Correspondence. 233 



have obliterated all the old features which formerly existed over the 

 areas where the plains are now ; and the subsequently formed rivers 

 would never have regained their channels in those ravines, which 

 would have been left as shallow passes through the hills at a greater 

 or less height above the plains. The plains have many broad open- 

 ings to the sea, without the intervention of any hills, and these 

 would have been the natural outlets of the rivers, if the '•' form of 

 the ground " had been made for them by the sea. 



The reason why the rivers choose to run through the hills by 

 deep ravines, instead of by much easier routes which are now open 

 to them, is that when they began to run these hills did not exist. 

 The hills were then buried, as it were, in much higher ground, by 

 which they were surrounded, and over which the rivers originally 

 ran. The rivers choosing, of course, the lowest ground they could 

 find in then- course to the sea, happened here and there to cross the 

 parts where these hills subsequently became disclosed by the waste 

 and erosion of the rock which surrounded them. The rivers, how- 

 ever, having once cut channels for themselves, have ever since kept 

 those channels open, and it is through those channels that the waste 

 of the interior has been carried off. Although, then, the interior was 

 worn down into a plain, while the hill ground resisted that action 

 and was left standing as a hill, the river channel, through that hill, 

 was always cut lower than any part of the plain, for it was only 

 in consequence of the deepening of that channel that the waste 

 could be carried off and the erosion of the surface of the plain 

 continued. 



In Ireland the rock that was thus wasted in the interior was Car- 

 boniferous Limestone, the ground that stood as a hill was Old 

 Eed Sandstone or some other siliceous rock. 



The calcareous rock was acted on both by mechanical erosion and 

 chemical solution, the siliceous rock only by mechanical erosion. 

 The siliceous rock therefore resisted the atmospheric action far more 

 than the calcareous rock did, but it would not have thus resisted the 

 sea, which would have cut into Old Eed Sandstone just as easily as 

 into Carboniferous Limestone. 



This alone is an argument in favour of atmospheric action, but 

 the great argument is the continued running of the rivers during 

 the denudation. Eivers only run over the land, therefore the denu- 

 dation took place upon the land. 



This conclusion, to which I found myself unconsciously and almost 

 reluctantly brought, acted on me like a sudden revelation. It con- 

 nected together and explained to me all that had been mysterious in 

 the " form of ground " in Wales and England, and other parts of the 

 world, during my observations of the last thirty years, including 

 many of the localities mentioned by Mr. Mackintosh. I saw how it 

 could be applied to the Weald, as my colleagues Professor Eamsay 

 and Dr. Foster and Mr, Topley have since applied it ; and, in fact, 

 that its application was universal. 



There are, doubtless, several difficulties to be got over in many 

 cases. Some of those instanced by Mr. Mackintosh are easily 



