EARTHQUAKES AND GEYSERS 359 
that sometimes a series of shocks occurs in the same 
place year after year. 
There are several causes which may produce besa 
jars of the rocks from which earthquake shocks 
originate, each cause probably accounting for some. 
The falling in of a cavern will effect a jar in the strata, 
and a landslip will do the same. In England some of 
the smaller earthquake movements have so originated. 
_ Struggles of gas, particularly steam, imprisoned within 
the rocks, will cause jarrings which are true earth- 
quakes; near volcanoes, this cause of shocks is con- 
stantly present and frequently active. 
A fourth way in which earthquake shocks may be 
produced is by the breaking or faulting of the rocks. 
As they snap and move, they cause a jar; and each 
slip starts a shock of great or small intensity. Such a 
movement would be along a plane, and the dislocation 
from which the jar originated might even be apparent 
at the surface (Figs. 170 and 219). From the constant 
slipping of the rocks along such a plane, innumerable 
shocks of more or less violence might originate, keeping 
the earth thereabout in a constant state of unrest; but 
only during a distinct and decided movement would a 
really severe earthquake shock occur. During the 
folding of mountains these faults are common; and 
hence, while mountains are growing, earthquakes must 
of necessity be frequent. 
