mantle of waste would differ in warm humid lands like India, in
frozen countries like Alaska, and in deserts such as the Sahara?

THE SOIL. The same agencies which produce the mantle of waste are
continually at work upon it, breaking it up into finer and finer
particles and causing its more complete decay. Thus on the
surface, where the waste has weathered longest, it is gradually
made fine enough to support the growth of plants, and is then
known as soil. The coarser waste beneath is sometimes spoken of as
subsoil. Soil usually contains more or less dark, carbonaceous,
decaying organic matter, called humus, and is then often termed
the humus layer. Soil forms not only on waste produced in place
from the rock beneath, but also on materials which have been
transported, such as sheets of glacial drift and river deposits.
Until rocks are reduced to residual clays the work of the weather
is more rapid and effective on the fragments of the mantle of
waste than on the rocks from which waste is being formed. Why?

Any fresh excavation of cellar or cistern, or cut for road or
railway, will show the characteristics of the humus layer. It may
form only a gray film on the surface, or we may find it a layer a
foot or more thick, dark, or even black, above, and growing
gradually lighter in color as it passes by insensible gradations
into the subsoil. In some way the decaying vegetable matter
continually forming on the surface has become mingled with the
material beneath it.

HOW HUMUS AND THE SUBSOIL ARE MINGLED. The mingling of humus and
the subsoil is brought about by several means. The roots of plants
penetrate the waste, and when they die leave their decaying