CHAP. VI.] CARBONIFEROUS PERIOD. 79 



limestone is again found. On the east side of the Wrekin 

 the basement beds consist of lava and volcanic ash, which 

 seem to have been the products of subaerial eruptions 

 from volcanoes situated on the border of the land ; on these 

 lies a thin representative of the Carboniferous Limestone 

 comprising 20 to 40 feet of limestone, overlain by about 

 the same thickness of shale and sandstone. The south end 

 of this outcrop is cut off by a fault, beyond which the 

 limestone is absent, and the Millstone Grit conglomerate 

 rests on the Silurian. It would appear, therefore, that the 

 greater part of Shropshire was land with open sea to the 

 north and north-west. 



We have now returned to the county from which we 

 started, and the logical conclusion to be drawn from the 

 facts which have been described is that the whole area 

 inside the localities mentioned was a continuous mass of 

 land (see map, Plate IV.). 



It only remains to give the evidence for the easterly ex- 

 tension of this land, and this was clearly given by Jukes 

 so long ago as 1853 in his " Memoir on the South Stafford- 

 shire Coalfield." He there describes the great uncon- 

 formity between the Coal-measures and the older rocks in 

 South Staffordshire and Warwickshire, these older rocks 

 being Silurian in the one case and Cambrian in the other. 

 The natural inference to be drawn from the facts cannot 

 be stated better than in Jukes' own words : "It is highly 

 probable that all this tract of country, together with much 

 of the adjacent district from Montgomeryshire to Leicester- 

 shire, became dry land after the close of the Silurian 

 period, rising perhaps very slowly, and undergoing a very 

 gradual and long- continued process of degradation as it 

 passed through the destructive plane of the sea-level ; 

 and that it remained above the waters during great part 

 of the period marked by the formation of the (Upper) 

 Old Bed Sandstone and Mountain Limestone, and that 



