36 RESUME OF SUBELl's STUDY OP 



and covered with vegetation over all their height ; and although they may 

 be stripped of trees, they are scarcely furrowed by a few thread-like streams. 

 In the Embrunais, on the contrary, where the forests have disappeared from 

 the sides of the mountain, these never fail to become the prey of the torrents. 



" Such is the hygrometrio action of the climate. There, where the soil is 

 constantly bathed in a humid atmosphere, the summits carpet themselves 

 with verdure, and the torrents have no more aliment. Here, where the 

 air is always dry, vegetation proceeds with more difficulty, and the. storms 

 of rain sweep from the surface the soil to the extent to which vegetation has 

 fixed it there. 



" Thus the moisture of the climate impedes the action of torrents in two 

 ways equally effective ; first, it makes the rain storms more rare and less 

 violent ; secondly, it renders the soil more fixed by covering it with more 

 vigorous vegetation ; it diminishes thus as by one stroke two causes of erosion. 



" If there stiU remain any doubt as to the active part played by the 

 climate in the production of torrents, I would cite a general observation 

 which has been made for a long time in these mountains : — When one 

 traverses the valleys running east and west, or the reverse, he sees that the 

 slopes on the north side are generally wooded, or carpeted with vegetation, 

 whilst those which look towards the south are denuded and arid. He sees, 

 at the same time, that the former are much less cut up with torrents than 

 the latter ; and, the contrast is often such that he sees the one slope horribly 

 disfigured by torrents over against another on which there exists not one, 

 as, for instance, in the valley of Orcieres, in the Valloniae. 



" Now it is evident that such a difference in the whole character of two 

 slopes, which are almost always formed of the same banks of earth, cannot 

 be explained but by the influence of the exposure. And how does the 

 exposure act but by moderating in the slopes directed to the north the effects 

 of the noon-tide sun 1 They protect for a longer time the snow, retain 

 more humidity, are protected from the scorching winds of the south, enjoy 

 all the advantages of shade and coolness, &c. All these effects combine 

 and actually submit these slopes to climatal influences different from those 

 which act on the opposite slopes, although they may both be situated under 

 the same atmosphere." 



Enumerating the geological formations of the High Alps, he shows that 

 the most abundant are comparatively recent formations, many of them so 

 friable that they crumble through exposure to the sun's rays, without the 

 super-added action of either frost or moisture ; that limestones presenting 

 all the appearance of great hardness, and selected on this account for 

 enrochements, were found to be reduced to earth in two yeai's ; that othei-s 

 were not only liable to be disintegrated, but, efflorescing with what seem 

 crystals of alum, lose at once their coherence and their chemical constitu- 

 tion. And the torrents are found to abound in the mountain chains of 

 unstable mineral composition ; they are more rare and less formidable in 

 mountains of more compact constituents ; and in mountains of primitive 

 rock they are altogether absent. 



Nowhere are torrents more fmious or mere numerous than in the valley 



of Embrun, extending over the whole land from Gap and Tallard to the 



village of St Crepin. Throughout the whole of this basin the base of the 



•mountam is of a slaty limestone, manifesting in a high degree the 



character given above. It is in this formation that innumerable ravines 



