other substances necessary for the maintenance of
high-grade organisms, depositing these materials in the growing
strata. Into these beds, which are buried as fast as they form, goes
not only these earthy materials, but a great store of the sea water as
well. The result would be in course of time a complete withdrawal into
the depths of the earth of those substances which play a necessary
part in organic development. The earth would become more or less
completely waterless on its surface, and the rocks exposed to view
would be composed mainly of silica, the material which to a great
extent resists solution, and therefore avoids the dissolving which
overtakes most other kinds of rocks. Here comes in the machinery of
the hot springs, the dikes, and the volcanoes. These agents, operating
under the influence of the internal heat of the earth, are constantly
engaged in bearing the earthy matter, particularly its precious more
solvent parts, back to the surface. The hot springs and volcanoes work
swiftly and directly, and return the water, the carbon dioxide, and a
host of other vaporizable and soluble and fusible substances to the
realm of solar activity, to the living surface zone of the earth. The
dikes operate less immediately, but in the end to the same effect.
They lift their materials miles above the level where they were
originally laid, probably from a zone which is rarely if ever exposed
to view, placing them near the surface, where the erosive agents can
readily find access to them.

Of the three agents which serve to export earth materials from its
depths, volcanoes are doubtless the most important. They send forth
the greater part of the water which is expelled from the rocks.
Various computations which the writer has made indicate that an
ordinary volcano, such as AEtna, in times of most intense explosion,
may send forth in the form of steam one fourth of a cubic mile or
more of water during each day of its discharge, and in a single great
eruption may pour forth several times this quantity. In its history
AEtna has probably returned to the atmosphere some hundred cubic miles
of water which but for the process would have remained permanently
locked up in its rock prison.

The ejection of rock material, though probably on the average less in
quantity than the water which escapes, is also of noteworthy
importance. The volcanoes of Java and the adjacent isles have, during
the last hundred and twenty years, deliv