ineyard there is a connected area
containing about thirty thousand acres which lies in a very favourable
position for tillage, but has been found substantially worthless for
such use. The farmers have found it more advantageous to clear away
the boulders from the coarser drift in order to win soil which would
give them fair returns.

Those areas which are occupied by soil materials which have been
brought into their position by the action of the wind may, as regards
their character, be divided into two very distinct groups--the dunes
and loess deposits. In the former group, where, as we have noted (see
page 123), the coarse sea sands or those from the shores of lakes are
driven forward as a marching hillock, the grains of the material are
almost always silicious. The fragments in the motion are not taken up
into the air, but are blown along the surface. Such dune accumulations
afford an earth which is even more sterile than that of the glacial
sand plains, where there is generally a certain admixture of pebbles
from rocks which by their decomposition may afford some elements of
fertility. Fortunately for the interests of man, these wind-borne
sands occupy but a small area; in North America, in the aggregate,
there probably are not more than one thousand square miles of such
deposits.

Where the rock material drifted by the winds is so fine that it may
rise into the air in the form of dust, the accumulations made of it
generally afford a fertile soil, and this for the reason that they are
composed of various kinds of rock, and not, as in the case of dunes,
of nearly pure silica. In some very rare cases, where the seashore is
bordered by coral reefs, as it is in parts of southern Florida, and
the strand is made up of limestone bits derived from the hard parts
which the polyps secrete, small dunes are made of limy material.
Owing, however, in part to the relatively heavy nature of this
substance, as well as to the rapid manner in which its grains become
cemented together, such limestone dunes never attain great size nor
travel any distance from their point of origin.

As before noted, dust accumulations form the soil in extended areas
which lie to the leeward of great deserts. Thus a considerable part of
western China and much of the United States to the west of the
Mississippi is covered by these wind-blown earths. Wherever the
rainfall is considerable these loess deposits have proved to have a
high agricultural value