266 CEUSHING FORCE OF TOEREISTTS. 



pebbles, extending several kilometres down tlie stream, but tliey 

 diminisli in size and weiglit so rapidly tliat above the moutli of 

 the river, wliicli is at a distance of thirty or thirty-five kilometrea 

 from the gorge, every trace of calcareous matter has disappeared 

 from the sands of the bottom, which are exclusively sihcious." * 



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

 stones against each other m 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 Yia Mala 

 is cai'ried receives, every year, from its own crumbling walls and 

 from the Hinter Rhein and its wild tributaries, enormous quanti- 

 ties of rock, in blocks and boulders.f In fact, the masses hurled 

 into it in a single flood like those of 1868 would probably fill it 

 up, at its narrow points, to the level of the road 400 feet above its 

 bottom, were not the stones crushed and carried off by the force 

 of the current. Yet below the outlet at Thusis only small rounded 

 boulders, pebbles and gravel, not rock, are found in the bed of 

 the river. The Swiss glaciers bring down thousands of cubic 

 yards of hard rock every season. Where the glacier ends in a plain 

 or wide valley, the rocks are accumulated in a terminal moraine, 

 but in numerous instances the water which pours from the ice- 

 river has force enough to carry down to larger streams the masses 

 delivered by the glacier, and there they, with other stones washed 

 out from the earth by the current, are ground down, so that few 

 of the affluents of the Swiss lakes deliver into them anything but 

 fine sand and slime. 



Great rivers carry no boulders to the sea, and, in fact, receive 

 none from their tributaries. Lombardini found, twenty years 

 ago, that the mineral matter brought down to the Po by its tribu- 

 taries was, in general, comminuted to about the same degree of 

 fineness as the sands of its bed at their points of discharge. In the 

 case of the Trebbia, which rises high in the Apennines and emp- 

 ties into the Po at Piacenza, it was otherwise, that river rolling 



* Avant-prajet pour la creation d'un sol fertile, p. 20. 



f At Riukenberg, on the right bank of the Vorder Rhein, in the flood of 

 1868, a block of stone computed to weigh nearly 9,000 cwt. was carried bodily 

 forwards, not rolled, by a torrent, a distance of three-quarters of a mile.— 

 Ck)Az, die Hochwasser im 1868, p. 54. 



