CHAP X.] CONTINUITY OF THE CHALK. 47] 
some of the animal groups whose remains enter most 
largely into the chalk both old and new, makes his 
opinion on such a subject particularly valuable. 
On our return from the ‘ Lightning’ cruise, during 
which we believed that our speculation had received 
strong confirmation, we used the expression,—perhaps 
somewhat an unfortunate one since it was capable of 
misconstruction,—that we might be regarded in a 
certain sense as still living in the cretaceous period. 
Several very eminent geologists, among whom were 
Sir Roderick Murchison and Sir Charles Lyell, took 
exception to this statement; but it seems that their 
censure was directed less against the opinion than 
the mode in which it was expressed; and I think I 
may say that the doctrine of the continuity of the 
chalk, in the sense in which we understood it, is now 
very generally accepted. 
I do not maintain that the phrase ‘ we are still 
living in the cretaceous epoch,’ is defensible in a 
strictly scientific sense, chiefly because the terms 
‘geological epoch’ and ‘geological period’ are 
thoroughly indefinite. We speak indifferently of 
the ‘ Silurian period,’ and the ‘ Glacial period,’ with- 
out consideration of their totally unequal value ; 
and of the ‘Tertiary period,’ and of the ‘ Miocene 
period,’ although the one includes the other. The 
expression is intended rather in a popular sense 
to meet what was certainly until very lately the 
general popular impression, that a geological period 
has, in the region where it has been studied and 
defined, something like a beginning and an end; 
that it is bounded by periods of change—elevation, 
denudation, or some other evidence of the lapse of 
