ect or incomplete, for it is so complex that the cases in
which all the necessary details have been eliminated are even now by no
means numerous. In fact, the want of a large number of thorough analyses
of soils of different kinds is a matter of some difficulty, and so soon
as a satisfactory mode of investigation can be determined upon, a full
examination of this subject would be of much importance.

_Origin of Soils._--The constituents of the soil, like those of the
plant, may be divided into the great classes of organic and inorganic.
The origin of the former has been already discussed: they are derived
from the decay of plants which have already grown upon the soil, and
which, in various stages of decomposition, form the numerous class of
substances grouped together under the name of humus. The organic
substances may therefore be considered as in a manner secondary
constituents of the soil, which have been accumulated in it as the
consequence of the growth and decay of successive generations of plants,
while the primeval soil consisted of inorganic substances only.

The inorganic constituents of the soil are obtained as the result of a
succession of chemical changes going on in the rocks which protrude
through the surface of the earth. We have only to examine one of these
rocks to observe that it is constantly undergoing a series of important
changes. Under the influence of air and moisture, aided by the powerful
agency of frost, it is seen to become soft, and gradually to
disintegrate, until it is finally converted into an uniform powder, in
which the structure of the original rock is with difficulty, if at all
distinguishable. The rapidity with which these changes take place is
very variable; in the harder rocks, such as granite and mica slate it is
so slow as to be scarcely perceptible, while in others, such as the
shales of the coal formation, a very few years' exposure is sufficient
for the purpose. These actions, operating through a long series of
years, are the source of the inorganic constituents of all soils.

Geology points to a period at which the earth's surface must have been
altogether devoid of soil, and have consisted entirely of hard
crystalline rocks, such as granite and trap, by the disintegration of
which, slowly proceeding from the creation down to the present time, all
the soils which now cover the surface have been formed. But they have
been produced by a succession of very complicated process