Introduction — Principles Involved. 5 



to their main. Let it be supposed, further, that the main stream 

 reaches such a stage of advancement that it has a wide valley- 

 plain at grade. The result is illustrated by Fig. 1, where CD 

 represents a valley plain of degradation, sloping gently toward 

 the master stream. On the upland, A to B, the original gravel 

 formation 1 remains, but its decomposable materials suffer 

 decay by oxidation, carbonation, hydration, etc. Under these 

 circumstances, the plain CD will be strewn with more or less 

 debris derived from the formations 1 and x, but the debris will 

 consist chiefly of the more resistant parts of these formations. 

 Thus if formation 1 contains soft pieces of shale, or pieces of 

 decayed granite, they will not be likely to reappear in the debris 



Fig. 1 



Fig 2 



on CD, unless in close proximity to outcrop of 1. At the same 

 time, the material of the lower plain will contain some detritus 

 from x. The result is that the debris on CD raay be distinctly 

 unlike the material of formation 1. If the slope between B and 



