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, \_ Lower Cretaceous. 2.  Lower Greensand or Neocomian, / 3.  Gault, \ 4.  Upper Greensand, |_ Upper Cretaceous. 5.  Chalk, | 6.  Maestricht beds, /

I. Wealden.—­The Wealden formation, though of considerable importance, is a local group, and is confined to the southeast 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 occasionally 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 inhabiting rivers.  We have, namely, fresh-water Mussels (Unio), 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 reason it has been proposed to substitute for Lower Greensand the name Neocomian, derived from the town of Neufchatel—­anciently called Neocomum—­in Switzerland.  If this name were adopted, as it ought to be, the Wealden beds would be called the Lower Neocomian.