y in the modern as
in the more ancient periods, we ought to see a greater abundance of
tertiary and secondary than of primary granite and gneiss; but if we
adopt the hypogene theory before explained, their rapid diminution in
volume among the visible rocks in the earth's crust in proportion as we
investigate the formations of newer date, is quite intelligible. If a
melted mass of matter be now cooling very slowly at the depth of several
miles beneath the crater of an active volcano, it must remain invisible
until great revolutions in the earth's crust have been brought about. So
also if stratified rocks have been subjected to Plutonic action, and
after having been baked or reduced to semi-fusion, are now cooling and
crystallizing far under ground, it will probably require the lapse of
many periods before they will be forced up to the surface and exposed to
view, even at a single point. To effect this purpose there may be need
of as great a development of subterranean movement as that which in the
Alps, Andes, and Himalaya has raised marine strata containing ammonites
to the height of 8000, 14,000, and 16,000 feet. By parity of reasoning
we can hardly expect that any hypogene rocks of the tertiary periods
will have been brought within the reach of human observation, seeing
that the emergence of such rocks must always be so long posterior to the
date of their origin, and still less can formations of this class become
generally visible until so much time has elapsed as to confer on them a
high relative antiquity. Extensive denudation must also combine with
upheaval before they can be displayed at the surface throughout wide
areas.

All geologists who reflect on subterranean movements now going on, and
the eruptions of active volcanoes, are convinced that great changes are
now continually in progress in the interior of the earth's crust far
out of sight. They must be conscious, therefore, that the
inaccessibility of the regions in which these alterations are taking
place, compels them to remain in ignorance of a great part of the
working of existing causes, so that they can only form vague conjectures
in regard to the nature of the products which volcanic heat may
elaborate under great pressure.

But when they find in mountain-chains of high antiquity, that what was
once the interior of the earth's crust has since been forced outwards
and exposed to view, they will naturally expect in the examination of
those mountainous