i;S THE STORY OF THE EARTH. 



area in the distribution of genera on land in the 



Tertiary period, continued as persistently as the 

 migrations of marine life in the Primary period. 



CHAPTER XXII. 



THE CRAG. 



Af n.R the great terrestrial epoch of the newer 

 Miocene period had passed away entirely unrepre- 

 sented by strata, in Great Britain, deposits, named 

 the Crag, are found, which fringe the coast in 

 Norfolk, Suffolk, and Essex, occur in a few places 

 in Kent ; and in Belgium. 



I e relative age of these beds was first deter- 

 mined by the method of counting the number of 

 existing - in each of the tertiary strata. On 



that basis the tertiary epoch had been divided into 

 Eocene, or lower Tertiary, Miocene, or middle 

 Tertiary, and Pliocene, or upper Tertiary. Sub- 



uently the bower Miocene was named 01 

 In the 1 e the fossils include n, 



than rent, of living species. 



In this great period the Crag finds a p 

 The older beds, named Coralline Crag, have 84 



per cent, of the shells still living; and in the 



r Red I per cent, of the shells still 



exi 



1 1 iralline ( »ts unconformably upon 

 the 1 Its l<>wcr part consists of yel- 

 low ed sands, which sometimes form a 



buil about 30 feel thick. This sand 



to have been derived from denudation of 



w hit h on. e extended over the 



