CHAP. X.] CRETACEOUS PERIOD. 171 



counties, but the oblique section along the line of the out- 

 crop reveals a still more remarkable lithological change. 

 The Gault in the south of Bedfordshire consists of clay 

 and micaceous marl, and is about 230 feet thick; thence to 

 the north-east it gradually diminishes in thickness, the 

 whole formation at Stoke Ferry in Norfolk being only 

 60 feet thick, and consisting of marly clay. Farther north, 

 at Roydon, it is only 20 feet, and includes layers of chalky 

 limestone and of red marl. At Dersingham there is only 



7 feet of grey, yellow, and red marl, and this appears to 

 pass into the red limestone of Hunstanton. 1 



In Lincolnshire and South Yorkshire there is a similar 

 red rock at the base of the Chalk, but as it is followed 

 through Yorkshire considerable changes take place ; at 

 the north-west extremity of the Wolds it is very thin, but 

 is still an essentially calcareous rock ; to the east and 

 north-east it thickens rapidly, becoming at the same time 

 much more argillaceous, till at Speeton it is 30 feet thick, 

 and Mr. W. Hill informs me that the material of the lower 



8 or 10 feet contains nearly equal quantities of insoluble 

 siliceous matter and carbonate of lime. 



The clays of the Lower G-ault seem to have been de- 

 posited in a shallow sea of 50 to 70 fathoms deep, which is 

 about the depth of the sea between England and Ireland, 

 while the fossils of the Upper Gault indicate a depth of 

 100 fathoms and upward. 2 The Gault and Greensand of 

 the Isle of Wight and the Midland counties was probably 

 all deposited in a sea of less than 100 fathoms, and the 

 depth of the water became less toward the west, where the 

 deposits exhibit abundant evidence of current action and 

 the vicinity of land. In the sands of Haldon Hill Dr. 

 Sorby found that only a proportion of one-tenth of the 



1 See " Quart. Journ. Geol. Soc.," vol. xliii. p. 550 et seq. 



2 See Price on the "Depth of the Gault Sea^ Proc. Geol. Assoc.,"" 

 vol. ir. p. 269. 



