120 GEOLOGY. 



When a stream flows over a rock bed, the wear which it accom- 

 phshes depends chiefly on the character of the rock, the velocity of 

 the stream, and the load it carries. If the rock be stratified and in 

 thin layers, and if these thin layers be broken by numerous joints at 

 high angles to the stratification planes, the impact of the water of a 

 clear stream of even moderate strength may be effective in dislodging 

 bits of the rock. This condition of things is often seen where streams 

 run on beds of shale or slate. If the rock be hard and without bed- 

 ding-planes and joints, or if its layers be thick and its joints few, clear 

 water will be much less effective. If the surface of the rock be rough, 

 the mechanical action of a swift stream of clear water might still pro- 

 duce some effect on it; but if massive hard rock presents a smooth 

 surface to a clear stream, the mechanical effect of even a swift current 

 is slight. 



This general principle is illustrated by the Niagara River. Just 

 above the falls the current is swift. When the river is essentially 

 free from sediment, the surface of the limestone near the bank beneath 

 it is sometimes distinctly green from the presence of the one-celled 

 plants (fresh-water algse) which grow upon it. The whole force of 

 the mighty torrent is not able to sweep them from their moorings. 

 Were the stream supphed with a tithe of the sand which it is capable of 

 carrying, it would not take many hours, and perhaps not many minutes, 

 to remove the last trace of vegetation. This illustration furnishes a 

 clue to the method by which the erosion of solid rock in a stream's 

 bed is effected. 



It has been seen that the ingathering waters which make a stream 

 often have abundant sediment before they reach well-defined stream 

 channels, and that the streams continue to gather sediment where- 

 ever their beds are composed of material which is readily detached. 

 The sediments which the stream carries are the tools with which it 

 works. Without them it is relatively impotent, so far as the abrasion 

 of solid rock is concerned ; with them, it may wear any rock over which 

 it passes (Fig. 102). 



We have next to inquire the methods by which running water uses 

 its tools in the excavation of valleys. When gravel is rolled along 

 in the channel of a stream there is friction between it and the bed 

 over which it moves. If the pebbles be as hard as the bed over which 

 they are rolled their movement must result in its wear, and even if they 

 be softer more or less wear takes place. As the moving stones wear the 



