6 SOILS. 



into the complex ones of modern mountain chains, to the 

 formation of valleys, plains, and basins out of the materials so 

 carried away, its effects are prodigious. The torrents and 

 streams in carrying silt, sand, gravel and bowlders, according 

 to velocity and volume, do not merely displace these materials ; 

 the rock fragments of all sizes not only score and abrade the 

 bed of the rill or stream, but by their mutual attrition produce 

 more or less of fine powder siirilar to that formed by glacier 

 action ; usually more mixed in its ingredients than the former, 

 because derived from a wider range of drainage surface. In 



FIG. 2. Erosion of Hawaiian Hills, near Honolulu. (Phot, by H. C Myers.) 



the glacier stream itself, it is easy to trace the gradual transi- 

 tion from the sharp stone fragments lying in the water as it 

 issues from the terminal ice cave at the lower end of the 

 glacier, to the rounded shingle found a few miles below. 

 On slopes where water flows only during rain or the melting 

 of snow, the same erosive effects may be seen as between the 

 heads of ravines and their outlets. (See figure 2.) It is 

 there too that the surprisingly rapid cutting-out of channels by 

 the aid of water charged with rock fragments or gravel, can 

 readily be observed, and the enormous power of water 

 erosion convincingly shown. In the United States the stu- 

 pendous gorges of the Columbia and Colorado rivers, the 



