ith
which springs are impregnated. The surface of the land, and portions of
the bottom of the sea, being thus raised, the external accessions due to
these operations would cause the dimensions of the planet to enlarge
continually, if the amount of depression of the earth's crust were no
more than equal to the elevation. In order, therefore, that the mean
diameter of the earth should remain uniform, and the unevenness of the
surface be preserved, it is necessary that the amount of subsidence
should be in excess. And such a predominance of depression is far from
improbable, on mechanical principles, since every upheaving movement
must be expected either to produce caverns in the mass below, or to
cause some diminution of its density. Vacuities must, also, arise from
the subtraction of the matter poured out from volcanoes and mineral
springs, or from the contraction of argillaceous masses by subterranean
heat; and the foundations having been thus weakened, the earth's crust,
shaken and rent by reiterated convulsions, must, in the course of time,
fall in.

If we embrace these views, important geological consequences will
follow; since, if there be, upon the whole, more subsidence than
elevation, the average depth to which former surfaces have sunk beneath
their original level must exceed the height which ancient marine strata
have attained above the sea. If, for example, marine strata, about the
age of our chalk and greensand, have been lifted up in Europe to an
extreme height of more than eleven thousand feet, and a mean elevation
of some hundreds, we may conclude that certain parts of the surface,
which existed when those strata were deposited, have sunk to an extreme
depth of _more than_ eleven thousand feet below their original level,
and to a mean depth of _more than_ a few hundreds.

In regard to faults, also, we must infer, according to the hypothesis
now proposed, that a greater number have arisen from the sinking down
than from the elevation of rocks.

To conclude: it seems to be rendered probable, by the views above
explained, that the constant repair of the land, and the subserviency of
our planet to the support of terrestrial as well as aquatic species, are
secured by the elevating and depressing power of causes acting in the
interior of the earth; which, although so often the source of death and
terror to the inhabitants of the globe--visiting in succession every
zone, and filling the earth with monuments of