392 
CRUSHING FORCE OF TORRENTS. 
over a surface as wide as the flow of the waters, and convert 
the most smiling valleys into scenes of the wildest desolation. 
c. Crushing Force of Torrents. 
There are few operations of nature where the effect seems 
more disproportioned to the cause than in the comminution of 
rock in the channel of swift waters. Igneous rocks are gen¬ 
erally so hard as to he wrought with great difficulty, and they 
hear the weight of enormous superstructures without yielding 
to the pressure; but to the torrent they are as wheat to the mill¬ 
stone. The streams which pour down the southern scarp of the 
Mediterranean Alps along the Riviera di Ponente, near Genoa, 
have short courses, and a brisk walk of a couple of hours or 
even less takes you from the sea beach to the headspring of 
many of them. In their heaviest floods, they bring rounded 
masses of serpentine quite down to the sea, but at ordinary 
high water their lower course is charged only with finely 
divided particles of that rock. Hence, while, near their 
sources, their channels are filled with pebbles and angular 
fragments, intermixed with a little gravel, the proportions are 
reversed near their mouths, and, just above the points where 
their outlets are partially choked by the rolling shingle of the 
beech, their beds are composed of sand and gravel to the 
almost total exclusion of pebbles. The greatest depth of the 
basin of the Ardeche is seventy-five miles, but most of its trib¬ 
utaries have a much shorter course. “ These affluents,” says 
Mardigny, “ hurl into the bed of the Ardeche enormous blocks 
of rock, which this river, in its turn, bears onward, and grinds 
down, at high water, so that its current rolls only gravel at its 
confluence with the Rhone.” * 
* Memoire sur les Inondations des Rivieres de VArdeche, p. 16. “ The 
terrific roar, the thunder of the raging torrents proceeds principally from 
the stones which are rolled along in the bed of the stream. This move¬ 
ment is attended with such powerful attrition that, in the Southern Alps, 
the atmosphere of valleys where the limestone contains bitumen, has, at 
the time of floods, the marked bituminous smell produced by rubbing 
