_heat_, all the other known phenomena, at and
about volcanic vents, become simple. Lavas and all other solid ejecta of
Volcanoes, from all parts of the earth's surface, as well as basalts,
present in chemical and physical constitution close resemblance, and may
be all referred to the melting of more or less fusible mixtures of
siliceous crystalloid rocks with aluminous (slates, etc.) and calcareous
rocks. Their general chemical composition, and the higher or lower
temperatures of fusion resulting therefrom, together with the higher or
lower temperatures to which they have been submitted at the different
volcanic foci, determine their difference of flow (under like surface
conditions) and of mineral character after ejection and cooling.

St. Clair de Ville and Fouque have shown that the gaseous ejections, of
which steam forms probably 99 per cent., are such as arise from water
admitted to a _pre-existent focus of high temperature_.

Whether sea or fresh water is not material, when we bear in mind that
the chemical constituents found in sea water and in natural fresh waters
that have penetrated the soil are, on the whole, alike in kind and only
differ in proportions. But I must pass almost without notice all the
varied and instructive phenomena which are presented by volcanic vents,
for to treat of these at all would be to more than double the size of
this sketch.

In the source that has been pointed out as that from which volcanic heat
itself is derived, viz., the secular cooling of our globe, and the
effects of that upon its solid shell, we are enabled to point to that
which is the surest test of the truth of any theory--that it not only
enables us to account for all the phenomena, near or remote, but to
predict them. We see here linked together as parts of one grand play of
forces, those of contraction by cooling, producing by _direct_
mechanical action the elevation of mountain chains, and by their
_indirect_ action, by transformation of mechanical work into heat, the
production of Volcanoes; and both by direct and by indirect action, of
Earthquakes, never previously shown to have thus the physical connection
of one common cause, but merely supposed, more or less, to be connected
by their distribution upon our earth's surface.

We now discern thus the physical cause _why_ Volcanoes are distributed,
viewed largely, linearly, and follow the lines of elevation; we see
equally why their action is uncertain, non-periodi