vel of the great
water comes against the ridges which fret the earth's surface. These
elevations are so disposed that about two thirds of the hard mass is
at the present time covered with water, and only one third exposed to
the atmosphere. This proportion is inconstant. Owing to the endless
up-and-down goings of the earth's surface, the place of the shore
lines varies from year to year, and in the geological ages great
revolutions in the forms and relative area of water and land are
brought about.

Noting the greater divisions of land and water as they are shown on a
globe, we readily perceive that those parts of the continental ridges
which rise above the sea level are mainly accumulated in the northern
hemisphere--in fact, far more than half the dry realm is in that part
of the world. We furthermore perceive that all the continents more or
less distinctly point to the southward; they are, in a word,
triangles, with their bases to the northward, and their apices,
usually rather acute, directed to the southward. This form is very
well indicated in three of the great lands, North and South America
and Africa; it is more indistinctly shown in Asia and in Australia. As
yet we do not clearly understand the reason why the continents are
triangular, why they point toward the south pole, or why they are
mainly accumulated in the northern hemisphere. As stated in the
chapter on astronomy, some trace of the triangular form appears in the
land masses of the planet Mars. There, too, these triangles appear to
point toward one pole.

Besides the greater lands, the seas are fretted by a host of smaller
dry areas, termed islands. These, as inquiry has shown, are of two
very diverse natures. Near the continents, practically never more than
a thousand miles from their shores, we find isles, often of great
size, such as Madagascar, which in their structure are essentially
like the continents--that is, they are built in part or in whole of
non-volcanic rocks, sandstones, limestones, etc. In most cases these
islands, to which we may apply the term continental, have at some time
been connected with the neighbouring mainland, and afterward separated
from it by a depression of the surface which permitted the sea to flow
over the lowlands. Geologists have traced many cases where in the past
elevations which are now parts of a continent were once islands next
its shore. In the deeper seas far removed from the margins of the
continents the i