THE CRETACEOUS PERIOD. 257 
As developed in Britain and Europe generally, the following 
leading subdivisions may be recognised in the Cretaceous 
series :— 
ae | Tower Cretaceous 
Lower Greensand or Neocomian, J TC 
Gault, 
Upper Greensand, 
Chalk, 
. Maestricht beds, 
Upper Cretaceous. 
Ou kw So 
I. Wealden.—The lWealden 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 ( U77zo), 
River-snails (Paludina), and other fresh-water shells, with 
numerous little bivalved Crustaceans, and some fishes. 
Il. Lower Greensand (Neéeocomien 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 
R 
