s of their channels, but that they 

 are the much diminished representatives of glaciers acting 

 along certain channels, which latter have been excavated 

 by the former and larger glaciers. A stream abrades not 



its load upon the channel structures. For corrasion, the 

 strength of the stream needs not to exceed the ultimate 

 crashing strength of the rock, although this phase is an 

 important one.' What the stream does need, however, for 

 purposes of quarrying and abrasion, is ability to accomplish 

 one or all of the following processes (1) to lift out and trans- 

 port large rock masses from their resting places, (2) to 

 break rock masses across the joint faces (3) to overcome 

 the coherence of the cementing material, (4) to sandpnper 

 the rock face itself. 



Action on lee seites and ft toss suites of obstacles. — 

 Consider the action of a stream [of ice or water) on a 

 thalweg such as O O' O" (Fig. 8). A represents a local 

 reversal of grade, and Band B' represent basins at the feet 

 of declivities or interruptions of the channel base. Now, 

 because the streams are due to the force of gravity, the 

 passage of the point A by the stream results in a net loss, 

 while the passage of the face O' B' results in a net gain to 

 the stream power. It is evident also that as the stream 

 descends the slope O' B' its velocity increases, and hence 

 its kinetic energy rises in a high geometrical ratio. Never- 

 theless because of this rapidly increasing velocity the pro- 

 portion of the kin. -tic energy expend..] ;is corrasion rapidly 

 diminishes. This is evident, at once from a consid.rat ion 



