CRUSHING FORCE OF TORRENTS. 265 



wliicli accompanies them in the floods of the French Alpine tor 

 rents is such, that large blocks of stone are hurled out of the bed 

 of the stream to the height of twelve or thirteen feet.* In the 

 great flood of 18G8, the torrent at Giornica, in the Canton 

 Ticino, carried rocks weighing many hundi'eds of pounds, and, it 

 is said, even tons, through the windows of houses, and at the 

 same time the Maggia conveyed huge boulders into the streets 

 and squares at Locai'no. 



The impulse of masses driven with such force overthrows the 

 most solid masonry, and their concussion can not fail to be at- 

 tended with the crushing of the rocks themselves. But the 

 crushing power of torrents appears to be chiefly exerted on rocks 

 while they are driven through narrow channels, where of course 

 the compression and friction of the rolling mass are very great. 

 "When they are once thrown out of the bed, or the swift water 

 current, they are subject to httle abrasion. 



The greatest depth of the basin of the Ard5che is seventy-five 

 miles, but most of its tributaries have a much shorter com-se. 

 " These afiiuents," says Mardigny, " hurl into the bed of the 

 Ardeche enormous blocks of rock, which this river, in its turn, 

 bears onwards, and grinds down, at high water, so that its cur- 

 rent rolls only gravel at its confluence with the Rhone." f 



Duponchel makes the following remarkable statement : " The 

 river Herault rises in a granitic region, but soon reaches calca- 

 reous formations, which it traverses for more than sixty kilome- 

 tres, rolling through deep and precipitous ravines, into which the 

 torrents are constantly discharging enormous masses of pebbles 

 belonging to the hardest rocks of the Jurassian period. These 

 debris, continually renewed, compose, even below the exit of the 

 gorge where the river enters into a regular channel cut in a ter- 

 tiary deposit, broad beaches, prodigious accumulations of rolled 



* SuRELL, j^tude sur Us Torrents, pp. 31-86. 



f Memoire sur les Inondations des Rivieres de VArdlche, 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 movement 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 pieces of such limestone to- 

 gether." — Wessely, Die Oesterreichischen Alpenldnder, i., p. 113. 

 12 



