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 



