CHAP. VII.] DYASSIC PERIOD. 113 



rivers that brought down the argillaceous material of the 

 Marl Slates and Middle Marls, derived doubtless from a 

 wide surface of Coal-measures then undergoing destruc- 

 tion, while the streams from the west, being smaller but 

 more rapid, may be credited with the introduction of the 

 Yorkshire " quicksands." 



