274 TRANSPORTING POWER OF WATER. 
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 numer- 
ous 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 
afiluents 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. LLombardini found, twenty years 
avo, that the mineral matter brought down to the Po by its 
tributaries was, in general, comminuted to about the same degree 
of fineness as the sands of its bed at their points of discharge. 
Jn the case of the Trebbia, which rises high in the Apennines 
and empties into the Po at Piacenza, it was otherwise, that 
river rolling pebbles and coarse gravel into the channel of the 
principal stream. The banks of the other affluents—excepting 
some of those which discharge their waters into the great lakes— 
then either retained their woods, or had been so long clear of 
them that the torrents had removed most of the disintegrated 
and loose rock in their upper basins. The valley of the Trebbia 
had been recently cleared, and all the forces which tend to the 
degradation and transportation of rock were in full activity.* 
Transporting Power of Water. 
But the geographical effects of the action of torrents are not 
* Since the date of Lombardini’s observations, many Alpine valleys have been 
stripped of their woods. It would be interesting to know whether any sensi- 
ble change has been produced in the character or quantity of the matter trans- 
ported by the rivers to the Po.—WNotice sur les Riviéres de la Lombardie, An- 
nales des Ponts et Chaussées, 1847, ler sémestre, p. 131. 
