42 PALEOZOIC TIME. [CHAP. IV. 



to this succeeds an immense series of shales, grits, and 

 flags, which are no less than 13,000 feet thick. Of this 

 total only 250 feet belong to the lower or Valentian group, 

 the great mass of the beds (11,000 feet) representing 

 the Salopian shale and limestone group, while the upper 

 2,000 feet represent the red passage beds of the Welsh 

 border. Although no Upper Silurian beds are found in 

 Cumberland north of their Coniston and Windermere out- 

 crop, yet they set in again on the north side of the Solway, 

 and, as the late Mr. J. C. Ward writes, 1 " there is every reason 

 to believe that the whole series once extended over the now 

 exposed volcanic rocks, for there is nothing in the deposits 

 themselves to indicate a land margin near their present 

 outcrop. Such a thickness of beds as that just described 

 implies a continued subsidence of the sea-bed throughout 

 the whole period of deposition." 



The prevalence of sandstones and flagstones, however, 

 shows that the water was never very deep or very far from 

 land, and we are guided to the direction in which some of 

 this land lay by considering the lithological changes which 

 the lowest group exhibits when traced in different direc- 

 tions. In Westmoreland it consists entirely of shale, with 

 a local conglomerate at the base ; to the south-east, near 

 Settle, it is represented by a band of calcareous mud- 

 stone resting on a few inches of grey shale, while to the 

 north and north-west it thickens into a great series of 

 shales and sandstones, which are spread over large areas 

 in the Southern Uplands of Scotland. 



In the Moffat district (Dumfries) the Birkhill and Gala 

 groups must be several thousand feet thick, and consist 

 largely of hard sandy and micaceous flagstones, but without 

 any actual shore-beds or conglomerates. In Ayrshire, 

 however, near G-irvan, they include thick beds of conglome- 

 rate ; the basal bed of this district is a boulder conglome- 



1 Geol. Mag.," 1879, Dec. 2, vol. vi. p. 55. 



