the Purbeck and Wealden Deposits of England and trance, 277 



which brought into existence deposits, first of brackish, and then 

 of freshwater origin, of great thickness, in the immediate neigh- 

 bourhood of the sea, that do not present any of the characters 

 which mark deltas properly so called. In the basin thus isolated, 

 we may easily conceive that deposits of limited thickness and of 

 brackish-water character would accumulate during that interval 

 in which the isolation was imperfect but progressing, and while 

 the land was becoming stationary. The thickness of the Purbeck 

 of England varies, according to the sections of the Geological Sur- 

 vey, between 180 and 380 feet. That of the Purbeck of the Cha- 

 rente is much inferior, being, according to M. Coquand, from 160 

 to 180 feet, a difference that would tend to show that the com- 

 plete isolation of the latter basin preceded that of the former. In 

 both, however, the amount of sediment is very small compared 

 with that forming the Wealden, and not more than a river of 

 moderate dimensions would furnish to a gradually shoaling basin. 

 It remains now to consider what were the conditions under 

 which, in the English basin, a depression ensued that permitted 

 the continuous accumulation of the far greater amount of sedi- 

 ment during the Wealden epoch, and under purely fluviatile 

 conditions, before the ocean re-entered the basin. In doing so, 

 it will be useful first to consider what is the thickness of the 

 beds in the South of England lying between the Portland sand 

 and the lower beds of the upper cretaceous group. 



The relative thickness of the Wealden beds at different points 

 of their exposure is uncertain. At the western end of the Isle 

 of Purbeck the section exposed is imperfect, through a fault 

 occurring there; at Swanage Bay and in the Isle of Wight the 

 lowest beds are not exposed ; while in the Weald of Kent a 

 similar difficulty exists, until it be determined whether the Ash- 

 burnham beds really belong to it or to the Purbeck. The 

 exposed beds of the Wealden section at Swanage Bay, from a 

 measurement furnished me by Mr. Jenkins of the Geological 

 Society, show a thickness of about 1000 feet; and the unex- 

 posed portion at the same spot would, assuming the same dip to 

 continue, give from 400 to 500 more. At the west of the Isle 

 of Purbeck, the sections of the Geological Survey would 

 appear to show considerably less than this, but the fault between 

 the Wealden and Purbeck renders the thickness uncertain. Mr. 

 Drew gives the entire thickness in Kent and Sussex, exclusive 

 of the Ashburnham beds, at about 1200 feet*. So far as we can 

 * Quart. Journ. Geol. Soc. vol. xvii. p. 271, divided as follows : — 



Weald clay 600 feet. 



Tunbridge Wells sand 160 „ 



Wadhurst clay 160 „ 



Ashdown sand 250 „ 



(Base of Ashdown sand not exposed.) 



Total H70 „ 



