488 
Van Hise—Earth Movements, 
portant horizontal movements in the crust of the earth, and ver¬ 
tical movements of the second order of magnitude. We may 
think of the standard illustration of the russet apple, with the 
smooth skin of the autumn. During the winter season there is 
a loss of water by evaporation, and the skin sinks to accommo¬ 
date itself to the lessened bulk of the apple. The chief move¬ 
ments of the skin with reference to the apple are centerward, 
but beside this movement the skin as it sinks becomes wrinkled, 
and parts of it are pushed farther from the center. In this 
same way the greater movements of the crust of the earth are 
centerward. But there are local areas in which the strata of 
the solid rock are elevated, bent, broken, crenulated, or even 
corrugated. Folds, joints, faults, and secondary structures are 
formed. Vulcanism and earthquakes result. The total effect 
is to produce elevated plateaus and mountain ranges. These 
areas correspond to the wrinkles upon the surface of the apple. 
The analogy between the orogenic earth movements and the 
wrinkling of the skin of an apple is only of the most general 
kind. The forces at work in the two cases are different, and 
their effects in one are much more complex than in the other. 
In mountain areas, because of the corrugations and thrust 
fractures, the average thickness of the strata is increased, just 
as in the case of ice ridges upon a lake formed at a time of 
relative warmth and consequent expansion. The mountain 
masses may rise far above the elevation required for isostatic 
equilibrium. The only theoretical limit is the sustaining power 
of the rocks, and so far as gravity determinations have gone, 
this limit is nowhere reached. In order that there shall be 
local elevation, there must be regional depression. Indeed, 
the vertical downward forces resulting in regional depressions 
are partly transformed, first into horizontal thrust, and this 
again into upward thrust, and therefore are believed to be the 
cause of local elevations. As stated above, since the active 
force which caused both the regional depressions and the local 
elevations is gravity, it cannot but be that in a combined epior- 
ogenic and orogenic movement the center of gravity of the mass 
moved shall be nearer the center of the earth after the move¬ 
ment than before, just as in the case of epiorogenic movements 
