Drainage Changes near Granville^ Ohio l6l 
survive the ’withdrawal of the ice, at times being confined in val- 
leys between two moraines. 
While the theory of the erosion of divides is quite plausible it 
seems not improbable that this may have been pushed too far. 
The overflowing water would be deprived of nearly all its cutting 
tools and would consequently be dependent almost entirely on cor- 
rosion for dissolving down its spillway. The time necessary for 
such a process would seem greater in certain instances^® than 
could be granted for the favorable position of the ice-front. 
Changes of drainage by glaciation may be ephemeral, lasting 
only through the period when immediately affected by the ice, or 
such changes may be of great permanency, outlasting indefinitely 
the period of glaciation. All degrees of endurance may be found 
between the two extremes. 
3. Diastrophism. This term includes all crustal movements. 
Diastrophism is the most potent and far-reaching of all the causes 
inducing drainage changes. In many examples explained by 
piracy or glaciation it is probably an unseen factor. Slight move- 
ments are difficult to determine, particularly when inland, and for 
this reason have not been accorded their full share of influence. 
M. S. Campbell has formulated the theoretical eiFects of land 
movements occurring under ideal conditions. He also describes 
specific instances of drainage changes and applies these theories 
to them.^® His ‘^Law of the Migration of Divides” is a brief sum- 
mary of the theoretical side of the question. It is as follows: 
‘‘When 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 move- 
ment; 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 
R. D. Salisbury: Loc. cit.^ p. 280. 
18 Harmer: Quarterly Journal of the Geological Society (London), vol. Ixiii, 
pp. 470-514, 1907- 
Journal of Geology, vol. iv, pp. 567-581, 1896. 
Ihtd., pp. 657-678. See also L. G. Westgate, American Geologist, vol. xi, pp. 
245-260, 1893. 
A phase of the migration of divides consequent on faulting by W. S. T. Smith, 
Journal of Geology, vol. v, pp. 809-812, 1897. 
