Vulcanism and Compressive Movements. 
497 
which is behind the great regional extrusions of igneous ma¬ 
terial, is gravity. Further it has been noted that these ex¬ 
trusions are contemporaneous with great crustal movements. 
The phenomenon may be considered under the headings of, vul¬ 
canism in connection with regional compressive movements, 
vulcanism in connection with regional tensile movements, and 
local vulcanism. 
Vulcanism in Connection with Regional Compressive Movements . 
During regional compressive movements the effects of great 
lateral forces are concentrated along certain belts, and there 
the rocks are thickened, plicated, and broken. Broad anticlinal 
mountain ridges are separated by wide synclinal depressions. 
The heaping up of the material gives sufficient pressure to cause 
the magma to reach the surface at the lower levels by gravita¬ 
tional adjustment, just as in a lake covered by a thick layer of 
ice, where on the depressed areas adjacent to expansion ice 
ridges which form during times of rising temperature, water 
rises to the surface of the lighter ice and floods it. Under this 
theory the material would be extruded through cracks in the 
depressed areas, or from the flanks of the folds, in which case 
it would flow toward the depressions. If the magma passes 
through cracks in the depressed area, it must be supposed that the 
mechanica movements or the local release of pressure at the cracks, 
or both, suffice to soften the deep-seated rock so that it rises. 1 
If the passage of the magma be through the flanks of the folds, 
in addition to the factors already mentioned tending to soften 
the rocks, there is still another. Such places are adjacent to 
the anticlinal ridges. There, because of the partial transmis¬ 
sion of the load along the limbs of the arches, under the prin¬ 
ciple of competent structure, the deep-seated material is under 
less pressure than the average, and this also tends to make it 
plastic or liquid. 
When once the extrusion of material has begun, its weight 
must be added to the load of the solid rock, and if deformation 
continues tending to depress the synclines and to raise the anti - 
1 Geikie thinks that cracks are not necessary, but that the magma may 
drill its way “ through rocks independently of faults.” (Ancient volca¬ 
noes of Great Britain, by Sir Archibald Geikie, Vol. II, 1897, p. 473.1 
32 
