load of rocks and soils, and as a result we find in that region of the
country great boulders and beds of sand and clay scattered over the
land.

_Work of the Air._--The air has helped in the work of wearing down the
rocks and making soils. If a piece of iron be exposed to moist air a
part of the air unites with part of the iron and forms iron rust. In
the same way when moist air comes in contact with some rocks part of
the air unites with part of the rock and forms rock rust which
crumbles off or is washed away by water. Thus the air helps to break
down the rocks. Moving air or wind picks up dust particles and carries
them from one field to another. On sandy beaches the wind often blows
the sand along like snow and piles it into drifts. The entire surface
of sandy regions is sometimes changed in this way. Sands blown from
deserts sometimes bury forests which with their foliage sift the fatal
winding sheet from the dust-laden winds.

_The Work of Plants._--Living plants sometimes send their roots into
rock crevices; there they grow, expand, and split off rock fragments.
Certain kinds of plants live on the surface of rocks. They feed on
the rocks and when they die and decay they keep the surface of the
rocks moist and also produce carbonic acid which dissolves the rocks
slowly just as the vinegar dissolved the limestone in our experiment.

Dead decaying roots, stems, and leaves of plants form largely the
organic matter of the soil. When organic matter has undergone a
certain amount of decay it is called humus, and these soils are called
organic soils or humus soils. The black soils of the woods, swamps and
prairies, contain large amounts of humus.