CHAP. -X. ] CONTINUITY OF THE CHALK. 495 
most cephalopods and all pteropods, heteropods, and 
other surface living animals of high type, even to 
their extinction. By oscillations of 500 feet up or 
down, the great mass of gasteropods, and all reef- 
building corals, would be forced to emigrate, would 
become modified, or would be destroyed,—and another 
hundred fathoms would exterminate the greater num- 
ber of bivalves; while elevations and depressions to 
ten times that amount might only slightly affect the 
region of brachiopods, echinoderms, and sponges. 
After a careful consideration of the results of recent 
investigations, we are strengthened in our confidence 
in the truth of the opinion which we previously held, 
that the various groups of fossils characterizing the 
tertiary beds of Europe and North America represent 
the constantly altering fauna of the shallower por- 
tions of an ocean whose depths are still occupied by 
a deposit which has been accumulating continuously 
from the period of the pre-tertiary chalk, and which 
perpetuates with much modification the pre-tertiary 
chalk fauna. I do not see that this view militates in 
the least against the ‘“‘ reasoning and classification”’ of 
that geology which we have learned from Sir Charles 
Lyell; our dredgings only show that these abysses of 
the ocean—abysses which Sir Charles Lyell admits in 
the passage quoted above, to have outlasted on account 
of their depth a succession of geological epochs—are 
inhabited by a special deep-sea fauna, possibly as persis- 
tent in its general features as the abysses themselves. 
I have said at the beginning of this chapter, that I 
believe the doctrine of the ‘continuity of the chalk,’ 
as understood by those who first suggested it, now 
meets with very general acceptance : and in evidence 
