438 ON THE ACTION OF RUNNING WATERS. 



1. Action of Torrents. 



Torrents have a true degrading and scooping action 

 upon the earth's surface, but, by the necessary conse- 

 quence of the sense which we attach to the word, this ac- 

 tion cannot be exercised upon spaces of great extent, for 

 a torrent is a water-course which has a great declivity. 

 Now, on account of the little height which the most ele- 

 vated summits of the globe have in comparison with the 

 extent of its surface, this action cannot be very exten- 

 sive ; it can only, therefore, produce short and narrow 

 ravines. This action, as ail who have visited high moun- 

 tain chains may have seen, is only often local and instan- 

 taneous ; it presents no remarkable effect but upon the 

 heaps of debris which cover the declivities of the moun- 

 tains, and on broken rocks, partially disintegrated by other 

 causes, and lastly on moveable deposits. The results of 

 this action contribute to confine it within narrower limits 

 still, by heaping up at the mouths of torrents in the val- 

 leys or plains, the debris carried down by these torrents. 

 The elevation of the soil, which necessarily follows from 

 the accumulation of these debris, diminishes with the de- 

 clivity, the rapidity, and consequently the power of these 

 water-courses. 



Great masses of water moving rapidly, have a marked 

 transporting power. Striking examples of this power 

 have but too often been seen in Holland, by the break- 

 ing down of the dikes, and in Alpine mountains, in con- 

 sequence of extraordinary rains during tempests, or from 

 the rupture of some of the natural barriers of lakes. In 

 these latter times (in 1818), the Vallee de Bagne expe- 

 rienced the terrible effects of this devastating power. 



