the Purbeck and Wealden Deposits of England and France, 279 



named. But this approximation would be very uncertain, for 

 two reasons : one, because the depression was not, as I am about 

 to attempt to show, equal over the two areas; and the other, 

 because the flux and reflux of water through the strait during 

 the formation of the lower greensand of Hampshire would impede 

 the deposit of sediment in the strait itself, and carry it into the 

 more open portions of the basin. 



It is evident that the depression, of which the commencement 

 marks the point when the elevatory movements of the oolitic age 

 terminated and the cretaceous movements of subsidence began, 

 had its focus upon the north-eastern sides of the Anglo-Frankish 

 basin, and that its effects were for a considerable lapse of time 

 confined to that part of the basiu. The lower greensand deposits 

 that overlie the Wealden in the Isle of Wight to the thickness 

 of 800 feet, and in Kent along the border of the palseozoic barrier 

 to a thickness of 400 feet, are, it is well known, either absent in 

 Dorsetshire, or exist in an attenuated form like the bed occur- 

 ring at the top of the Wealden at Punfield in Swanage Bay, 

 referred to by Mr. Godwin-Austen ; so that by the close of the 

 lower cretaceous age the palaeozoic barrier had sunk to the same 

 level as the opposite shore of the basin in Dorsetshire, where 

 the upper cretaceous overlie alike the oolite and the Wealden, 

 being a subsidence of some 1500 feet in excess of the one region 

 over the other during the lower cretaceous epoch. A similar 

 effect is to be observed in the French portion of the basin. M. 

 Triger, in his description of the cretaceous deposits of La Sarthe*, 

 shows that as we advance southwards towards the edge of the 

 basin, the lower beds of cretaceous age disappear, and the creta- 

 ceous beds covering the Jurassic formations become successively 

 newer and newer, — the lower greensand, which at Cape la Heve 

 reposes on the Jurassic, being at Honfleur replaced by thegault, 

 which reposes thereon the Kimmeridge clay; while more south- 

 wards, in the department of La Sarthe itself, the upper greensand 

 or the chalk marl is the deposit which reposes on the oolitic 

 formations; so that the depression which permitted the sea 

 during the cretaceous age to reoccupy the oolitic basin, palpably 

 diminished in amount as the distance from the line of the oolitic 

 barrier increased. 



In Norfolk, according to Mr. Rosef, the beds which repre- 

 sent the chalk marl and lower and middle chalk attain a thick- 

 ness at Norwich of 1150 feet, while, according to the same 

 authority, the lower greensand at its outcrop some 40 miles 

 west from Norwich has a thickness of only from 70 to 90 

 feet. This circumstance appears to show the effect of the flux 



* Bull. vol. xv. p. 543. 



t Proceedings of Geologists' Association, vol. i. p. 227- 



