272 CRUSHING FORCE OF TORRENTS. 
beach, their beds are composed of sand and gravel to the 
almost total exclusion of pebbles. 
Guglielmini argued that the gravel and sand of the beds of 
running streams were derived from the trituration of rocks by 
the action of the currents, and inferred that this action was 
generally sudiicient to reduce hard rock to sand in its passage 
from the source to the outlet of rivers. Frisi controverted this 
opinion, and maintained that river-sand was of more ancient 
origin, and he inferred from experiments in artificially grinding 
stones that the concussion, friction, and attrition of rock in 
the channel of running waters were inadequate to its com- 
minution, though he admitted that these same canses might re. 
duce silicious sand to a fine powder capable of transportation 
to the sea by the currents.* Frisi’s experiments were tried 
upon rounded and polished river-pebbles, and prove nothing 
with regard to the action of torrents upon the irregular, more 
or less weathered, and often cracked and shattered rocks which 
lie loose in the ground at the head of mountain valleys. The 
fury of the waters and of the wind which accompanies them 
in the floods of the French Alpine torrents is such, that large 
blocks of stone are hurled out of the bed of the stream to the 
height of twelve or thirteen feet.| The impulse of masses 
driven with such force overthrows the most solid masonry, and 
their concussion cannot fail to be attended with the crushing 
of the rocks themselves. 
The greatest depth of the basin of the Ardeéche is seventy- 
five miles, but most of its tributaries have a much shorter 
course. “These affluents,’ says Mardigny, “hurl into the bed 
of the Ardéche enormous blocks of rock, which this river, in its 
* Frist, Del modo di regolare i Fiumi et Torrenti, pp. 4-19. See in Lom- 
BARDINI, Sulle Inondazioni in Francia, p. 87, notices of the action of cur- 
rents transporting only fine material in wearing down hard rock. In the 
sluices for gold-washing in California having a grade of 1 to 144, and paved 
with the hardest stones, the wear of the bottom is at the rate of two inches 
in three months.—RAYMOND, Minera! Statistics, 1870, p. 480. 
¢{ SuRELL, Ltude sur les Torrents, pp. 31-36. 
