CHAP. X.] CONTINUITY OF THE CHALK. 491 
the mineral accumulations and the fauna of the 
margin of some sea. We may say that they have been 
deposited in the shallow water of tertiary seas whose 
deep-sea fauna is unknown, and this mode of expres- 
sion is most in accordance with previous ideas; but if 
the view here advocated be correct, we must regard 
the tertiaries as the deposits formed and exposed by 
depressions and upheavals of the borders of the cre- 
taceous sea; of a sea which, with many changes of 
condition produced by the same oscillations which 
alternately exposed and submerged the tertiaries, 
existed continuously, depositing conformable beds of 
chalk-mud from the period of the ancient chalk. 
Mollusca are chiefly shallow-water forms, although 
some of them are special to deep water, and others 
have a great vertical range. As I have already said, 
considering the many changes in the conditions which 
most affect animal life which have occurred during 
later geological times, we cannot expect to find any 
animals of the higher groups specifically identical 
with chalk fossils; the difficulty in the case seems 
rather to be to account for the identity of many 
living deep-water species with species found in the 
Tertiaries. I think, however, that we can find a clue. 
Most of the species common to the modern Atlantic 
and to tertiary beds are now found in the Atlantic at 
much greater depths than those at which they were 
imbedded in the tertiary seas. This we know by the 
species from shallower water which are associated 
with them in the Tertiaries. They are, therefore, 
species which had a considerable vertical range; and 
probably while many of the shallower water forms 
were exterminated by elevations or other change 
