DNA INA GE MODIFICA TIONS 663 



region. These streams instead of showing the regularity in 

 direction which would be expected if the cause is the rotation of 

 the earth, exhibit an irregular arrangement which clearly indi- 

 cates that the cause is local and not general. As will be shown 

 later these modifications have a certain definite relation to the 

 great uplift along the Appalachians and seem without much 

 doubt to owe their origin to the tilting which accompanied this 

 uplift. The writer would also suggest that even in the coastal 

 regions there may yet be found evidence of decided elevation, 

 or tilting, of which the visible coastal sediments give no indica- 

 tions. 



We have then two theories to account for this class of facts : 

 terrestrial rotation, and the natural adjustment of the drainage 

 upon a tilted surface caused by crustal movements. It has been 

 demonstrated in the previous part of this paper that the latter 

 theory is entirely adequate to produce the given effect, and also 

 that these earth movements have been of frequent occurrence in 

 the past. On the other hand, the sufficiency of the former 

 hypothesis is a matter of doubt, even in the minds of the eminent 

 scientists who have investigated it. 



When the practical application of the law of the migration of 

 divides is made, the question becomes very complex. Alterna- 

 ting hard and soft beds, even when they are approximately 

 horizontal, tend to modify the result ; but they are infinitely 

 more potent in shaping the lines of drainage, when they are 

 tilted at various angles, or bent into great folds. At first 

 sight it would seem impossible in such regions to detect 

 any change due to slight surface warpings, but careful 

 study reveals the fact that even here the streams are affected by 

 it, showing by their arrangement the position of the axis of 

 movement and something of the relative steepness of the 

 slopes. 



(3) MODIFICATIONS DIFFER ACCORDING TO AGE. 



{a) Recent movc))ients sliozvn by barriers and chajiges of grade. — 

 The most recent uplifts have not yet affected the alignment of 



