erfectly connected steps back to a time when what is now only an
occasional and rare phenomenon was the normal condition of our earth;
when those internal fires were inclosed in 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.

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 molten 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, where matter exists only in its essential elements, 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