498 
Van Else—Earth Movements. 
dines, the extrusion of magma may continue until a great thick¬ 
ness of volcanic material is accumulated. 
If the force behind the magma be sufficient to drive it to the 
surface, it may be sufficient to force it along the partings be¬ 
tween the layers, or other fractures or planes of weakness, and 
thus form sills, laccolites, batholites, or other intrusive masses. 
As stated by Gilbert 1 this is especially likely to be the case if 
the magma has a greater specific gravity than the upper part of 
the solid crust; for, disregarding the forces necessary to flexure 
the rocks overlying the intrusive, the work required to intro¬ 
duce a given quantity of magma below the surface is less than 
that required to carry it to the surface. In either case the 
boundary between the rock and air is at the same level, but if 
intruded, the center of gravity of the sill or laccolite and the 
overlying solid rock is lower than the center of gravity of this 
same mass of solid rock and the extruded lava spread over an 
equivalent area of the surface. Because of this principle it 
happens in many regions that after the forces are too feeble to 
press the magma to the surface, vast quantities may still be in¬ 
truded into the rocks near the surface. This is illustrated by 
the ancient vulcanism of Great Britain. 2 
To some extent the expansive force of the dissolved waters 
known always to be contained in the magma may assist in the 
process of regional eruptions. 3 A portion of the water is prob¬ 
ably original. Another part may be derived from underground 
1 Geology of the Henry mountains, by G. K. Gilbert: Kept. U. S. Geol. 
Survey, 2nd ed., 1880, pp. 66-74. 
2 Loc. cit., p. 474. 
3 It is not my purpose to here only discuss the source of the water. It is 
probable that an important source is by percolation through the surface 
rocks. This is especially true of local volcanoes. In regional extrusions 
it, however, appears highly probable that the occluded water has been held 
largely by the magma from the first. The gas pressure resulting from its 
presence would depend upon the quantity of water occluded per unit mass 
and the temperature of the occluding rock. Since both of these factors 
are unknown, it is useless to speculate upon the resulting pressure, but 
unless the amount of water be assumed to be great, and the temperature 
be assumed to be very high, there is no doubt that the pressure of the 
superincumbent rock would vastly surpass that of the gas pressures result¬ 
ing from the water contained, except at very superficial depths. 
