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_ (_Néocomien_ 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 Neufchâtel--anciently called _Neocomum_--in Switzerland. If this name were adopted, as it ought to be, the Wealden beds would be called the Lower Neocomian. The Lower Greensand or Neocomian of Britain has a thickness of about 850 feet, and consists of alternations of sands, sandstones, and clays, with occasional calcareous bands. The general colour of the series is dark brown, sometimes red; and the sands are occasionally green, from the presence of silicate of iron.