CHAP. X.] CRETACEOUS PERIOD. 185 



dependent on the movements of the land as influencing 

 rainfall, erosion, and transportation of material ? 



From the considerations already stated, it is probable 

 that the actual deltas would attain their maximum develop- 

 ment in early Wealden times, while the land was rising or 

 stationary, and it is even possible that at one time a large 

 portion of the lacustrine area may have been silted up, and 

 the whole made very shallow ; but when the land began to 

 subside erosion and transportation would be checked, 

 though the volume of water filling the river-channels 

 might still . be large ; the level of the lake-waters would 

 probably rise, and the surface extent of the deltas would 

 be greatly reduced, if they were not submerged altogether. 

 Toward the close of the lake's existence this seems to have 

 been the case, the shaly clays of the Upper Wealden indi- 

 cating quiet deposition in the still waters of a lake that 

 was being gradually lowered toward the level of the sea. 



As already stated, the deposition of the Wealden Beds 

 was contemporaneous with the formation of the marine 

 Neocomian strata of France and Germany. The Germanic 

 sea was extending itself eastward, and a gulf connected 

 with it lay over the east of Yorkshire and Lincolnshire. 

 At the same time the southern sea was gradually creeping 

 up the valley of the river which ran from the Wealden 

 lake, and the distance between the lake and the sea was 

 gradually lessened, till at length the last barrier was 

 broken through, and the Wealden lake became the Vectian 

 gulf or estuary. The change from the lacustrine shales of 

 the Wealden to the marine clay of the Lower Vectian is a 

 sudden one, and in this respect is like the change from the 

 Triassic marls to the Ehaetic beds ; but the actual condi- 

 tions of the two cases were very different, the area of the 

 Wealden lake was very much smaller, and its conversion 

 into a gulf was not accompanied by the tremendous cliinatal 

 change which took place in the earlier time, when the 



