500 
Van Hise—Earth Movements. 
to have been poured out over a depressed area. Moreover, 
these extrusions and those of the Tertiary of Britain 1 are very 
largely amygdaloidal. The cavities resulted from steam sep¬ 
aration, and the specific gravity of the magmas was consider¬ 
ably lowered. That this change partly or largely took place 
after the extrusions is highly probably, but it is not unlikely 
that the formation of bubbles occurred somewhat extensively at 
a considerable depth, and if so, as already explained, it was an 
important factor in the eruption of the lavas. Whether the 
bubbles were originally formed before or after the extrusions, 
they made their way toward the upper and lower surfaces of 
the lava flows before solidification. Many of the bubbles which 
once existed may have escaped. Only those which were re¬ 
tained in the solidified lava furnished cavities for the formation 
of amygdules. 
Whether or not in the lavas of the other areas of regional 
vulcanism amygdaloidal lavas are abundant, I have been unable 
to ascertain from the literature. 
Vulcanism in Connection with Regional Tensile Movements .— 
It is believed that the same principle of gravitative rise, per¬ 
haps through the assistance of steam is also applicable to reg¬ 
ions in which the deformation is largely that of tension. The 
plateau region of the western United States belongs to this class. 
As explained by Gilbert and others this is a region of normal 
faulting and block tilting. The fractures are believed in such 
cases to be due to tension. If this be so, it is favorable to their 
extension to a great depth. The fractures are generally not ex¬ 
actly vertical. In the case of the rock masses upon opposite 
sides of a given inclined fracture, the base of the overhanging 
mass carries a greater weight per unit area than the average of 
the region, and the base of the opposite mass a less weight. 
The deep-seated rock is therefore under greater stress than the 
average for the region, in the first place, and under less stress, 
in the second place. As a consequence, the mass on one side 
sinks, and on the other side rises. This necessitates flowage 
from below one mass to below the other. The potentially liq¬ 
uid rock far below the surface is largely released from pressure 
1 Loc. cit., p. 187. 
