506 
Van Hise—Earth Movements . 
elevation would occur. The process above outlined might con¬ 
tinue until approximate isostatic equilibrium had again been 
reached. We thus have a real cause for a very long continued 
growth. When the hypogene forces causing more rapid subsid¬ 
ence of the sea beds than of the continents, and consequently 
relative upheaval of the continental masses, shall have finally 
exhausted themselves, it cannot be doubted that the epigene 
forces will win, and that the continental masses will be re¬ 
duced to an even platform slightly below the level of the sea. 
Rock Structures Resulting from Earth Movements .—Returning 
now to the corrugations of the earth, I would direct attention 
more closely to the structures and rock alterations resulting 
from mountain-making. 
During these movements the strata are bent into great com¬ 
plex flexures or folds; they are faulted, jointed, and fractured; 
new structures are produced in them; their composition and 
mineral character are changed. 
In the outer part of the surface where the rocks are not suffi¬ 
ciently loaded, numerous joints and faults are formed; very 
numerous parallel fractures and slight displacements may occur 
which break the rock up into thin leaves, these leaves being 
parallel to the bedding or intersecting it; and finally, the rock 
may be broken into irregular fragments, or even into rubble. 
This is the zone of rock fracture. Deeper down, where the load 
is great, even the most rigid rocks are bent like paper or are 
mashed like dough, showing no macroscopic evidence of crevice 
or fracture. It has been calculated that for even the strongest 
rocks this depth can not exceed 40,000 feet. 1 This is the 
zone of rock flowage. It is in this deep-seated zone that true flex¬ 
ure folds are produced. In the zone of fracture, folds of similar 
form may be developed by slight rotations of the small blocks 
between multiple parallel joints. The result is similar to that 
of an arch made of rectangular bricks, there being slight open¬ 
ings between the bricks on the convex side of the arch. 
Folds in rocks may be compared with waves of the sea. Each 
1 Principles of North American pre-Cambrian geology, by C. R. Van 
Hise: Sixteenth Ann. Kept. U. S. Geol. Survey, Pt. I, 1896, p. 592. 
