e. More steam would then be liberated, carrying up
with it jets of melted rock, which being hurled up into the air may fall
in showers of ashes on the surrounding country, and at length, by the
arrival of lava and water more and more heated at the orifice of the
duct or the crater of the volcano, expansive power may be acquired
sufficient to expel a massive current of lava. After the eruption has
ceased, a period of tranquillity succeeds, during which fresh accessions
of heat are communicated from below, and additional masses of rock fused
by degrees, while at the same time atmospheric or sea water is
descending from the surface. At length the conditions required for a new
outburst are obtained, and another cycle of similar changes is renewed.

_Causes of earthquakes--wave-like motion._--I shall now proceed to
examine the manner in which the heat of the interior may give rise to
earthquakes. One of the most common phenomena attending subterranean
movements, is the undulatory motion of the ground. And this, says
Michell, will seem less extraordinary, if we call to mind the extreme
elasticity of the earth and the compressibility of even the most solid
materials. Large districts, he suggests, may rest on fluid lava; and,
when this is disturbed, its motions may be propagated through the
incumbent rocks. He also adds the following ingenious speculation:--"As
a small quantity of vapor almost instantly generated at some
considerable depth below the surface of the earth will produce a
vibratory motion, so a very large quantity (whether it be generated
almost instantly, or in any small portion of time) will produce a
wave-like motion. The manner in which this wave-like motion will be
propagated may, in some measure, be represented by the following
experiment:--Suppose a large cloth, or carpet (spread upon a floor), to
be raised at one edge, and then suddenly brought down again to the
floor; the air under it, being by this means propelled, will pass along
till it escapes at the opposite side, raising the cloth in a wave all
the way as it goes. In like manner, a large quantity of vapor may be
conceived to raise the earth in a wave, as it passes along between the
strata, which it may easily separate in a horizontal direction, there
being little or no cohesion between one stratum and another. The part of
the earth that is first raised being bent from its natural form, will
endeavor to restore itself by its elasticity; and the parts n