rely absent, and their decomposition
is due to the presence of protoxide of iron, which readily absorbs
oxygen from the air, when the magnesia is separated and a ferruginous
clay left.

The minerals just referred to, constitute the great bulk of the mountain
masses, but they are associated with many others which take part in the
formation of the soil. Of these the most important are the zeolites
which do not occur in large masses but are disseminated through the
other rocks in small quantity. They form a large class of minerals of
which Thomsonite and natrolite may be selected as examples--

Thomsonite. Natrolite.

Silica 38.73 48.68
Alumina 30.84 26.36
Lime 13.43 --
Potash 0.54 0.23
Soda 3.85 16.00
Water 13.09 9.55
---- ----
100.48 100.83

They are chiefly characterized by containing their silica in a soluble
state, and hence may yield that substance to the plants in a condition
particularly favourable for absorption.

It is obvious from what has been stated that all these minerals are
capable, by their decomposition, of yielding soft porous masses having
the physical properties of soils, but most of them would be devoid of
many essential ingredients, while not one of them would yield either
phosphoric acid, sulphuric acid, or chlorine. It has, however, been
recently ascertained that certain of these minerals, or at least the
rocks formed from them, contain minute, but distinctly appreciable
traces of phosphoric acid, although in too small quantity to be detected
by ordinary analysis; and small quantities of chlorine and sulphuric
acid may also in most instances be found.

Still it will be observed that most of these minerals would yield a soil
containing only two or three of those substances, which, as we have
already learned, are essential to the plant. Thus, potash felspar, while
it would give abundance of potash, would be but an inefficient source
of lime and magnesia; and labradorite, which contains abundance of lime,
is altogether deficient in magnesia and potash.

Nature has, however, provided against this difficulty, for she has so
arranged it that these minerals rarely occur alone, the rocks which form
our great mountain masses being composed of intimate mixtures of two or
more of them, and that in such a manner that the deficiencies of the one
compensate those of the other. We shall sho