EFFECT OF RUNNING WATER. 49 



feet it will roll along stones as big as an egg ; while mountain 

 torrents which have dwindled to mere threads, swell in a few 

 hours to such a size that they will carry before them sand, 

 mud, rocks, and trees, will sweep away bridges, and bury 

 meadows ten or fifteen feet deep in rubbish. 



Then, again, a mere dribble of water flowing perpetually 

 over limestone rocks will, in time, produce results such as 

 seem to be quite out of proportion to its size ; and we may, in 

 fact, safely affirm that even the most sluggish stream wears 

 its bed more or less, in one way or another, either chemically 

 or mechanically, 



It is when they are provided with tools of just the right 

 sort, however, that streams and rivers get through most work 

 in the way of carving and grinding. 



From the banks, cliffs, &c., between which they flow, 

 they receive constant supplies of mineral matter, which, 

 having been loosened by alternate frost and thaw, are 

 washed and blown into them by rain and wind. The more 

 rapid the stream, the more quickly these contributions are 

 carried along and the harder blows they give to the sides and 

 bed of the stream, as well as to one another. The larger 

 fragments are rolled along the bottom, deepening the bed, 

 wearing off one another's sharp corners, rounding the angles 

 of any rocks they may meet, undermining the cliffs and 

 pounding the smaller fragments like so many pestles in 

 a mortar, until they are reduced to gravel, sand, or mud, 

 which is then floated farther down the stream. 



It is by the gravel and sand, though they look so much 

 less important than the boulders, that the river's chief work 

 is done ; but much depends upon the inclination of the bed 

 E 



