DIASTROPHISM AND THE FORMATIVE PROCESSES 583 
developed beneath the shell when it has suffered orogenic move- 
ments and that this facilitated folding and at the same time tended 
to limit it below. The shear is probably distributive through some 
depth and covers the horizons at which the tendency to creep is 
chiefly felt. The shear planes thus formed by the forceful orogenic 
agencies may serve as planes of movement for the less forceful 
creep afterward. The occasion for creep springs in some part from 
the elevation involved in the folding and allied diastrophic processes, 
and the creep itself is of the nature of a reversal of the elevating 
process, and so it may not unnaturally be facilitated by the shear 
planes already developed by the antecedent diastrophism. 
Complicated by these modifying factors whose values are uncer- 
tain, it does not appear that the problem of creep is at present 
susceptible of satisfactory computative treatment; it can be dealt 
with now only in a naturalistic way on the basis of the evidence and 
the probabilities of the case. In time, when suitable geodetic 
surveys shall have been repeated, there will come positive demon- 
stration. The creep movement, if it is appreciable, must result 
in a spreading of the geodetic stations, and exact measurements 
after sufficient intervals will give unequivocal evidence of the move- 
ment or of its absence, as also of its nature and its rate. Some few 
first steps in this direction have already been taken; notably the 
geodetic re-survey of the region of the recent California earthquake. 
Horizontal strain followed by horizontal movement were there 
shown, but they were too limited and their interpretation too uncer- 
tain to contribute much to the general question of creep. The 
demonstration that the movement was horizontal is, so far forth, 
favorable. The recognition that this and certain other earth- 
quakes were due to differential horizontal movements—in dis- 
tinction from the vertical movements to which earthquakes have 
generally been referred—is perhaps a step toward the demonstration 
of a wider system of horizontal movements of secular prevalence. 
It would be going too far from our immediate purpose to set 
forth in full the phenomena that seem to point to glacier-like con- 
tinental creep as one of the prevailing movements of the earth’s 
crust. We are here merely seeking its possible effects on sea- 
transgressions and the stratigraphic terranes dependent on these. 
