[224-A] See a paper by the author, Quart. Journ. Geol. Soc., vol. i. p. 55. [225-A] Proceed. Geol. Soc. iv. p. 391. [225-B] See Forbes, Quart. Geol. Journ. vol. i. p. 79. CHAPTER XVIII. WEALDEN GROUP. The Wealden divisible into Weald Clay, Hastings Sand, and Purbeck Beds--Intercalated between two marine formations--Weald clay and Cypris-bearing strata--Iguanodon--Hastings sands--Fossil fish--Strata formed in shallow water--Brackish water-beds--Upper, middle, and lower Purbeck--Alternations of brackish water, freshwater, and land--Dirt-bed, or ancient soil--Distinct species of fossils in each subdivision of the Wealden--Lapse of time implied--Plants and insects of Wealden--Geographical extent of Wealden--Its relation to the cretaceous and oolitic periods--Movements in the earth's crust to which it owed its origin and submergence. Beneath the cretaceous rocks in the S.E. of England, a freshwater formation is found, called the Wealden (see Nos. 5. and 6. Map, p. 242.), which, although it occupies a small horizontal area in Europe, as compared to the chalk, is nevertheless of great geological interest, not only from its position, as being interpolated between two great marine formations (Nos. 7. and 9. Table, p. 103.), but also because the imbedded fossils indicate a grand succession of changes in organic life, effected during its accumulation. It is composed of three minor divisions, the Weald Clay, the Hastings, and the Purbeck Beds, of which the aggregate thickness in some districts may be 700 or 800 feet; but which would be much more considerable (perhaps 2000 feet), were we to add together the extreme thickness acquired by each of them in their fullest development. The common name of Wealden was given to the whole, because it was first studied in parts of Kent, Surrey, and Sussex, called the Weald, (see Map, p. 242.), and we are indebted to Dr. Mantell for having shown in 1822, in his Geology of Sussex, that the whole group was of fluviatile origin. In proof of this he called attention to the entire absence of Ammonites, Belemnites, Terebratulae, Echinites, Corals, and other marine fossils, so characteristic of the cretaceous rocks above, and of the Oolitic strata below, and to the presence of Paludinae, Melaniae, and various fluviatile shells, as well as the bones of terrestrial reptiles and the trunks and leaves of land plants. [Illustration: Fig. 227. Position of the Wealden between two marine formations.]