the surface of the land in regions where there are no rivers
to effect that action in the normal way.

Notwithstanding its swift motion when impelled by differences in
weight, the movements of the air have had but little direct and
immediate influence on the surface of the earth. The greater part of
the work which it does, as we shall see hereafter, is done through the
waters which it impels and bears about. Yet where winds blow over
verdureless surfaces the effect of the sand which they sweep before
them is often considerable. In regions of arid mountains the winds
often drive trains of sand through the valleys, where the sharp
particles cut the rocks almost as effectively as torrents of water
would, distributing the wearing over the width of the valley. The dust
thus blown, from a desert region may, when it attains a country
covered with vegetation, gradually accumulate on its surface, forming
very thick deposits. Thus in northwestern China there is a wide area
where dust accumulations blown from the arid districts of central Asia
have gradually heaped up in the course of ages to the depth of
thousands of feet, and this although much of the _debris_ is
continually being borne away by the action of the rain waters as they
journey toward the sea. Such dust accumulations occur in other parts
of the world, particularly in the districts about the upper
Mississippi and in the valleys of the Rocky Mountains, but nowhere are
they so conspicuous as in the region first mentioned.

Where prevailing winds from the sea, from great lakes, and even from
considerable rivers, blow against sandy shores or cliffs of the same
nature, large quantities of sand and dust are often driven inland
from the coast line. In most cases these wind-borne materials take on
the form of dunes, or heaps of sand, varying from a few feet to
several hundred feet in height. It is characteristic of these hills of
blown sand that they move across the face of the country. Under
favourable conditions they may journey scores of miles from the shore.
The marching of a dune is effected through the rolling up of the sand
on the windward side of the elevation, when it is impelled by the
current of air to the crest where it falls into the lee or shelter
which the hill makes to the wind. In this way in the course of a day
the centre of the dune, if the wind be blowing furiously, may advance
a measurable distance from the place it occupied before. By fits and
starts t