portion of 
what is now European land was below the sea-level. It was 
only when this period terminated that large areas in 
several parts of Europe became submerged and received the 
earliest Tertiary deposits known as Eocene. If, therefore, 
1 Geological Magazine, 1871, p. 426. 
