540 HISTORICAL PALEONTOLOGY. 



CHAPTER LIII. 



CRETACEOUS PERIOD. 



ROCKS OF THE PERIOD. 



THE next series of rocks in ascending order is the great and 

 important series of the Cretaceous Rocks, so called from the 

 general occurrence in the system of chalk (Lat. creta, chalk). 

 As developed in Britain and Europe generally, the following 

 leading subdivisions may be recognised in the Cretaceous 

 series : 



1. Wealden, } T ^ 



T f? j XT v / Lower Cretaceous. 



2. Lower Greensand or Neocomian, j 



3. Gault, -I 



4 * r j? P i? Greensand ' 1 Upper Cretaceous. 



5. Chalk, 



6. Maastricht beds, J 



I. The Wealden formation, though of considerable impor- 

 tance, is a local group, and is confined to the south-east of 

 England, France, and some other parts of Europe. Its name 

 is derived from the Weald, a district comprising parts of 

 Surrey, Sussex, and Kent, where it is largely developed. Its 

 lower portion, for a thickness of from 500 to 1000 feet, is 

 arenaceous, and is known as the Hastings Sands. Its Upper 

 portion, for a thickness of 150 to nearly 300 feet, is chiefly 

 argillaceous, consisting of clays with sandy layers, and occa- 

 sionally courses of limestone. The geological importance of the 

 Wealden formation is very great, as it is undoubtedly the 

 delta of an ancient river, being composed almost wholly of 

 fresh-water beds, with a few brackish-water and even marine 

 strata, intercalated in the lower portion. Its geographical 

 extent, though uncertain, owing to the enormous denudation 

 to which it has been subjected, is nevertheless great, since it 

 extends from Dorsetshire to France, and occurs also in North 

 Germany. Still, even if it were continuous between all these 

 points, it would not be larger than the delta of such a modern 

 river as the Ganges. The river which produced the Wealden 

 series must have flowed from an ancient continent occupying 

 what is now the Atlantic Ocean ;' and the time occupied in 

 the formation of the Wealden must have been very great, 

 though we have, of course, no data by which we can accu- 

 rately calculate its duration. 



