English Formation*. 305 



atmospheric agencies, and also by the action of the 

 waves of the sea of a younger Silurian period, the 

 evidence of which is seen in the conglomerates of the 

 Upper Llandovery beds, which, mingled with marine 

 shells, lie unconformably on the denuded edges of the 

 Cambrian and Lower Silurian strata of the Longmynd 

 in Shropshire, like a consolidated sea beach. Slow 

 submergence then took place beneath the Upper Silurian 

 sea, in which the Upper Silurian rocks were gradually 

 accumulated unconformably till, perhaps, they entirely 

 buried the Lower Silurian strata (2, fig. 57), for in 

 places they attained a thickness of from three to six 

 thousand feet. 



As shown in Chapter VIII. the uppermost Upper 

 Silurian beds of Wales pass insensibly into a newer 

 series, known as the Old Eed Sandstone (3, fig. 57), 

 formed, if we include the entire formation, of beds of 

 red marl, sandstone, and conglomerate, which in all the 

 British areas by the absence of marine shells, and the 

 occasional presence of crocodilians, land reptiles, and of 

 fish (whose nearest allies live in the rivers and lakes of 

 America and Africa, or in the brackish pools of Australia), 

 seem to have been deposited in lakes. In Wales these 

 strata again pass upwards into the Carboniferous Lime- 

 stone, which is overlaid in Wales, Derbyshire, and 

 Lancashire, by the Millstone Grrit and the Coal- 



measures. 



In Yorkshire, Durham, Northumberland, and Scot- 

 land, the Carboniferous Limestone has no pretension to 

 be ranked as a special formation, for it is broken up 

 into a number of bands interstratified with masses of 



1 This is not shown in fig. 57, but the Carboniferous Limestone No. 

 4 is shown in fig. 67, p. 330, lying, as it does in North Wales, uncon- 

 formably on Silurian rocks. 



