DRA IN A GE MODIFI CA TIONS 665 



that the changes induced in the drainage systems are inclined to 

 be jicriodic in their occurrence. This is extremely important, 

 since it enables us to locate in time many of the changes which 

 otherwise we would be unable to fix definitely. 



Streams are so susceptible to prevailing conditions that they 

 do not always respond to the tilting of the surface. There are 

 times when a tilt produces but little effect ; then again a slight 

 movement will produce the most profound modifications. If 

 the tilting occurs while the streams are in their youth, it will 

 have but little effect upon them unless it is excessive. In that 

 period of its existence the stream is active, it is cutting its chan- 

 nel vertically, and it is well entrenched in its position, hence a 

 slight tilt of the surface will produce no appreciable effect. If 

 the streams are in their old age, the surface of the land will con- 

 stitute a peneplain, and if in extreme old age, this peneplain will 

 approach ver_y closely to baselevel. At such times the drainage 

 basins are delicately balanced against each other; not alone are 

 the systems so balanced, but each individual stream is pitted 

 against its neighbors in a balance so delicate that the least out- 

 side inflluence may turn the scale, and the favored stream conquer 

 the ground now occupied by its neighbors. It is at such times 

 that crustal movements are accompanied by the most profound 

 results ; consequently we find that a large majority of the changes 

 in the alignment of the drainage systems of the Appalachian 

 region have occurred after a period of extensive baseleveling ; 

 they were caused by the first movement which terminated the 

 cjuiet of the long period of uninterrupted erosion. 



(5) CRITERIA FOR DETERMINING THE COINCIDENCE OF LINES OF 

 UPLIFT WITH PREEXISTING DIVIDES. 



Since, under favorable conditions, the final result of all long- 

 continued local uplifts has been the migration of the divides to 

 the axial line of the uplift, it would seem difficult, if not impos- 

 sible, to establish criteria by which to separate the uplifts which 

 originally coincided with divides, from those which were differ- 

 ently located. In a measure this is true, but there are certain 



