CHAP. X.] CRETACEOUS PERIOD. 173 



which only occur together in the Chloritic Marl and in the 

 sands just beneath it in Dorset. We may conclude, there- 

 fore, that the sea did not reach Ireland till this horizon 

 was reached, and it is possible that the Irish Pecten asper 

 zone is really the time equivalent of part of our Chalk 

 Marl, the fauna of this zone being a shore-fauna which 

 advanced as the shore receded. Consequently it may have 

 prevailed at a western locality after the sediments contain- 

 ing it to the eastward had been covered by a considerable 

 depth of the succeeding Chalk Marl. 



In this connection the Cenomanien deposits of Sarthe in 

 Brittany are instructive, for here the Chalk Marl (Craie de 

 Rouen) passes westward into a sandstone containing some 

 of the same fossils, but mixed with a shallow- water fauna 

 which includes Pecten asper. 1 



From the G-reensand to the Chalk Marl of England the 

 transition is always rapid and sometimes sudden. There 

 is evidence of strong current action at this epoch, conse- 

 quent probably on the submergence of certain barriers, 

 and these currents seem to have swept away portions of 

 the deposits which then formed the sea-bottom, sifting the 

 soft marls and sands, washing out such fossils as were 

 hardened by the deposition of phosphate of lime, and in- 

 corporating them in the basement bed of the new forma- 

 tion. This bed is generally known as the Chloritic Marl, 

 and it frequently lies on a plane of contemporaneous 

 erosion. In the south of England it contains fossils 

 derived from the Pecten asper zone, but in Bedford and 

 Cambridge the derived fossils have been obtained from 

 the G-ault, and the bed is sometimes called the Cambridge 

 G-reensand. 



The Cambridge G-reensand has also yielded a number of 

 rock-fragments which have evidently been brought from 



1 A. Guillier, Geologie de la Sarthe," PaJis, 1886, p. 213. 



