332 Correspondence. \ 



First let me acknowledge my own debt of g-ratitude to Mr. Poulett 

 Scrope, whose clear exposition, more than thirty years ago, of the 

 purely subaerial origin of the valleys of Central France has had, 

 perhaps, even more influence on my mind than I was altogether 

 conscious of, in leading me to correct views of the amount of the 

 action of atmospheric forces. I used, indeed, to suppose that this 

 was an exceptional case, or one applicable only to soft Tertiary rocts 

 and thin lava streams, till my investigations into the origin of the 

 river valleys of the South of Ireland showed me that it was only a 

 normal example of a universal rule. 



The subject of the production of the " external form of the ground" 

 is one which has been so little discussed, that it is difficult to write 

 on it without being misunderstood. 



By the "form of ground" I would mean, not its altitude above or 

 below the level of the sea, still less the position, (horizontal, inclined, 

 bent or broken) of the rocks composing it, — -but simply the form of 

 its external surface : and I think Mr. Poulett Scrope will agree with 

 me that internal forces have never any direct effect upon that, except 

 to a slight extent and only for a brief period of time. 



Even a volcanic cone could not stand ten years without having its 

 sides more or less washed or gullied by rains, abraded by winds, or 

 modified in some way and to some extent, however slight. 



The direct effects of earthquakes in cracking, or bending the surface, 

 are surely very insignificant, and the features thus produced are 

 externally modified almost as soon as they are made. 



Great elevation and depression of land might occur, and apparently 

 has occurred, without any alteration of '•' the form of the ground," 

 though of course some change of slope about the boundaries of areas 

 thus acted on must be produced even at the surface. 



I hope Mr. Poulett Scrope will pardon me if I say that I do not 

 know whether "the basin of Switzerland" be "a synclinal valley 

 between the elevated ridges of the Alps and Jura" or not. Still 

 more ignorant am I of the Italian side of the Alps, a region I can 

 never hope to visit till the Geological Survey of Ireland is finished, 

 and long before that time I shall probably myself be passing into 

 the inorganic condition. 



The hills in the central valley of Switzerland are, I believe, " hills 

 of circumdenudation : " that is, are hills solely because of the removal 

 of the matter which once surrounded them. 



The hills of the Jura and the Alps are, doubtless, " hills of uptilt- 

 ing, " that is, the rocks composing them are at that altitude and in 

 that position in consequence of having been thrust up by forces 

 acting from the interior. But the rocks which we now see at the 

 surface were not at, or near, the surface at the time they were thus 

 thrust up. 



The present surface cuts across the edges of beds having an aggre- 

 gate thickness of many thousand feet. If that surface was formed 

 before the rocks were disturbed and while they were still horizontal, 

 there must have been an excavation in order to form it, sufficient to 

 show a section of that depth, and the subsequent elevation must have 

 been confined to that previous hollow. 



