ON THE NATURE OF IGNEOUS INTRUSIONS Ig! 
in the crust itself, is in harmony with the gradual bending of 
strata thousands of feet in thickness, as seen in the Black Hills, 
Big Horn Mountains, etc., without producing fractures through 
which the igneous material could escape. 
Certain broad physical features of the earth seem in harmony, 
also, with the views here advanced. It has long been recognized 
that volcanoes are arranged about the borders of continents, and 
on the ocean’s floor. So far as the association of oceanic waters 
is concerned with the origin of volcanoes, this arrangement is 
now considered accidental. But as shown by Dana, volcanoes 
mark lines of weakness in the earth’s crust. These lines of 
weakness may reasonably be supposed to be the direction taken 
by the fractures during an early stage in the cooling of the globe 
and to have continued to be the lines of weakness along which 
movements have taken place from time to time, down to our own 
day. In the central portions of continental areas, active and 
recently extinct volcanoes are much less numerous than near 
continental margins. In the case of those that do occur far 
inland, as in the Great Basin region, there is plain evidence 
that the rocks of the earth’s crust have been broken, and the 
region is as much a line of weakness as if it chanced to be 
adjacent to the sea. 
While active and recently extinct volcanoes are notably absent 
from the central portions of continental areas, it is equally true 
so far as can be judged from available data, that subtuberant 
mountains are confined to such central regions. This follows 
also from the conclusions that broad areas of horizontal and 
unbroken stratified rocks are favorable for the formation of 
extensive intrusions. 
The central portions of continental areas although not lines 
of weakness in the sense used by Dana, are regions of denudation 
and, as has been recognized by several American geologists, may 
be considered as relatively light areas, in comparison with con- 
tinental borders, where maximum sedimentation takes place. It 
is in regions where the earth’s crust is relatively light, when 
fractures are absent, and where the strata are essentially horizon- 
