Marine Denudation. Escarpments. 499 



only the relic of an average general gentle slope, repre- 

 sented by the straight line (b 6) drawn from the inland 

 heights towards the sea.' l Mr. Jukes applied and ex- x 

 tended the scope of the same kind of reasoning to the 

 south of Ireland, with great success. In various parts 

 of Europe, notably in those regions that have been 

 longest above the water on the banks of the Moselle 

 and of the Rhine, and in the great coalfield west of the 

 Appalachian chain in North America we find unnum- 

 bered valleys intersecting tablelands, of a form that leads 

 us to believe that they also have been made by the 

 long-continued action of atmospheric waste and running 

 waters ; and I believe that the valleys of South Wales 

 have been formed in the same way, and in their origin 

 are even often of latest palaeozoic dates. 



Nothing is more remarkable in the history of rivers 

 than the circumstance that very frequently they run 

 straight through bold escarpments, which at first sight 

 we might suppose ought to have barred the course of 

 the streams. 2 The Wye in South Wales, for example, 

 runs through a bold escarpment of Old Red Sandstone 

 hills ; and the same is the case with the Usk. 



For long it was customary to attribute such breaches 

 in escarpments, and indeed valleys in general, to dis- 

 turbances and fractures of the strata, producing a wide 

 separation, and actually making hills. But when we 

 realise that thousands of feet of strata have often been 

 removed by denudation since the great disturbances of 

 the Welsh strata took place, it becomes clear that the 

 present valleys are in no way immediately connected 

 with them ; for even if there be dislocations or faults 



1 Reports, British Association, p. 66, 1847. 



2 This has already been alluded to in the case of the rivers of the 

 Wealden, pp. 108-119. 



K K 2 



