THE WORK OF STREAMS 133 



CORRASION 



How streams wear rock. Like clear air, clear water can 

 do little in the way of mechanically wearing firm rocks. Per- 

 haps the most striking illustration of this is afforded at 

 Niagara Falls. Seven thousand tons of essentially clear water 

 rush over the brink of the falls each second, and yet certain 



FIG. 123. The tools of a river. Stream-worn pebbles in the bed of the 

 Potomac River at Barnum, Md. (Md. Geol. Sun.) 



tiny plants grow in the water, clinging to the rocks at the 

 very edge. Were erosion actively in progress at the edge, the 

 plants would, of course, be swept away. The St. Lawrence 

 River leaves Lake Ontario as clear as the lake waters them- 

 selves, and for many miles is unable to corrade effectively, 

 even where its current has great velocity and washes the shores 

 of islands whose banks are of clay. Many other streams 

 which flow from lakes illustrate the same thing. They often 

 have mossy channels in spite of swift currents. Streams, like 

 winds, wear rocks by means of the rock fragments which they 

 transport (Fig. 123). Sand grains, pebbles, etc., that are 

 swept along by the main current, rub, rasp, and strike the bed 

 and sides, breaking and wearing pieces from them. Material 

 B. & B. GEOL. 9 



