
FAULTS 175 
denudation of the country these thrust-planes have now and again given 
rise to marked surface-features. As lines of weakness, for example, they 
are sometimes followed by streams—a good instance being shown in 
Plate XLII. 
Transverse thrusts are obviously due to the same cause as overthrusts 
—tangential pressure,—and may be looked upon as contemporaneous 
with the folds and overthrusts of the region in which they occur. When 
the strata of a growing mountain chain were being compressed and 
pushed forward in some particular direction, there might well be in- 
equalities in the crustal creep. Some portions of the moving mass would 
advance more rapidly than others, and thus cause strain and tension, to 
which the crust would necessarily yield in the direction of the horizontal 
movement. The vertical fissures and fractures or glide-planes thus formed 
would, therefore, traverse the dominant folds and overthrusts of the chain 
at right angles. 
The general conclusion, then, to which the evidence leads is simply 
this, that faults are usually connected with folds, or, at all events, with 
horizontal movements of the crust. When strata are sufficiently com- 
pressed, they usually double up, and with continued pressure eventually 
yield, and overthrusts take place. In some cases, however, in place of 
becoming folded, they at once shear, and approximately horizontal or 
steeper overthrusts come into existence. Again, yielding takes place, 
and transcurrent faults make their appearance between contiguous 
masses which are moving horizontally at different rates in the same 
direction. 
The overthrusts of a highly disturbed region are often cut across by 
a series of normal faults, and such phenomena seem to suggest that, 
while overthrusts and transverse thrusts are the result of horizontal 
movements, the normal faults in question may have come into existence 
when the tangential pressure was relieved. During a great horizontal 
movement, the rocks are not only subjected to enormous compression, but 
the crust is bulged up—in other words, mountains of elevation are formed, 
When the forward movement ceases and pressure is relieved, the pro- 
tuberant rock-masses will tend to settle down to some extent along the 
rents and fissures opened during the elevatory process, and normal faults 
will thus be produced. In the case of normal faults traversing horizontal 
and gently inclined strata, all we can say is that, like joints, they probably 
owe their origin to torsion, and since they are so commonly related to 
strike and dip, it seems highly probable that they also are the direct 
result of crustal folding. But we must not forget that many normal faults 
have been formed during movements of direct subsidence—which may 
not necessarily have been connected with any horizontal movements of 
the crust. Not a few extensive basin- or trough-shaped depressions are 
bounded by normal faults, and are obviously the result of direct subsidence 
or collapse of the crust. 
Crustal deformation would appear usually to have been slowly effected. 
Under sufficient pressure solids can be compelled to flow, and hard rocks 
may be bent without fracturing, provided the pressure be applied gradually. 

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