o form a by no
means unimportant part of their mass. They do not occur in the oldest
sedimentary rocks, but as we ascend to the more recent geological
epochs, they increase in abundance, until, in the greensands and other
recent formations, whole beds of coprolites and other organic remains
are met with. Great differences are observed in the quality of the soils
yielded by different rocks. In general, those formed by the
disintegration of clay slates are cold, heavy, and very difficult and
expensive to work; those of sandstone light and poor, and of limestone
often poor and thin. These statements must, however, be considered as
very general; for individual cases occur in which some of these
substances may produce good soils, remarkable exceptions being offered
by the lower chalk and some of the shales of the coal formation. Little
is at present known regarding the peculiar nature of many of these
rocks, or their composition; and the cause of the differences in the
fertility of the soil produced from them is a subject worthy of minute
investigation.

_Chemical Composition of the Soil._--Reference has been already made to
the division of the constituents of the soil into the two great classes
of organic and inorganic. And when treating of the sources of the
organic constituents of plants, we entered with some degree of
minuteness into the composition and relations of the different members
of the former class, and expressed the opinion that they did not admit
of being directly absorbed by the plant. But though the parts then
stated lead to the inference that, as a direct source of these
substances, humus is unimportant, it has other functions to perform
which render it an essential constituent of all fertile soils. These
functions are dependent partly on the power which it has of absorbing
and entering into chemical composition with ammonia, and with certain of
the soluble inorganic substances, and partly on the effect which the
carbonic acid produced by its decomposition exerts on the mineral
matters of the soil. In the former way, its effects are strikingly seen
in the manner in which ammonia is absorbed by peat; for it suffices
merely to pour upon some dried peat a small quantity of a dilute
solution of ammonia to find its smell immediately disappear. This
peculiar absorptive power extends also to the fixed alkalies, potash and
soda, as well as to lime and magnesia, and has an important effect in
preventing these subst