DRA IN A GE MODIFICA TIONS 5 7 7 



its effects are somewhat feeble ; but as the divide migrates toward 

 the axial line the acceleration of the stream will become greater 

 and its most effective work will be accomplished when the divide 

 approaches close to the axis of elevation. In balancing the two 

 prot:esses, it seems jsrobable that the former is the controlling 

 element and that the divide migrates more and more slowly as 

 it approaches the axis ; and the duration of the last stage may 

 be manv times that of the first. 



If the uplift continues indefinitely the divide will certainly 

 reach the axis and there it will remain so long as the uplift con- 

 tinues, unless some more potent force causes it to change. 



Under the last assumption the rate of the uplift is, at least, 

 equal to the rate of corrasion. We must now consider the case 

 when it is much less. This probably approaches more nearly 

 the actual condition which has accompanied each movement in 

 this province. Since the rate of elevation is less than that of 

 corrasion, the streams can more than keep pace with the rising 

 fold, and hence their profiles will change only in a nearer 

 approach to baselevel and the migration of the divide toward 

 the axis of uplift. The conditions remain practically the same 

 as in the previous case, except that now the divides will migrate 

 more slowly and will constantly approach baselevel. 



If the axis of uplift corresponds with the original divide 

 (Fig. i), there will be no migration, for each stream will be 

 accelerated equally, and each will concentrate its energies at the 

 same point — the divide C. The position of the divide C will 

 be maintained as long as the uplift continues, unless some 

 external cause exerts a more powerful influence in an opposing 

 direction. 



[h] Effects of depressio7i. — As we have already seen, any pro- 

 nounced tilting of the surface of the earth tends to produce 

 a migration of the divides in the region affected by the tilt, 

 hence the principle of the migration of divides will apply equally 

 well whether the movement be elevation or depression. And 

 while in the great majority of cases of earth movements the 

 effect is to elevate the land, there are well-marked cases of local 



