324 ELEMENTARY GEOLOGY 
Among mountains there is commonly evidence not 
merely of vertical, but also of horizontal movement. 
In the course of this there must, also, have been a 
resultant crushing of the materials, and sometimes an 
actual flow of the minerals of the rocks. It is as if 
the strata had been pushed by a horizontal stress from 
one side, just as we might crumple paper by pushing 
it against some immovable object. Applying this 
method of mountain formation, it has been possible 
to artificially make actual mountain folds by crumpling 
layers of wax, etc.; and among these folds, many of 
the phenomena of mountains have been very closely 
reproduced (Fig. 194). Evidence of horizontal move- 
ment is found in the presence of overturned folds, and 
of the reverse and overthrust faults (pp. 290-292), in 
some of which the rocks have been moved horizontally 
for thousands of feet. 
While compression and horizontal movement are the 
common features, mountains exist in which the reverse 
appears to be true; and here the uplift seems to have - 
been accompanied by a stretching, so that imstead of 
reverse faults, the slipping has been such as to produce 
normal faults (p. 292). Such a condition is found in 
the Great Basin between the Sierra and the Rockies. 
Contraction Theory. Any theory of mountains 
which is universally accepted, must explain all these 
phenomena; and it is because many believe that no 
