156 NEOZOIC TIME. [CHAP. IX. 



testimony to the shallowness of the Great Oolite waters 

 were needed, the scarcity of Ammonites in the rocks of 

 that group would supply it, only three species being known 

 to occur in the Forest Marble and Cornbrash. The change 

 from Cornbrash to Kellaways is particularly striking in 

 Yorkshire, where the latter contains no fewer than forty- 

 one species of Ammonites, and only one is common to the 

 two deposits ; twenty of these species pass up into the 

 Oxford clay. Mr. Hudleston, who has made such a careful 

 study of the Yorkshire Oolites, 1 remarks : " Probably this 

 great sandbank was deposited during a submergence of 

 this region far more continuous in time and extended in 

 space than those more partial depressions which during 

 the period of the Lower Oolites (Middle Jurassic) had in 

 this region intercalated the spoils of the sea with those of 

 the estuary and the marsh. This more continuous descent 

 seems at length to have removed or lowered barriers which 

 had hitherto kept out the waters of a sea swarming with 

 strange Cephalopoda." 



One of these barriers was undoubtedly that tract or pro- 

 montory of land which occupied the North Sea, and stretched 

 from Belgium into the eastern part of the British area 

 throughout the Liassic and Middle Jurassic periods. The 

 southern part of this ridge began to be submerged during 

 the formation of the Great Oolite, and though the Oxford 

 clay is not found beneath London, it occurs below Chatham 

 in Kent, and its absence further north is doubtless due to 

 the great denudation which took place in Cretaceous times. 

 We may therefore assume that a large part of the eastern 

 land was completely submerged during the formation of 

 the Oxford clay, and that by the close of this epoch a con- 

 tinuous deposit of the clay was spread over the whole of 

 the eastern and midland portion of the British area, and 



1 " Proc. Geol. Assoc.," vols. iv. and v., and " Quart. Journ. Geol. 

 Soc.," vol. xxxiii. p. 272. 



