Ch. XVIII.] WEALDEN FORMATION. 345 



Gault leave 110 doubt that the upper formation lies actually uncon- 

 formable on the lower, and the time occupied by the denudation has 

 been with us uurepreseuted by any stratified formation." * Yet 

 while there is so much difference between the organic remains of the 

 Upper and Lower Cretaceous rocks, the Cretaceous series, palseon- 

 tologically considered, forms an independent whole, having' scarcely 

 any species in common with the Oolitic series which preceded it, or 

 with the Eocene which followed. Thus, by referring to the tables 

 above mentioned, we observe that 521 species are enumerated as 

 known in the Upper Chalk of England, all of which, with the excep- 

 tion of Terebratula caput-serpentis, and a few Foraminifera, had become 

 extinct before the beginning of the Eocene epoch, as represented by 

 the Thanet sands. 



On the other hand, when the lowest marine strata or Atherfielcl 

 beds of the Cretaceous series are compared with the marine forma- 

 tions of the Upper Oolite, we find that no British species pass from one 

 to the other, and we know that this change in the organic world com- 

 cides in date with that enormous lapse of time during which the fresh- 

 water formations of the Wealden and Purbeck, more than 1500 feet in 

 thickness, were deposited. 



WEALDEN FORMATION. 



Beneath the Lower Greensand in the S.E. of England, a freshwater 

 formation is found, called the Wealdon (see Nos. 5 and 6, Map, fig. 

 3 5 5, p. 357), which, although it occupies a small horizontal area in 

 Europe, as compared to the White Chalk and Greensand, is neverthe- 

 less of great geological interest, since the imbedded remains give us 

 some insight into the nature of the terrestrial fauna and flora of the 

 Lower Cretaceous epoch. The name of Wealden was given to this 

 group because it was first studied in parts of Kent, Surrey, and Sus- 

 sex, called the Weald (see Map, p. 357) ; 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, Terebratulse, 

 Echinites, Corals, and other marine fossils, so characteristic of the 

 Cretaceous rocks above, and of the Oolitic strata below, and to the 

 presence in the Weald of Paludinse, Melaniae, and various fluviatile 

 shells, as well as the bones of terrestrial reptiles and the trunks and 

 leaves of land-plants. 



The evidence of so unexpected a fact as the infra-position of a dense 

 mass of purely freshwater origin to a deep-sea deposit (a phenomenon 

 with which we have since become familiar) was received, at first, with 

 no small doubt and incredulity. But the relative position of the beds 



* Ramsay, Anniversary Address, Geol. Quart. Journ., vol. xx. p. 58. 



