nomenon was the normal condition of our earth;
when the internal fires were enclosed by an envelope so thin that it
opposed but little resistance to their frequent outbreak, and they
constantly forced themselves through this crust, pouring out melted
materials that subsequently cooled and consolidated on its surface. So
constant were these eruptions, and so slight was the resistance they
encountered, that some portions of the earlier rock-deposits are
perforated with numerous chimneys, narrow tunnels as it were, bored by
the liquid masses that poured out through them and greatly modified
their first condition.

[Illustration: IDEAL SECTION OF A VOLCANO IN ACTION.]

The question at once suggests itself, How was even this thin crust
formed? what should cause any solid envelope, however slight and filmy
when compared to the whole bulk of the globe, to form upon the surface
of such a liquid mass? At this point of the investigation the
geologist must appeal to the astronomer; for in this vague and
nebulous border-land, where the very rocks lose their outlines and
flow into each other, not yet specialized into definite forms and
substances,--there the two sciences meet. Astronomy shows us our
planet thrown off from the central mass of which it once formed a
part, to move henceforth in an independent orbit of its own. That
orbit, it tells us, passed through celestial spaces cold enough to
chill this heated globe, and of course to consolidate it externally.
We know, from the action of similar causes on a smaller scale and on
comparatively insignificant objects immediately about us, what must
have been the effect of this cooling process upon the heated mass of
the globe. All substances when heated occupy more space than they do
when cold. Water, which expands when freezing, is the only exception
to this rule. The first effect of cooling the surface of our planet
must have been to solidify it, and thus to form a film or crust over
it. That crust would shrink as the cooling process went on; in
consequence of the shrinking, wrinkles and folds would arise upon it,
and here and there, where the tension was too great, cracks and
fissures would be produced. In proportion as the surface cooled, the
masses within would be affected by the change of temperature
outside of them, and would consolidate internally also, the crust
gradually thickening by this process.

[Illustration: A VOLCANO.]

But there was another element without the g