sTATKMF.NT HY MH M.\f?STI. 'I'M 



■wasted, that is, to abandon them for a certain time to spontaneous 

 vegetation, which was not slow in ranking its appearance.' " 



Mr Marsh next treats thus of two subjects intimately connected, 

 the crushing effects of torrents, and the transporting power of water : 

 — " I must here notice a mechanical eifect of the rapid flow of the 

 torrent, which is of much importance in relation to the desolating 

 action it exercises by covering large tracts of cultivated ground with 

 infertile material. The torrent, as we have seen, shoots or rolls 

 forward, with great velocity, masses and fragments of rock, and 

 sometimes rounded pebbles from more ancient formations. Every 

 inch of this violent movement is accompanied with crushing con- 

 cussion, or, at least, with great abrasion of the mineral material, and, 

 as you follow it along the course of the waters which transport it, 

 you find the stones gradually rounding off in form, and diminishing in 

 size, until they pass successively into gravel, and, in the beds of the 

 rivers to which the torrents convey it, sand, and lastly impalpable 

 slime. 



" There are few operations of nature where the effect seems more 

 disproportioned to the cause than in this crushing and comminution 

 of rock in the channel 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 millstone. 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 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 run- 

 ning streams were derived fi-om the trituration of rocks by the action 

 of the currents, and inferred that this action was generally sufficient 

 to reduce hard rock to sand in its passage from the source to the 

 outlet of rivers. Frisi controverted this opinion, and maintained 



