» ACTION OF TOERENTS. 259 



broken rock wluch overhangs the valley is full of cracks and fis- 

 sures, and really in such a state of disintegration that every frost 

 must bring down tons of it. If he compute the area of the basin 

 which finds here its only discharge, he will perceive that a sudden 

 thaw of the wintei*'s deposit of snow, or one of those terrible dis- 

 charges of rain so common in the Alps, must send forth a deluge 

 mighty enough to sweep down the largest masses of gravel and 

 of rock. The simple measurement of the cubical contents of the 

 semicircular hillock which he climbed before he entered the gorge, 

 the structure and composition of which conclusively show that it 

 must have been washed out of this latter by torrential action, will 

 often account satisfactorily for the disposal of most of the matter 

 which once filled the ravine. 



"WTien a torrent escapes from the lateral confinement of its 

 mountain walls and pours out of the gorge, it spreads and divides 

 itself into numerous smaller streams which shoot out from the 

 mouth of the ravine as from a centre, in different directions, like 

 the ribs of a fan from the pivot, each carrying with it its quota 

 of stones and gravel. The plain below the point of issue fi'om 

 the mountain is rapidly raised by newly-formed torrents, the ele- 

 vation depending on the inclination of the bed and the form and 

 weight of the matter transported. Every flood both increases the 

 height of this central point and extends the entire circumference 

 of the deposit. 



Other things being equal, the transporting power of the water 

 is greatest where its flow is most rapid. This is usually in the 

 direction of the axis of the ravine. The stream retaining most 

 nearly this direction moves with the greatest momentum, and con- 

 sequently transports the solid matter with which it is charged to 

 the greatest distance. 



The untravelled reader will comprehend this the better when 

 he is informed that the southern slope of the Alps generally rises 

 suddenly out of the plain, with no intervening hill to break the 

 abruptness of the transition, except those consisting of compar- 

 atively small heaps of its own debris brought down by ancient 

 glaciers or recent torrents. The torrents do not wind down val- 

 leys gradually widening to the rivers or the sea, but leap at once 

 from the flanks of the mountains upon the plains below. This 

 arrangement of surfaces naturally facihtates the formation of vast 



