Ch. XYIII.] 



WEALDEN" FOEMATION". 



259 



or Ancyloceras^ wliicli has been aptly described as an ammonite more or 

 less uncoiled ; also a furrowed Nautilus^ N. pUcatus (fig. 298), Trigonia 

 caudata^ likewise found in the Blackdown beds (see above, p. 251), and 

 Gervillia, a bivalve genus allied to Avkula. 



Fig. 299. 



FiS. 300. 



Fis. 801. 



Trigonia caudata, Agass. 



Gervillia anceps, Desh. 



Terebratula sella. Sow. 



WEALDEN FORMATION. 



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

 formation is found, called the Wealden (see Nos. 5 and 6, Map, fig. 320, 

 p. 2*71), which, although it occupies a small horizontal area in Europe, 

 as compared to the White Chalk and Greensand, is nevertheless 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 Sussex, called the Weald (see Map, p. 271) ; 

 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, Be- 

 lemnites, Terebratulae, Echinites, Corals, and other marine fossils, so char- 

 acteristic of the cretaceous rocks above, and of the Oolitic strata below, 

 and to the presence in the Weald of Paludinae, Melanige, and various flu- 

 viatile 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 is un- 

 equivocal ;■ the Weald Clay being distinctly seen to pass beneath the Lower 

 Greensand in various parts of Surrey, Kent, and Sussex, and to reappear 

 in the Isle of Wight at the base of the Cretaceous Series, being, no doubt, 

 continuous iav beneath the surface, as indicated by the dotted lines in the 

 annexed diaa:ram, fio;. 302. 



Isle of Wight. 



Fig. 302. 



Hants. 



Sussex. 



a. Cballc I. Greensand. 



c. Weald Clay. d. Hastings Sand. 



e. Purbeck beds. 



