DRA INA GE MODIFI CA TIONS 573 



the (jucstioii, however, we can eliminate the local disturbing 

 conditions and thus determine the true value of surface warpings 

 as stream modifiers. 



We will then assume a land area in which the strata are 

 horizontal and perfectly homogeneous and, for farther simplifica- 

 tion, will su[)pose that its surface has a regular descent from a 

 low, interior water-parting toward the sea on either side. If in 

 such an area the climatic conditions are the same on either side 

 of the dividing ridge, the rate of erosion would be the same and 

 the opposing streams would be held in a delicate balance against 

 each other. The amount of energy expended by one stream in 

 corrading its channel would necessarily be the same as that of 

 its antagonist and the gradient of the streams, in their various 

 portions, would show a close correspondence. 



(^) Effects of elevation. — If then an uplift should occur 

 across one stream a short distance from the divide, it would not 

 materially change the ratio of energy expended by the streams 

 in the corrasion of their channels ; each would be increased by 

 the increased gradient, but the energies would be expended in 

 very different portions of the channels. The stream which is 

 not crossed by the axis of uplift would have its gradient near 

 headwaters increased, and consequently the greater portion of 

 its corrasive energy would be concentrated upon the divide at 

 its head. The stream which is crossed by the axial line would 

 have its gradient near headwaters diminished, while below that 

 line its gradient would be increased, consequently a large 

 amount of its energy would be transferred to that part of its 

 course which is below the axis of uplift, and almost none would 

 be used on the divide against which its antagonist is concentrat- 

 ing all of its force. There can be only one result, and that is the 

 gradual eating away of the divide on the side opposite the 

 uplift, and the consequent migration of this divide toward the 

 axis of elevation. It matters not how slow nor how slight is this 

 uplift, its effect will be the same in character, though differing 

 in amount of modification produced. The principle is funda- 

 mental and must apply in all cases. 



