40C| PEOCEEDIJfGS OP THE GEOLOGICAI, SOCIETY. [JunO 18, 



action of the rain running down the slopes of the hills, and dissolving 

 the limestone immediately at the fooo, at a greater rate than where 

 that limestone was merely acted upon by the rain falling directly 

 from the heavens. 



Thus have been produced the present low limestone plains of the 

 centre of Ireland and the longitudinal limestone valleys of the south ; 

 and simultaneously with their production the ravines have been cut 

 through the ridges of other rocks by which the drainage of these 

 plains and valleys has always escaped to the sea. 



PosTSCEiPT. — The ravine of the Avon at Bristol, those of the Wye, 

 and others in South Wales, as also numerous deep erosions in the 

 Palaeozoic rocks of the north of England, many of which were com- 

 plete before the Triassic Period, as shown by the horizontal beds of 

 New Eed Sandstone that lie in them, naturally occur to the mind 

 in connexion with this subject. 



Atmospheric denudation, however, has produced marked eifects, 

 not solely upon Palaeozoic rocks, but, as I believe, upon all forma- 

 tions, in proportion to their age, their composition, and their duration 

 as dry land. 



My acquaintance with the Weald of Kent is too superficial to 

 allow me to express an opinion ; but perhaps I may venture to ask the 

 question whether the Chalk, when once bared by marine denudation, 

 which perhaps removed it entirely from the centre of the district, has 

 not been largely dissolved by atmospheric action, and whether the 

 lateral river-vaUeys that now escape through ravines traversing the 

 ruined walls of Chalk that surround the Weald may not be the ex- 

 pression of the former river-vaUeys that began to run down the 

 slopes of the Chalk from the then dominant ridge that first appeared 

 as dry land during or after the Eocene Period ? If this question be 

 answered in the affirmative, as I suspect it may, I think it reasonable 

 to suppose that the mode of action in the production of river-valleys, 

 which I have here endeavoured to establish, wiU ultimately be found 

 applicable to all river-valleys in aU parts of the woild. Atmospheric 

 denudation or degradation wiU then have to be taken into account as 

 one of the most important geological agencies in the production of the 

 '' form of ground " on all the dry lands of the globe. 



I may also be allowed to ask whether it wiU not turn out to be a 

 general law in all mountain-ranges in the world, that the lateral 

 valleys are the first formed, running directly from the crests of the 

 ranges down the steep slopes of the mountains, while the longitu- 

 dinal valleys are of subsequent origin, gradually produced by atmo^ 

 spheric action on the softer and more easily eroded beds that strike 

 along the chains. 



I may venture perhaps to instance the Upper Phone as an illus- 

 tration. From the central mountain-mass traversed by the Eurca 

 Pass, the Ehone runs in a nearly straight line E.S.E. for about 67 

 miles, to Martigny, in a deep valley between the mountains of the 

 Oberland and those of the main Alpine watershed, of which Mont 

 Blanc and Monte Eosa are the most conspicuous eminenceSc At 



