THE CRETACEOUS PERIOD. 257 



As developed in Britain and Europe generally, the following 

 leading subdivisions may be recognised in the Cretaceous 

 series : 



1. Wealden, ) T 



2. Lower Greensand or Neocomian, ' Lower Cetaceous. 



3. Gault, 



4. Upper Greensand, 



5. Chalk, 



6. Maestricht beds, 



Upper Cretaceous. 



I. Wealden. The Wealden formation, though of consider- 

 able importance, 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 accurately 

 calculate its duration. 



The fossils of the Wealden series are, naturally, mostly the 

 remains of such animals as we know at the present day as in- 

 habiting rivers. We have, namely, fresh-water Mussels ( Unto), 

 River-snails (Paludina), and other fresh - water shells, with 

 numerous little bivalved Crustaceans, and some fishes. 



II. Lower Greensand (Neocomien of D'Orbigny). The 

 Wealden beds pass upward, often by insensible gradations, 

 into the Lower Greensand. The name Lower Greensand is 

 not an appropriate one, for green sands only occur sparingly 

 and occasionally, and are found in other formations. For this 



