crystallization of a dense fluid, or the accumulation of
pent-up gases, in whatever conjectures we indulge, we can never doubt
for a moment, that at some unknown depth beneath Sweden and the Baltic,
the structure of the globe is in our own times becoming changed from day
to day, throughout a space probably more than a thousand miles in
length, and several hundred in breadth.

CHAPTER XXXI.

CAUSES OF EARTHQUAKES AND VOLCANOES.

Intimate connection between the causes of volcanoes and
earthquakes--Supposed original state of fusion of the
planet--Universal fluidity not proved by spheroidal figure of the
earth--Attempt to calculate the thickness of the solid crust of the
earth by precessional motion--Heat in mines increasing with the
depth--Objections to the supposed intense heat of a central
fluid--Whether chemical changes may produce volcanic heat--Currents
of electricity circulating in the earth's crust.

It will hardly be questioned, after the description before given of the
phenomena of earthquakes and volcanoes, that both of these agents have,
to a certain extent, a common origin; and I may now, therefore, proceed
to inquire into their probable causes. But first, it may be well to
recapitulate some of those points of relation and analogy which lead
naturally to the conclusion that they spring from a common source.

The regions convulsed by violent earthquakes include within them the
site of all the active volcanoes. Earthquakes, sometimes local,
sometimes extending over vast areas, often precede volcanic eruptions.
The subterranean movement and the eruption return again and again, at
irregular intervals of time, and with unequal degrees of force, to the
same spots. The action of either may continue for a few hours, or for
several consecutive years. Paroxysmal convulsions are usually followed,
in both cases, by long periods of tranquillity. Thermal and mineral
springs are abundant in countries of earthquakes and active volcanoes.
Lastly, hot springs situated in districts considerably distant from
volcanic vents have been observed to have their temperature suddenly
raised, and the volume of their water augmented, by subterranean
movements.

All these appearances are evidently more or less connected with the
passage of heat from the interior of the earth to the surface; and where
there are active volcanoes, there must exist, at some unknown depth
below, enormous masses of m