Atherfield Clay. 213 



Bay, in Yorkshire, we have the actual marine represent- 

 atives of the continental Neocomian strata. These 

 Yorkshire beds were formerly called Speeton Glay, 

 and lie between the uppermost Oolitic strata of the 

 district, called by Mr. Judd, Portlandian, and the Red 

 Chalk or Hunstanton Limestone, which, according to 

 that author, cannot be of later age than the Upper 

 Greensand, and may be as early as the Gault. 1 



The area occupied by the Purbeck and Wealden 

 strata underwent a long period of slow depression, during 

 which these fresh- water strata with occasional marine 

 interstratincations were deposited ; and by sinking still 

 further, the purely marine beds of the Atherfield Clay 

 began to be formed. In fact, but for the presence in it of 

 marine fossils, it is hard to draw any line between the 

 Wealden and the Atherfield Clays, and no doubt the mud 

 that formed the latter was at first carried seaward by the 

 same great river, in the manner, for example, that muddy 

 sediments are now deposited at and near the mouth of the 

 Amazons on the east coast of South America. 



The Atherfield Clay takes its name from Atherfield, 

 on the south-west coast of the Tsle of Wight, where it is 

 well seen overlying the Weald Clay, and is overlaid by 

 the Lower Greensand. Its lowest beds form a kind of 

 passage from the fresh-water strata of the Weald into 

 the overlying marine beds of the Lower Greensand, both 

 in the Isle of Wight and in the Wealden district, round 

 which it circles at the edge of the Lower Greensand ; for 

 at Atherfield there seems to have been a depression of the 

 fresh-water area and an influx of the sea, accompanied 

 by the appearance of Cerithium carbonarium, accom- 

 panied by Pinna and Panopcea standing vertically in 

 the position in which they lived. Many other shells 

 1 ' Journal of the Geological Society,' 1868, vol. xxiv., p. 218. 



