ely associated with the volcano the
causes of the latter are in great measure the causes of the former, and
the forces at work frequently produce a more or less violent quaking of
the earth's surface before they succeed in opening a channel of escape
through the mountain's heart. One agency of great potency, and one whose
work never ceases, has doubtless much to do with earthquake action.
In the description of this we cannot do better than to quote from "The
Earth's Beginning" of Sir Robert S. Ball.

CAUSE OF EARTHQUAKES

"As to the immediate cause of earthquakes there is no doubt considerable
difference of opinion. But I think it will not be doubted that an
earthquake is one of the consequences, though perhaps a remote one, of
the gradual loss of internal heat from the earth. As this terrestrial
heat is gradually declining, it follows from the law that we have
already so often had occasion to use that the bulk of the earth must be
shrinking. No doubt the diminution in the earth's diameter due to the
loss of heat must be exceedingly small, even in a long period of time.
The cause, however, is continually in operation, and, accordingly, the
crust of the earth has from time to time to be accommodated to the fact
that the whole globe is lessening. The circumference of our earth at
the equator must be gradually declining; a certain length in that
circumference is lost each year. We may admit that loss to be a quantity
far too small to be measured by any observations as yet obtainable, but,
nevertheless, it is productive of phenomena so important that it cannot
be overlooked.

"It follows from these considerations that the rocks which form the
earth's crust over the surface of the continents and the islands, or
beneath the bed of the ocean, must have a lessening acreage year
by year. These rocks must therefore submit to compression, either
continuously or from time to time, and the necessary yielding of the
rocks will in general take place in those regions where the materials
of the earth's crust happen to have comparatively small powers of
resistance. The acts of compression will often, and perhaps generally,
not proceed with uniformity, but rather with small successive shifts,
and even though the displacements of the rocks in these shifts be
actually very small, yet the pressures to which the rocks are subjected
are so vast that a very small shift may correspond to a very great
terrestrial disturbance.

"Suppose,