A NOTE ON TEE MIGRATION OF DIVIDES: 
SAN CLEMENTE ISLAND, one of a group lying off the southern 
coast of California, is a typical orogenic block,? formed by fault- 
ing in geologically recent times. Its drainage is still in its 
infancy, and is therefore very simple. A study of the topo- 
graphical features of this isolated mass has led the writer to 
consider the effect on a previously established drainage system, 
of faulting with consequent migration of divides. 
In the case of San Clemente, as the slope toward the line of 
faulting is by far the steeper, erosion on that side is much more 
rapid, and. consequently the main watershed of the unmodified 
crust-block migrates away from that side toward the other. In 
this case then, the movement of the divide is from the line 
along which the elevation takes place—that is, from the line of 
faulting. This differs from an uplift along an axis, without 
faulting, in a topographically simple region of homogeneous 
rocks, as in the latter case the axis of uplift itself forms the 
ultimate divide, while in the former case the resulting divide is 
situated at some distance to one side of the line of faulting. 
Thus the effect on a simple drainage system already established 
will be different for the two kinds of crustal movements, and the 
law for the migration of divides as given by Campbell? must be 
modified in order to make it applicable to an uplift accompanied 
by pronounced faulting. 
This may be illustrated as follows, assuming, as Campbell 
has done, the simplest possible conditions, in order to eliminate 
disturbing elements from the problem: In Figs. 1, 2 and 3, 
C represents the divide between the symmetrical drainage slopes 
«Published by permission of the Director of the United States Geological 
Survey. 
2 Bull. Dept. Geol. Univ. Cal., Vol. I, No. 4, p. 129. 
3 Jour. Geol. Vol. IV, No. 5, July-August 1896, p. 580. 
809 
