lected back toward its point of origin. These
earthquake echoes sometimes give rise to very destructive movements.
It often happens that before the original tremors of a shock have
passed away from a point on the surface the reflex movements rush in,
making a very irregular motion, which may be compared to that of the
waves in a cross-sea.

The foregoing account of earthquake action will serve to prepare the
reader for an understanding of those very curious and important
effects which these accidents produce in and on the earth. Below the
surface the sensible action of earthquake shocks is limited. It has
often been observed that people in mines hardly note a swaying which
may be very conspicuous to those on the surface, the reason for this
being that underground, where the rocks are firmly bound together, all
those swingings which are due to the unsupported position of such
objects as buildings, columnar rocks, trees, and the waters of the
earth, are absent. The effect of the movements which earthquakes
impress on the under earth is mainly due to the fact that in almost
every part of the crust tensions or strains of other kinds are
continually forming. These may for ages prove without effect until the
earth is jarred, when motions will suddenly take place which in a
moment may alter the conditions of the rocks throughout a wide field.
In a word, a great earthquake caused by the formation of an extensive
fault is likely to produce any number of slight dislocations, each of
which is in turn shock-making, sending its little wave to complicate
the great oscillation. Nor does the perturbing effect of these jarring
movements cease with the fractures which they set up and the new
strains which are in turn developed by the motions which they induce.
The alterations of the rocks which are involved in chemical changes
are favoured by such motions. It is a familiar experience that a
vessel of water, if kept in the state of repose, may have its
temperature lowered three or four degrees below the freezing point
without becoming frozen. If the side of the vessel is then tapped with
the finger, so as to send a slight quake through the mass, it will
instantly congeal. Molecular rearrangements are thus favoured by
shocks, and the consequences of those which run through the earth are,
from a chemical point of view, probably important.

The reader may help himself to understand something of the complicated
problem of earth tensions, an