EARTHQUAKES AND GEYSERS 361 
lava flows into the crevices, each break reaches the 
surface as a shock. It is no doubt for this reason that 
volcanic eruptions are preceded and accompanied by 
earthquakes, each ineffectual attempt of the lava to 
escape at the surface sending a jar through the rocks. 
The last three causes probably explain the majority 
of the earthquakes of the world, and nearly all those 
of serious consequence. ‘This being true, earthquake 
shocks will naturally be most common near volcanoes 
and among growing mountains; but any one of these 
causes may appear at any time in any part of the crust, 
although not likely to exist far away from their com- 
mon source. For great earthquakes there may be other 
causes, though none as yet appear. For minute jar- 
rings, even the passing of a heavily loaded wagon is a 
sufficient cause. 
From what has been said, it will be seen that most, 
if not all, great earthquakes are merely another expres- 
sion of the influence of the internal heat of the earth, 
possibly exerted through the action of - contraction. 
They are certainly a result of mountain folding; for 
both faulting and the intrusion of lava are commonly 
associated with this. Hence the source of the im- 
mediate causes for the majority of great earthquakes 
is mountain formation; and they may therefore be 
considered among the secondary phenomena of moun- 
tain growth. | 
