ext to it
being to have their weight supported by the vapor, which will insinuate
itself under them, will be raised in their turn, till it either finds
some vent, or is again condensed by the cold into water, and by that
means prevented from proceeding any farther."[782] In a memoir published
in 1843, on the structure of the Appalachian chain, by the Professors
Rogers,[783] the following hypothesis is proposed as "simpler and more
in accordance with dynamical considerations, and the recorded
observations on earthquakes."--"In place," say they, "of supposing it
possible for a body of vapor or gaseous matter to pass horizontally
between the strata, or even between the crust and the fluid lava upon
which it floats, and with which it must be closely entangled, we are
inclined to attribute the movement to an _actual pulsation_, engendered
in the _molten matter itself_, by a linear disruption under enormous
tension, giving vent explosively to elastic vapors, escaping either to
the surface, or into cavernous spaces beneath. According to this
supposition, the movement of the subterranean vapors would be _towards_,
and not from, the disrupted belt, and the oscillation of the crust would
originate in the tremendous and sudden disturbance of the previous
pressure on the surface of the lava mass below, brought about by the
instantaneous and violent rending of the overlying strata."

This theory requires us to admit that the crust of the earth is so
flexible, that it can assume the form, and follow the motion of an
undulation in the fluid below. Even if we grant this, says Mr. Mallet,
another more serious objection presents itself, viz. the great velocity
attributed to the transit of the wave in the subterranean sea of lava.
We are called upon to admit that the speed of the wave below equals that
of the true earthquake shock at the surface, which is so immense, that
it is not inferior to the velocity of sound in the same solids. But the
undulation in the fluid below must follow the laws of a tidal wave, or
of the great sea-wave already spoken of. "Its velocity, like that of the
tidal wave of our seas, will be a function of its length and of the
depth of the fluid, diminished in this case by certain considerations as
to the density and degree of viscidity of the liquid; and although it
would be at present impossible, for want of data, to calculate the exact
velocity with which this subterraneous lava-wave could move, it may be
certai