232 ACTION OF FORESTS ON THK FLOW OF RIVPRS. 



that river-sand was of more ancient origin, and he inferred from 

 experiments in artifically-grinding stones that the concussion, friction, 

 and attrition of rock in the channel of running waters were inadequate 

 to its comminution, though he admitted that these same causes might 

 reduce 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-pebbies, 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 length of the basin of the Ardfeche is seventy-five 

 miles, but most of its tributaries have a much shorter course. ' These 

 afSuents,' says Mardigny, ' hurl into the bed of the Ardfeche enormous 

 blocks of rook, which this river, in its turn, bears onwards, and grinds 

 down, at high water, so that its current rolls only gravel at its con- 

 fluence with the Rhone.' 



" Duponchel makes the following remarkable statement : ' The river 

 Herault rises in a granitic region, but soon reaches calcareous forma- 

 tions, which it traverses for more than sixty kilometres, 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 tertiary deposit, broad beaches 

 prodigious accumulations of rolled pebbles, extending several kilome- 

 tres down the stream, but they diminish in size and weight so rapidly 

 that above the mouth of the river, which is at a distance of thirty or 

 thirty-five kilometres from the gorge, every trace of calcareous matter 

 has disappeared from the sands of the bottom, which are exclusively 

 silicious,' 



" Similar effects of the rapid flow of water and the concussion of 

 stones against each other in river-beds may be observed in almost 

 every Alpine gorge which serves as the channel of a swift stream. 

 The tremendous cleft through which the well-known Via Mala is 

 carried receives, every year, from its own crumbling walls and from 



