124 E. C. ANDREWS. 



rock masses from their unstable moorings on declines. It 

 is also evident that a stream must so work as to be enabled 

 to handle its load in the easiest manner, so as to minimise 

 friction and nevertheless so as to take the line of quickest 

 descent. To handle its rock load as a whole, the stream 

 volume must greatly exceed that of its load. The stones 

 by their superior weight will also occupy the more basal 

 portions of the streams. The individual pebbles and other 

 classes of channel debris will all attempt to take the line 

 of quickest descent, but from considerations of friction and 

 transportation the channel can not be V-shaped. On the 

 other hand the stream is forced to concentrate its volume 

 as much as possible, so as also to minimise friction. Inas- 

 much then as the stream load is small as compared with 

 the stream volume, as the stream channel is designed so 

 as to minimise friction, and as the streams take the lines 

 of quickest descent, so the channel floor, in homogeneous 

 structures as a whole, must be flattish nevertheless possess- 

 ing a gentle slope to one deepest point, and its sides must 

 be steep as compared with the floor. The reason for the 

 last condition is plain. The individual boulders and pebbles 

 of the load by minimising of friction tend to roll smoothly 

 over each other, and yet to occupy only a small portion of 

 the stream volume. They thus abrade the rock structures 

 and form a flattish floor and sap the walls of the channel 

 which are also abraded in a minor degree by the sand and 

 smaller pebbles of the higher flood waters. The width and 

 depth of a stream channel are thus functions of the volume, 

 the velocity, and the load of the stream. This applies in 

 the main to all streams alike. Thus if a glacier has exca- 

 vated a valley as its channel, and such channel be examined 

 at a moderate distance from its head then its base may be 

 wide and fairly flat. If now the slowly moving glacier 

 disappear and a much smaller mobile waterstream occupy 

 its valley, then the new stream cannot occupy the whole 



