Physical Geography. 135 



The prevalence of corals in the thick masses of 

 Carboniferous Limestone, and sometimes the rapid thin- 

 ning out of these masses in opposite directions, point 

 to the conclusion that they were true coral reefs, of the 

 nature of the Barrier Eeefs of Australia and the Pacific 

 Ocean, and that they thinned away on one side to a 

 feather edge in the direction of the land, and on the 

 other more steeply towards the deep sea. These len- 

 ticular masses were probably formed round outlying 

 islands, large and small, undergoing a process of slow 

 depression, or otherwise on the shores of some old con- 

 tinent, the details of the original shape of which are 

 now lost to our knowledge. One part of this land, 

 however, consisted of that area now known as the moun- 

 tainous parts of Wales, and the adjacent Silurian and 

 Cambrian territory that underlies the Coal-measures of 

 South Staffordshire, Warwickshire, and Leicestershire, 

 Derbyshire, Cumbria, and the South of Scotland, while 

 far north the Grampian mountains and the whole of the 

 North Highlands stood higher above the level of the 

 sea than they do now, for ever since they have suffered 

 from denudation. 



But while in the south, coral reefs of the nature of 

 Barrier Eeefs or Atolls were being formed, in the north 

 the case was different ; for there, as in parts of the 

 modern Pacific, volcanic action was rife, and this is 

 witnessed by the lavas and ashes, intermingled and in- 

 terstratified with the whole of the Carboniferous series 

 in Scotland. This area, together with the north of 

 what is now England, was therefore more or less an 

 area of elevation, accompanied by oscillations of 

 partial depression. Thus it happens that in these 

 regions, the bands of Carboniferous Limestone are quite 

 insignificant when compared with the thick interstrati- 



