2 78 Mr. S. V. Wood on the Events which produced and terminated 



judge, therefore, the thickness seems to vary from about 1200 

 feet at its eastern edge, to nearly 1500 at its more central por- 

 tion. We should expect to find the greatest thickness towards 

 the centre of the basin, where the most sediment would be swept 

 by the current ; and in the case of the overlying marine deposit 

 of the lower greensand, that is conspicuously the case. The 

 sections of Mr. Simms* show the lower greensand at Hythe as 

 having a thickness of 405 feet; and those of Dr. Fittonf at 

 Atherfield give 805 feet, being nearly double that at Hythe. 

 Atherfield is placed about the central part of the gulf, while 

 Hythe is, as I conceive, close to the palaeozoic barrier that, until 

 the upper cretaceous epoch, shut in the basin J. 



The sum of the thickness of the deposits, from the base of the 

 Wealden to the base of the upper cretaceous (which are absent 

 at Harwich), would amount to about 2000 feet; and in the Pas 

 de Calais and Departement du Nord it would be more than 

 this. We thus see that the shore of the Wealden lake was, 

 after the close of the Purbeck epoch, skirted by a range of hills 

 attaining a height of about 2000 feet above the bottom of the 

 lake ; and if, as we are compelled to do by the succession of 

 footprint horizons, we consider the lake as shallow enough, 

 during a considerable part of its existence, to be laid dry along 

 its Sussex margin at intervals, then the greater part of that 

 elevation would at the time referred to be above the surface of 

 the lake. To what height above the sea the elevation of the 

 ground in the region of the north-east opening extended, which, 

 shutting out the sea, converted the basin into a lake, we have no 

 means of forming any approximation. If the depression pro- 

 ceeded at an equal rate in the region of the north-east opening, 

 that is, in Buckinghamshire and Bedfordshire, with what it did 

 over the rest of the palaeozoic barrier, we might, by comparing the 

 thickness of any lower greensand beds present in those counties 

 with that of the similar deposit in Kent and Surrey, estimate 

 the elevation above the sea, of the land closing the north-east 

 opening, by deducting the thickness of the lower greensand in 

 the two latter counties from the thickness in the two first- 



* Proc. Geol. Soc. vol. iv. p. 206. 



t Quart. Journ. Geol. Soc. vol. iv. p. 327. Forbes and Ibbetson, ibid, 

 p. 407, give 843.feet. 



X A somewhat similar proportion between the northern and more cen- 

 tral parts of the basin is exhibited in the relative thicknesses of the lower 

 greensand in the Haute Marne and in the Aube — that in the former being 

 about 280 feet, and that in the latter about 4/0. See Cornuel, in Mem. 

 Ge'ol. Soc. France, vol. iv. ; and Leymerie, in the same. Both the Haute 

 Marne and the Aube, being near the south-east shore of the basin, show r a 

 great attenuation of deposit from that in the more central portions of the 

 basin lying in the South of England. 



