at they better the
conditions by affording dwelling places for a host of lowly species,
such as the forms which are known as bacteria. The organisms probably
aid in the decomposition of the mineral matter, and in the conversion
of nitrogen, which abounds in the air or the soil, into nitrates of
potash and soda--substances which have a very great value as
fertilizers. Some effect is produced by the decay of the foreign
matter brought into the soil, which as it passes away leaves channels
through which the soil water can more readily pass.

By far the most general and important effect arising from the decay
of organic matter in the earth is to be found in the carbon dioxide
which is formed as the oxygen of the air combines with the carbon
which all organic material contains. As before noted, water thus
charged has its capacity for taking other substances into solution
vastly increased, and on this solvent action depends in large part the
decay of the bed rocks and the solution of materials which are to be
appropriated by the plants.

Having now sketched the general conditions which lead to the formation
of soils, we must take account of certain important variations in
their conditions due to differences in the ways in which they are
formed and preserved. These matters are not only of interest to the
geologist, but are of the utmost importance to the life of mankind, as
well as all the lower creatures which dwell upon the lands. First, we
should note that soils are divisible into three great groups, which,
though not sharply parted from each other, are sufficiently peculiar
for the purposes of classification. Where the earth material has been
derived from the rocks which nearly or immediately underlie it, we
have a group of soils which may be entitled those of immediate
derivation--that is, derived from rocks near by, or from beds which
once overlaid the level and have since been decayed away. Next, we
have alluvial soils, those composed of materials which have been
transported by streams, commonly from a great distance, and laid down
on their flood plains. Third, the soils the mineral matters of which
have been brought into their position by the action of glaciers; these
in a way resemble those formed by rivers, but the materials are
generally imperfectly sorted, coarse and fine being mingled together.
Last of all, we have the soils due to the accumulation of blown dust
or blown sand, which, unlike the others, occupy bu