580 MARIUS R. CAMPBELL 



In the cases so far considered we have assumed that the 

 process continues to its completion — that the uplift or depres- 

 sion was of such duration that the streams became perfectly 

 adjusted to their changed conditions, and that the divides in all 

 cases reached the highest point on the tilted surface. This is 

 the ideal condition, but in reality it is probable that the move- 

 ments were seldom of sufficient duration to produce this result. 

 Hence in applying these principles in the field we must expect 

 to find cases where the migration was but partial and the streams 

 continue to head across the former axis of uplift. Also we have, 

 in the foregoing cases, assumed the simplest conditions possible. 

 Nowhere in actual practice will the physiographer have to deal 

 with so simple a case as we have here assumed ; he will find 

 instead of homogeneous rocks a mass of alternating sandstone, 

 shale and limestone which will greatly modify the results, and 

 he will find complex geologic structure instead of the horizontal 

 rocks in the ideal case. While the actual conditions in the 

 field seem so different from those which we have assumed, the 

 determination of the ideal case furnishes us with a law which 

 applies to all cases, but under complex conditions its results 

 are difificult to distinguish from those produced by other forces. 



(4) LAW OF THE MIGRATION OF DIVIDES. 



Whenever local radial movements occur in any region the 

 stream divides in that area will tend to migrate ; the direction 

 in which they move will be determined by the character of the 

 crustal movement ; and the extent of the migration will depend 

 upon the amount of movement and the local obstacles which 

 the streams may encounter. If the movement is upward the 

 divide will tend to migrate toward the axis of uplift; and if the 

 movement continues long enough, and other conditions are 

 favorable, it will reach the axial line and there remain. If 

 the axis coincides with a divide already established it will hold 

 the latter stationary, unless some stronger influence causes it to 

 migrate. 



If the movement is one of subsidence the divide will tend 



