urely imaginary line, and no striking
difference can be observed either in their chemical or physical
characters. In such cases it has been the practice with some persons not
to limit the term soil to the upper portion, but to apply it to the
whole depth, however great it may be, which agrees in characters with
the upper part, and only to call that subsoil which manifestly differs
from it. This principle is perhaps theoretically the more correct, but
great practical advantages are derived from limiting the name of soil to
the depth actually worked in common agricultural operations. The subsoil
is always analogous in its general characters to a soil, but it may be
either identical with that which overlies it or not. Of the former,
striking illustrations are seen in the wheat subsoils, the analyses of
which have been already given. In the latter case great differences may
exist, and a heavy clay is often found lying on an open and porous sand,
or on peat, and _vice versa_. Even where the characters of the subsoil
appear the same as those of the soil, appreciable chemical differences
are generally observed, especially in the quantity of organic matter,
which is increased in the soil by the decay of plants growing upon it
and by the manure added. In general, then, all that we have said
regarding the characters of soils both chemically and physically, will
apply to the subsoils, except that, owing to the difficulty with which
the air reaches the latter, some minor peculiarities are observed. The
most important is the effect of the decay of vegetable matter, without
access of air, which is attended by the reduction of the peroxide of
iron to the state of protoxide, and not unfrequently by the production
of sulphuret of iron, compounds which are extremely prejudicial to
vegetation, and occasionally give rise to some difficulties when the
subsoil is brought to the surface, as we shall afterwards have to
notice.

The physical characters of the subsoil are often of much importance to
the soil itself. As, for instance, where a light soil lies on a clay
subsoil, in which case its value is much higher than if it reposed on an
open or sandy subsoil. And in many similar modes an important influence
is exerted; but these belong more strictly to the practical department
of agriculture, and need not be mentioned here.

_Classification of Soils._--Numerous attempts have been made to form a
classification of soils according to their chara