134 PHYSIOGRAPHY. [chap. 



chiefly that of transporting sob'd matter which has been 

 carried into it by rain and other denuding agents. But the 

 river is, itself, a powerful agent of direct denudation — 

 fliaiiatile denudation as it is sometimes termed. It is true 

 that running water, alone, can do but little towards abrading 

 a hard rock : but the pebbles, sand and other detrital matter 

 carried along by the stream, rub against every hard point 

 with wliich they come in contact, and thus enable the river 

 to wear away the hardest rocks in its course, as surely as 

 though they were being ground and scoured with sand- 

 paper. The grinding action of pebbles, when set in motion 

 by water, is strikingly shown in the formation oi potholes. 

 These are roundish cavities, perhaps several feet in depth, 

 not uncommon in the hard bed of a mountain-stream. A 

 few pebbles, lodging in a small cavity, get whirled round and 

 round by the eddies of the stream, until, at length, they 

 excavate deep holes of considerable size. In such cases, the 

 grinding effect of the pebbles is generally assisted by the 

 sand and finer particles in the water, which scour the walls 

 of the hole as effectually as though they were well rubbed 

 with fine sand-paper. 



Aided by its burden of detrital matter, the river frets 

 away the rocks along its banks and thus tends to widen its 

 channel ; while, at the same time, the coarse sediment scratch- 

 ing along the bottom, helps to tear it up and thus deepen 

 the bed of the river. Every stream, with sufficient fall, is 

 in this manner continually at work, gnawing away the rocks 

 through which it flows, so tliat a channel which is, to begin 

 with, narrow and shallow may gradually become widened and 

 deepened. The amount of excavation which can be wrought 

 in a given time, by means of running water, is well seen in 

 volcanic regions, where rivers have cut through sheets of 

 lava which have been poured forth at known dates. 



