he Purbeck and Wealden Deposits of England and France. 269 



depressions of the bed of the sea of the last preceding epoch, 

 but by the elevation of land reduced to the condition of inland 

 basins, in which beds first of a brackish, and afterwards of purely 

 freshwater character, were accumulated, and that these, by the 

 subsequent depression of the area, were, in common with the 

 intervals of dry land which severed the basin from the sea, over- 

 spread by the marine deposits of the succeeding epoch. I pro- 

 pose in this paper to endeavour to show in detail the applicabi- 

 lity of this theory, with some modifications, to the case of the 

 Wealden proper of the South-east of England. 



Mons. Coquand has* described the fluviatile beds of the two 

 Charentes, which, containing an intermixture of fluviatile and 

 estuarine mollusca, overlie the oolite of Portlandian age, and 

 underlie the upper cretaceous beds in those departments, and, 

 following and recognizing the division established by Prof. E. 

 Forbes, refers them to the age of the Purbeck rather than to 

 that of the Wealden. He also derives these beds from lacustrine 

 conditions rather than from the operation of a delta — conditions 

 which he considers were produced by the elevation of the bed of 

 the Portlandic sea, converting depressions in that bed into closed 

 basins, much in the same manner as that suggested by Mr. 

 Robertson. 



It would also seem, from the description of M. Eomerf, that 

 most of the conditions which mark the Purbeck and Wealden 

 respectively of England are repeated in the Purbeck- Wealden of 

 Hanover. Before entering, however, upon the discussion of the 

 causes w T hich, I conceive, produced the features exhibited by the 

 Purbeck and Wealden deposits of England, there is one other 

 peculiarity which the beds of the Wealden in England exhibit 

 that should be noticed ; and that is the occurrence, at horizons 

 separated from each other by a considerable vertical thickness 

 of deposit, of footprints of air-breathing animals J. These im- 

 pressions, it is obvious, could only have occurred upon surfaces 

 left dry by receding water, and require the assumption of a con- 

 siderable and continuous depression of the bed of the lake in 

 such a manner as to permit of a shallowness of bed (at least 

 in the parts where the footprints occur) sufficient to allow of its 

 being, by a reduction of the river supply, laid dry for an interval, 

 at the same time that no such general depression of level took 

 place as would admit of the sea regaining its place in the basin. 

 It seems clear, from the features exhibited by modern deltas, that 



* Bull, de la Soc. Geol. de France, vol. xv. p.. 577- 



t Proc. Geol. Soc. vol. iii. p. 323. 



X See the papers of Mr. Beckles, Quart. Journ. Geol. Soc. vol. vii. p. 117; 

 vol. viii. p. 396 ; vol. x. p. 456 ; vol. xviii. p. 443 ; of Mr. Tylor, vol. xviii. 

 p. 247. 



