200 



LYELL'S ELEMENTS OF GEOLOGY. 



Wealden Group. 



CHAPTER XVL 



WEALDEN GROUiP. 



The Wealden, including the Weald clay, Hastings sand, and Purbeck beds — 

 Intercalated between two marine formations — Fossil shells freshwater, with a 

 few marine — Cypris — Fish — Reptiles — Birds — Plants— Section showing passage 

 of Wealden beneath chalk — Junction of Wealden and OoUte — Dirt-bed — Theory 

 of gradual subsidence — Proofs that the Wealden-strata, notwithstanding their 

 thickness, may have been formed in shallow water — Geographical extent of 

 Wealden — Bray near Beauvais — Relation of the Wealden to the Lower Green- 

 sand and Oolite. 



Beneath the cretaceous rocks in the S. E. of England, a 

 freshwater formation is found, called the Wealden, which, al- 

 though it occupies a small area in Europe, as compared to the 

 chalk, is nevertheless of great interest, as being intercalated be- 

 tween two marine formations. It is composed of three minor 

 groups, of which the aggregate thickness in some places cannot 

 be less than 800 feet.* These subdivisions are. 



Thickness. 



1st. Weald clay, sometimes including thin beds of sand and 



shelly hmestone 140 to 280 ft. 



2d. Hastings sand, in which occur some clays and calcareous 



grits ; — between 400 and 500 ft. 



3d. Purbeck beds, consisting of various kinds of limestones and 



marls , about 250 ft. 



To all these subdivisions, the common name of the Wealden 

 * has been given, because they may be best studied in part of 

 Kent, Surrey, and Sussex, called the Weald. 



We have seen that the fossils of the chalk and green-sands 

 which repose upon the Wealden are all marine, and the species 

 numerous ; and the same remark applies to the Portland stone 

 and other members of the Oolitic series which lie immediately 

 beneath (see Fig. 181.). But in the Wealden itself, although 

 the fossils are abundant as to quantity, the number of different 

 species is comparatively small, and by far the greater part of 

 them show that they were deposited in a freshwater lake, or es- 

 tuary communicating with the sea.f 



* Dr. Fitton, Geol. Trans, vol. iv. p. 320. Second Series. t Ibid. p. 104. 



