DRA IN A GE MODTFICA 7 'IONS 5 7 1 



rock, will iKiturallv tend to migrate down the slope of the beds, 

 jjroducing a ciiang'e in the alignment of the streams. Davis has 

 fully demonstrated that streams flowing over folded and faulted 

 rocks will first have their positions determined by the synclinal 

 folds of the structural surface, and then they will migrate to the 

 anticlines, or in technical terms will change from consequent to 

 subsequent streams. 



Changes due to these conditions are of great importance in 

 drainage studies, but, since complicated geologic structure is 

 limited to small areas compared with the continental mass, such 

 conditions can prevail only in a prescribed area, and conse- 

 (]uently affect but a limited number of streams. Hence in a gen- 

 eral study of drainage changes, this class does not deserve the 

 prominence that has been attached to it. 



C/dssj. Rearrangements caused by radial cnistal movements. — 

 If the earth's crust remained entirely free from movement, the 

 history of the drainage would be extremely simple, consisting of 

 but a single cycle ; and its barrenness of striking features would 

 only be equaled by the monotonous expanse of baseleveled 

 plain which would be produced during the cycle. The present 

 diversity of surface features is positive evidence that such has 

 not been the case — -that the crust of the earth has suffered 

 repeated oscillations which have prevented the formation of 

 such an extensive baseleveled plain, and at the same time have 

 complicated the drainage history to a remarkable extent. 



Recent studies of the Mesozoic and Cenozoic peneplains of 

 the southern Appalachians seem to demonstrate that this region 

 has suffered two kinds of crustal movements in post-Palaeozoic 

 time. Both of these come under the class of radial movements, 

 but they differ in the intensity of the deformation of the base- 

 level and in their lateral extent. For convenience of study we 

 may divide them into general and local oscillations. As the 

 name implies, the first class embraces those movements of eleva- 

 tion or depression which are of continental or semi-continental 

 extent. Since the amount of movement is slight compared with 

 the horizontal extent, the deformation will be so slight as to be 



