CHAP. X.] CRETACEOUS PEEIOD. 187 



the actual shore did not lie very far beyond the present 

 limits of the Vectian sands, but ran in a N.N.E. direction 

 from near the position of Oxford through Northampton, 

 Eutland, and Lincoln. This phase of Cretaceous geography 

 is represented in Plate VIII. 



Upper Cretaceous Time. We have now to chronicle 

 the phases of the great subsidence which commenced at 

 the epoch of the G-ault, and continued throughout the 

 remainder of the Cretaceous period, until a thickness of 

 some 1,300 feet of sediment had been accumulated over 

 the Lower G-reensand. The thickness of the Upper Cretaceous 

 sediments, however, as compared with those of the lower 

 division, is no criterion of the relative duration of the two 

 eras, for the coarser sediments of the earlier portion of the 

 period must have been deposited far more rapidly than the 

 fine materials of the later era. Some idea of the time 

 occupied in the formation of the Chalk may be gained if 

 we remember that the accumulation of the Atlantic ooze 

 the modern analogue of the Chalk is a process so slow, 

 that it is doubtful whether a foot's thickness of it is de- 

 posited in a century ; at this rate 1,000 feet of chalk would 

 require 100,000 years for its formation, and if we assume 

 that chalk was accumulated twice as rapidly as the Atlantic 

 ooze is supposed to be, the figures (50,000 years) still re- 

 present an enormous length of time. 



Considering the depth of Gault clay which overlies the 

 eastern Palaeozoic area, and the manner in which the Gault 

 and Greensand overstep the members of the Jurassic system 

 westward, it is evident that the eastern part of the British 

 region subsided much more rapidly than the western, so 

 that the Jurassic strata were bent down, as it were, beneath 

 the advancing Cretaceous sediments. The waves of the 

 Cretaceous sea cut obliquely across the older strata, form- 

 ing a plane of marine denudation which was carried rapidly 

 westward, and had a gentle slope or inclination eastward. 



