CHAP. VI.] CARBONIFEROUS PERIOD. 71 



The only district which presents evidence of differential 

 movement is that of the Old Red Sandstone tract of 

 Shropshire, for though the Upper Old Eed exists there, 

 the Lower Carboniferous beds are absent, and the several 

 stages of the Upper Carboniferous rest unconformably on 

 the Old Red Sandstone and Silurian rocks ; whence we 

 may infer that though this was low ground at the close of 

 Devonian time, it was not submerged beneath the Lower 

 Carboniferous sea, but was undergoing elevation which 

 kept it above water till late in the Carboniferous period. 



In dealing with the evidence derivable from, the litho- 

 logical changes in the Lower Carboniferous rocks, I propose 

 to follow a different method from that adopted in previous 

 chapters. Although there can be no doubt that the greater 

 part of England and Ireland was submerged during the 

 formation of the Carboniferous Limestone, I think there are 

 grounds for believing that a large island of irregular shape 

 existed over the area now covered by St. George's Channel, 

 and that it stretched northward into Scotland, and eastward 

 through the centre of Wales and the midland counties of 

 England. Instead therefore of discussing the conclusions 

 to be deduced from a study of the English, Scotch, and 

 Irish rocks respectively, I propose to adopt a more syn- 

 thetic method, and, taking the existence of this island as a 

 theorem, to state the facts which may be regarded as 

 strong evidence, even if they do not amount to proof of the 

 proposition. 



It will be convenient to start with Shropshire, as the 

 evidence for the existence of land in that county has just 

 been alluded to ; it is corroborated by the rapid northerly 

 thinning of the Carboniferous Limestone series in Gloucester 

 and Honmouth ; near Bristol this series has a total thick- 

 ness of about 2,600 feet, including the lower and upper 

 shales ; near Chepstow it is about 1,500 feet thick, and in 

 the Forest of Dean it is only 840 feet, having lost 1,760 feet 



