TOBBENTS OF THE HIGH ALPS. 39 



seems paradoxical facts, which do not appear to be in accordance with the 

 law he thinks he has discovered. A modification of that law may, in some 

 instances, be necessary to enable him to embrace by it all the facts of the 

 case; but there may be other instances in which a more comprehensive view 

 of the matter may show that the apparently paradoxical fact, so far from 

 vitiating, establishes the law. Thus is it here. It is mentioned by M. 

 SureU that there may be named a good many rivers which were navigable 

 formerly, but are no longer so on account of the condition of their lower 

 stream ; this may seem to be inconsistent with the general law which has 

 just been propounded, but the study of the phenomena presented by some 

 torrents supplies a solution of the paradox. 



To cite a case in point, the revers on the left bank of the Durance, from 

 Savines to the river Ubage, is formed, it has been stated, by a succession of 

 beds of dejection belonging to ancient torrents, which became extinct after 

 a time. The whole district was covesed with forests, but these have been 

 cleared away in a great measure, and the torrents resumed their ravages. 



Many rivers have attained to the state of stability, in the same way that 

 many torrents have done so — by the spread of vegetation over the whole 

 area of the grounds, through the midst of which their waters flow. If this 

 vegetation were destroyed by any means, the soil being again left free, the 

 stability would be interrupted, and devagation would be recommenced by the 

 rivers, with effects similar to those connected with the devastations of the 

 torrents. So that the undesirable change which has taken place in the per- 

 manent flow of some rivers may be attributed to the denudation of their basin. 



This explanation, he says, has been frequently given, but without power 

 to adduce direct proof of its correctness. But now the rekindling of extinct 

 torrents by deforesting operations supplies the desiderated demonstration of 

 an analogous fact. It may be considered, in some sorts, a special experi- 

 ment on a small scale under exaggerated conditions, to render the effects 

 more striking and more quickly produced. And thus may we obtain, from 

 what has been termed the study of these torrents, information which may 

 be turned to practical account in dealing with torrential floods in other 

 lands, and in other circumstances. 



The peculiar characteristics of the torrents of the High Alps, consequent 

 on the combination of atmospherical influences on the mineral composition 

 of the mountains, seems at first to place them apart from all other analogous 

 water-courses. But the study of these has revealed the homology which 

 subsists and seems to run through the whole of these, making it appear 

 that in the torrents of the High Alps we have only one excessive develop- 

 ment of what is common to all, — which, having arrested the attention of 

 SureU, has enabled him by this excessive development to study it without 

 difficulty in all its details, and to show in them what may be seen in a 

 degree less manifest, and it may be less developed, but not the less really 

 existent, in all mountain streams, and to show that rivers also are only 

 homologues of these. 



Comparing rivers with torrents, he finds and shows that the law of 

 development of both is the same, marked by the same three stages, posses- 

 sing the same characterestics, attained in the same way, the most stable in 

 their course, having attained this stability after and by means of similar 

 devagations, or changes of channel. And he goes on to say, — " When we 

 consider the wide-stretching valleys in which flow the Khine, the Nile, we 

 Mississippi, and the greater part of the large rivers which diversify the 



