an
unsupported hollow would be left under the earth's crust. Let us now
suppose a continental area to sink, and an adjacent oceanic area to
rise, it will be seen that the greater part of the land will disappear
long before the new land has approached the surface of the ocean. This
difficulty will not be removed by supposing a portion of a continent to
subside, and the immediately adjacent portion of the ocean on the other
side of the continent to rise, because in almost every case we find that
within a comparatively short distance from the shores of all existing
continents, the ocean floor sinks rapidly to a depth of from 2000 to
3000 fathoms, and maintains a similar depth, generally speaking, over a
large portion of the oceanic areas. In order, therefore, that any area
of continental extent be upraised from the great oceans, there must be a
subsidence of a land area five or six times as great, unless it can be
shown that an extensive elevation of the ocean floor up to and far
above the surface could occur without an equivalent depression
elsewhere. The fact that the waters of the ocean are sufficient to cover
the whole globe to a depth of two miles, is alone sufficient to indicate
that the great ocean basins are permanent features of the earth's
surface, since any process of alternation of these with the land areas
would have been almost certain to result again and again in the total
disappearance of large portions, if not of all, of the dry land of the
globe. But the continuity of terrestrial life since the Devonian and
Carboniferous periods, and the existence of very similar forms in the
corresponding deposits of every continent--as well as the occurrence of
sedimentary rocks, indicating the proximity of land at the time of their
deposit, over a large portion of the surface of all the continents, and
in every geological period--assure us that no such disappearance has
ever occurred.

_Oceanic and Continental Areas._

When we speak of the permanence of oceanic and continental areas as one
of the established facts of modern research, we do not mean that
existing continents and oceans have always maintained the exact areas
and outlines that they now present, but merely, that while all of them
have been undergoing changes in outline and extent from age to age, they
have yet maintained substantially the same positions, and have never
actually changed places with each other. There are, moreover, certain
physical and b