576 MARIUS R. CAMPBELL 



keeping its channel free at the point D. The actual amount of 

 corrasion accomplished by the two streams is probably not far 

 from the same. The stream A, B, C, is accelerated greatly, but 

 this acceleration is limited to its headwaters where its volume of 

 water is small, hence its power of corrasion is not proportion- 

 ately increased : on the other hand the stream E, D, C, is retarded 

 at its headwaters, but accelerated in its lower course, and since its 

 volume of water is greater at the point of acceleration, its power 

 of corrasion is considerably increased, although its gradient is 

 but slightly changed. This transfer of active corrasion from C 

 to D is what weakens the stream E, D, C, and gives the stream 

 A, B, C, its great advantage ; for nearly all of the cutting of the 

 latter stream is confined to the immediate vicinity of the gap. 

 When the point D is elevated to /)' the profile of the streams 

 will be A, B, F, E, instead of A, B, C, D, E, and the divide will 

 have migrated from C to F. 



This process is continued as the point D is elevated. When 

 it reaches D'^ (Fig- 3) the stream E, i^ will have been so handi- 

 capped by the uplift across its course and the stream A, B,F, so 

 accelerated by the same movement that the divide will have 

 migrated still further toward the axis of uplift and will occupy 

 some such position as F'^. Again the uplift continues and the 

 point D reaches D^, and the divide F'' reaches the position F'^. 



The figures (i to 4) would seem to indicate that the rate of 

 migration is the same for a given amount of elevation whether 

 the divide is near or far from the axis of uplift. Such can 

 hardly be the case, for if the axis crosses the stream at some 

 distance from its source it will be at a point where the gradient 

 of the channel is less than if the axis were near the headwaters 

 of the stream ; consequently the stream which is retarded will 

 suffer most at the first uplift and as a consequence the divide 

 will tend to migrate rapidly. Later uplifts will occur at points 

 where the gradient is steep and consequently they will have but 

 little effect. In the case of the stream which is accelerated, the 

 tendency will be different. At the beginning of the uplift the 

 drainage basin is so far removed from the axis of the uplift that 



