CHAP. x.J CONTINUITY OF THE CHALK. 471 



some of the animal groups whose remains enter most 

 largely into the chalk hoth 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 e we are still 

 living in the cretaceous epoch, 5 is defensible in a 

 strictly scientific sense, chiefly because the terms 

 ' geological epoch 5 and ' geological period 5 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, 5 and of the 'Miocene 

 period, 5 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 



