CHAPTER XIX 
EARTHQUAKES AND GEYSERS 
HARTHQUAKES 
Nature of the Shock. — An earthquake is a jar pass- 
ing through the rocks. Any shock will cause such a 
jar. An explosion of gunpowder in a quarry causes a 
tiny earthquake to spread out from the centre of the 
discharge; and some years ago, an explosion of a great 
quantity of dynamite at Hell Gate, near New York 
City, sent a jar through the rocks which was measured 
by delicate instruments at Cambridge on the one side, 
and Washington on the other. There are many real 
earthquake shocks of less intensity than this; but on 
the other hand, there are some of extreme violence. 
Really the rocks of the earth are in a constant state of 
strain, and this causes numerous slippings, so that 
slight jars are frequent, though strong ones are rare, 
occurring only in a few parts of the earth. 
If the rocks were perfectly homogeneous, like glass, 
the jar would pass through them with true wave motion. 
2A 353 
