■-. — .—._-"- 



340 



RESUME AND THEORETICAL DISCUSSION. 





the region of sufficient strength to sweep the material away after it has fallen 

 and been sufficiently ground up by the action of the water. But, again, a 

 sinking of the land will also bring fresh surfaces into contact with the waves, 

 and thus allow the work of erosion to be continuously carried on, even where 

 the abraded material accumulates at the base of the elevation from which the 

 supply has been obtained. In this way, probably, the larger portion of the 

 work of the ocean in levelling off the surface of the land has been performed. 

 The tidal wave has beaten incessantly against an ever-sinking mass of land. 



Again, second, in regions of great precipitation, there, other things be- 

 ing equal, the erosion will be most rapid. This statement is one of which 

 the truth is so evident that it is not necessary to enlarge upon it. It needs 

 only to recall what devastation streams swollen to very much enlarged 

 dimensions have been capable of effecting. 



Third, there are to be taken into account all those varied conditions, with 

 regard to the character of the rocks themselves, by which rapidity of erosion 





is affected. As a general rule, the accumulation of debris is likely to be a 

 somewhat slow process; the sweeping of such material away may, under 

 favorable circumstances, be very rapidly accomplished. Much will depend, in 

 regard to the amount of time required for detrital material to accumulate, 

 on such circumstances as these : the character and position of the joints, 

 cleavage planes, and lines of stratification by which the mass is intersected ; 

 the tendency of the rock to undergo decomposition on exposure to the at- 

 mosphere ; the amount and rapidity of the variations of temperature ; the 

 violence of earthquake shocks, etc. 



And, finally, the original form of the depression in which the water begins 

 to run, as the result of the preceding orographic disturbances, will have a 

 most marked effect on both the character and the rate of the erosion. 



In applying the above considerations more specially to the resulting forms 

 of valley sections, we arrive at the following conclusions. 



As a general rule, large streams run in correspondingly broad valleys, with 

 cross-sections closely approaching straight lines ; the amount of the depres- 

 sion is but trifling compared with the linear extent included within the edges 



4 



of the water-shed. The higher we ascend into the mountains, and conse- 

 quently the smaller the stream, — because nearer its source, — the more the 

 valley acquires a pronounced form, capable of being represented in cross- 

 section. To this, there are, of course, striking exceptions ; but these have 

 usually special orographic causes for their existence. 







