in a
considerable proportion of the undecomposed mineral, which continues for
a long period to yield a supply of alkalies to the plants which grow on
them.

Mica is a very widely distributed mineral, and two varieties of it are
distinguished by mineralogists, one of which is characterised by the
large quantity of magnesia it contains. Different specimens are found to
vary very greatly in composition, but the following analyses may
represent their most usual composition:

MICA.
|----------------|
Potash. Magnesia.
Silica 46.36 42.65
Alumina 36.80 12.96
Peroxide of iron 4.53 ---
Protoxide of iron --- 7.11
Oxide of manganese 0.02 1.06
Magnesia --- 25.75
Potash 9.22 6.03
Hydrofluoric acid 0.70 0.62
Water 1.84 3.17
---- ----
99.47 99.35

Mica undergoes decomposition with extreme slowness, as is at once
illustrated by the fact that its shining scales may frequently be met
with entirely unchanged in the soil. Its persistence is dependent on the
small quantity of alkaline constituents which it contains; and for this
reason it is observed that the magnesian micas undergo decomposition
less rapidly than those containing the larger quantity of potash.
Eventually, however, both varieties become converted into clay, their
magnesia and potash passing gradually into soluble forms.

Hornblende and augite are two widely distributed minerals, which are so
similar in composition and properties that they may be considered
together. Of the former two varieties, basaltic and common have been
distinguished, and their composition is given below:--

Hornblende.
|----------------|
Common. Basaltic. Augite.

Silica 41.50 42.24 50.12
Alumina 15.75 13.92 4.20
Protoxide of iron 7.75 14.59 11.60
Oxide of manganese 0.25 0.33 --
Lime 14.09 12.24 20.55
Magnesia 19.40 13.74 13.70
Water 0.50 -- --
---- ---- ----
99.24 97.05 99.67

In these minerals alkalies are enti