so perfectly preserve their original structure that when
cut and polished they may be used for decorative purposes. Conspicuous
as is this work of the dunes, it is in a geological way much less
important than that accomplished by the finer dust which drifts from
one region of land to another or into the sea. Because of their
weight, the sand grains journey over the surface of the earth, except,
indeed, where they are uplifted by whirl storms. They thus can not
travel very fast or far. Dust, however, rises into the air, and
journeys for indefinite distances. We thus see how slight differences
in the weight of substances may profoundly affect the conditions of
their deportation.

THE SYSTEM OF WATERS.

The envelope of air wraps the earth completely about, and, though
varying in thickness, is everywhere present over its surface. That of
the waters is much less equally distributed. Because of its weight, it
is mainly gathered in the depths of the earth, where it lies in the
interstices of the rocks and in the great realm of the seas. Only a
very small portion of the fluid is in the atmosphere or on the land.
Perhaps less than a ten thousandth part of the whole is at any one
time on this round from the seas through the air to the land and back
to the great reservoir.

The great water store of the earth is contained in two distinct
realms--in the oceans, where the fluid is concentrated in a quantity
which fills something like nine tenths of the hollows formed by the
corrugations of the earth's surface; and in the rocks, where it is
stored in a finely divided form, partly between the grains of the
stony matter and partly in the substance of its crystals, where it
exists in a combination, the precise nature of which is not well
known, but is called water of crystallization. On the average, it
seems likely that the materials of the earth, whether under the sea or
on the land, have several per cent of their mass of the fluid.

It is not yet known to what depth the water-bearing section of the
earth extends; but, as we shall see more particularly hereafter when
we come to consider volcanoes, the lavas which they send up to the
surface are full of contained water, which passes from them in the
form of steam. The very high temperature of these volcanic ejections
makes it necessary for us to suppose that they come from a great
depth. It is difficult to believe that they originate at less than a
hundred mi