26 THEORY OF THE EARTH. 



the principal use of those hills of debris, is to fur- 

 nish materials for the ravages of torrents. 



Alluvial Formations *. 



The rains which fall, the vapours which are 

 condensed, and the snows which are melted, up- 

 on the ridges and summits of mountains, de- 

 scend, by an infinite number of rills, along their 

 slopes, carrying with them some portions of the 

 materials of which these slopes are composed, and 

 tracing slight furrows by their passage. These 

 rills soon unite in the deeper gutters with which 

 the surface is marked, run off by the deep valleys 

 which intersect their bottom, and thus form streams 

 and rivers, which carry back to the sea the waters 

 it had formerly supplied to the atmosphere. On 

 the melting of the snows, or when a storm takes 

 place, these mountain torrents become suddenly 

 swollen, and rush down the declivities with a ve- 

 locity proportioned to their steepness. They dash 

 violently against the bases of those taluses of 

 fallen fragments which cover the sides of all the 

 high valleys, carrying off the already rounded 

 fragments of which they are composed, and which 

 thus become smoothed, and still farther po- 

 lished, by attrition. But in proportion as 

 they reach the more level valleys, where their 



* Note F. 



