chap, x.] CONTINUITY OF THE CHALK. 471 



some of the animal groups whose remains enter most 

 largely into the chalk both old and new, makes his 

 opinion on such a question 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 



