Local Vulcanism. 
501 
at the immediate place of fracture, and to a less extent under the 
mass giving less than the average pressure per unit area. Where 
the pressure is lessened, as a result of this cause, and perhaps as a 
result of mechanical movement, the rock is softened and it be¬ 
gins to rise and overflow the subsiding area. The process once 
begun, the weight of the ejected material must be added to that 
of the overhanging block, and thus tends to continue the process. 
Furthermore, as in the case of regional eruption by crustal com¬ 
pression, the formation of steam bubbles may be an important 
assistance in the uprise of the lava. It is not supposed that the 
disturbed gravitative equilibrium is entirely compensated for by 
extrusions. It is partly compensated for by the movements of 
the blocks in opposite directions. The position of the extensive 
lava fields and cones at various places in the plateau country 
of western America appears to accord with this explanation. 
The lavas have flowed from fissures and in many places they 
also built up rows of cones. On one side of each fissure there 
probably has been depression, and upon the other side equiva¬ 
lent uprise. However, a part of the subsidence of the great 
basin blocks have been compensated for by extrusions of the lavas. 
During tensile movements, exactly as in the case of com¬ 
pressive movements, much or all of the magma formed or moved 
may be intruded instead of extruded, and thus dikes, sills, 
laccolites and batholites be produced. 
Local Vulcanism .— In the case of volcanoes such as now 
exist, it may be supposed that the expansive force of steam is 
a relatively more important factor than in regional eruptions. 
In most districts of living volcanoes, as a consequence of various 
eruptions, volcanic mountains have been built. In such a case the 
weight of the mountain is to be added to that of the adjacent crust. 
The total stress is sufficient to raise the lava part way up the fun¬ 
nel. If as a result of earth movements fissures form on the 
flank or at the base of a mountain, gravity may be sufficient 
to cause an extrusion. When any considerable quantity of lava 
is extruded during a given eruption, it is generally through low- 
lying fissures. Through them lava may easily escape. How¬ 
ever, many extrusions are over the lower lips of the craters. It 
is not to be expected that the downward pressure of the solid 
