312 SUMMARY OF GEOGRAPHICAL EVOLUTION. [CHAP. XIV 



surrounding land was submerged at the time of greatest 

 depression, are points upon which we must be content to 

 confess our ignorance. The greater part of England wa 

 probably submerged, but the submarine ridge which 

 stretched across the central part of the sea still influenced 

 deposition, for only 500 feet of sediment was deposited over 

 it during Upper Jurassic time, while the southern basins 

 received from 1,300 to 2,000 feet, and the northern lasin 

 about 1,000 feet of sediment (in Yorkshire). Possiblj this 

 unequal distribution of material reacted on the initial 

 cause, and the intervening ridge was kept from sinking to 

 the same extent as the basins on either hand Ijy the 

 pressure arising from the weight of the sediment poured 

 into those basins. However this may have been, it is 

 certain that the whole sea-bed was not levelled up when 

 subsidence ceased and upheaval set in, for it was the 

 central ridge which first became land and formed an 

 isthmus separating the northern and southern gulfs of the 

 Portlandian Sea (see Plate VIII.). The southern gulf 

 being the deepest, this remained in the form of a large 

 lake when the rest of the Jurassic sea-bed was converted 

 into the Purbeck and Wealden land. 



The land of early Cretaceous times therefore differed in 

 some important particulars from that of the Trias. The 

 western arm of the Triassic lake and Jurassic sea became 

 a broad and fertile plain which opened south-eastward on to 

 a still wider and probably slightly higher undulating tract 

 of country, the watershed of which stretched across what 

 are now the Midland counties to the eastern tract of Palaeo- 

 zoic rocks, which we may call the eastern uplands. Whether 

 these uplands were originally as lofty as the mountains 

 which lay to the west of the Jurassic sea we have no means 

 of knowing, but it would certainly appear that their com- 

 parative elevation in Cretaceous times was very much less 

 then it had previously been. Either the post-Jurassic up- 



