should
occur a sinking down to a trifling amount, and such ravines should be
formed as might be produced by a few earthquakes, not more considerable
than have fallen within our limited observation during the last 150
years, the waters of the Sea of Azof would pour rapidly into the
Caspian, which, according to the measurements lately made by the Academy
of St. Petersburg, is 84 feet below the level of the Black Sea.[984] The
Sea of Azof would immediately borrow from the Black Sea, that sea again
from the Mediterranean, and the Mediterranean from the Atlantic, so that
an inexhaustible current would pour down into the low tracts of Asia
bordering the Caspian, by which all the sandy salt steppes adjacent to
that sea would be inundated. An area of several thousand square leagues,
now below the level of the Mediterranean, would be converted from land
into sea.

_Illustration derived from the elevation of land._--Let us next imagine
a few cases of the elevation of land of small extent at certain critical
points, as, for example, in the shallowest part of the Straits of
Gibraltar, where the deepest soundings from the African to the European
side give only 220 fathoms. In proportion as this submarine barrier of
rock was upheaved, the whole channel would be contracted in width and
depth, and the volume of water which the current constantly flowing from
the Atlantic pours into the Mediterranean would be lessened. But the
loss of the inland sea by evaporation would remain the same; so that
being no longer able to draw on the ocean for a supply sufficient to
restore its equilibrium, it must sink, and leave dry a certain portion
of land around its borders. The current which now flows constantly out
of the Black Sea into the Mediterranean would then rush in more rapidly,
and the level of the Mediterranean would be thereby prevented from
falling so low; but the level of the Black Sea would, for the same
reason, sink; so that when, by a continued series of elevatory
movements, the Straits of Gibraltar had become completely closed up, we
might expect large and level sandy steppes to surround both the Black
Sea and Mediterranean, like those occurring at present on the skirts of
the Caspian and the Lake of Aral. The geographical range of hundreds of
aquatic species would be thereby circumscribed, and that of hundreds of
terrestrial plants and animals extended.

A line of submarine volcanos crossing the channel of some strait, and
gradua