ed of
the sea more shallow, it will displace a certain quantity of water, and
thus tend to submerge low tracts.

Astronomers having proved (see above, p. 129) that there has been no
change in the diameter of the earth during the last two thousand years,
we may assume it as probable, that the dimensions of the planet remain
uniform. If, then, we inquire in what manner the force of earthquakes
must be regulated, in order to restore perpetually the inequalities of
the surface which the levelling power of water tends to efface, it will
be found, that the amount of depression must exceed that of elevation.
It would be otherwise if the action of volcanoes and mineral springs
were suspended; for then the forcing outwards of the earth's envelope
ought to be no more than equal to its sinking in.

To understand this proposition more clearly, it must be borne in mind,
that the deposits of rivers and currents probably add as much to the
height of lands which are rising, as they take from those which have
risen. Suppose a large river to bring down sediment to a part of the
ocean two thousand feet deep, and that the depth of this part is
gradually reduced by the accumulation of sediment till only a shoal
remains, covered by water at high tides; if now an upheaving force
should uplift this shoal to the height of 2000 feet, the result would be
a mountain 2000 feet high. But had the movement raised the same part of
the bottom of the sea before the sediment of the river had filled it up;
then, instead of changing a shoal into a mountain 2000 feet high, it
would only have converted a deep sea into a shoal.

It appears, then, that the operations of the earthquake are often such
as to cause the levelling power of water to counteract itself; and,
although the idea may appear paradoxical, we may be sure, wherever we
find hills and mountains composed of stratified deposits, that such
inequalities of the surface would have had no existence if water, at
some former period, had not been laboring to reduce the earth's surface
to one level.

But, besides the transfer of matter by running water from the continents
to the ocean, there is a constant transportation from below upwards, by
mineral springs and volcanic vents. As mountain masses are, in the
course of ages, created by the pouring forth of successive streams of
lava, so stratified rocks, of great extent, originate from the
deposition of carbonate of lime, and other mineral ingredients, w