264 CEUSHiisrG foece of toeeekts. 



tion of rock in tlie cliannel of swift waters. Igneous rocks are 

 generally so hard as to be wrought with great difficulty, and they 

 bear 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 ser- 

 pentine 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 

 roUing shingle of the 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 gen- 

 erally sufficient to reduce hard rock to sand in its passage from 

 the source to the outlet of rivers. Frisi controverted this opin- 

 ion, 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 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- 

 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 



* Frisi, Del modo di regolare i Fiumi e i 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 14^, and paved 

 with the hardest stones, the wear of the bottom is at the rate of two inches in 

 three months. — Raymond, Mineral Statisticis, 1870, p. 480. 



