Potash 0.27
Water 12.44
------
100.00
------

The chief difference here is the almost total loss of potash and a
portion of the silica, and the gain of water. The other constituents
practically remain insoluble.

Another important mineral is _Mica_. Its composition is not unlike
felspar. It contains silica, alumina, and iron, in considerable
quantities, also magnesia and potash. There are two kinds of mica--that
containing potash, and that containing magnesia, in excess. The analyses
of these two kinds are as follows (by the late Dr Anderson):--

MICAS.
(_a_) Potash. (_b_) Magnesia.
Silica 46.36 42.65
Alumina 36.80 12.96
Peroxide of iron 4.53 none
Protoxide of iron none 7.11
Oxide of manganese 0.02 1.06
Magnesia none 25.75
Potash 9.22 6.03
Hydrofluoric acid 0.70 0.62
Water 1.84 3.17
----- -----
99.47 99.35
----- -----

The decomposition of mica is very slow, however, as it is a peculiarly
hard mineral.

Other important minerals are _Hornblende_ and _Augite_. These are
composed of silica, alumina, iron oxide, manganese oxide, lime and
magnesia. These are the chief minerals out of which soils are formed. It
is scarcely necessary to say that few soils are made up out of any of
these three minerals alone. Nearly all rocks are formed out of a mixture
of these minerals. Where, however, any one mineral predominates over the
rest, the nature of the soil will be thereby affected. In order to
illustrate this, it may be well to mention the composition of one or two
of the commoner rocks.

1. _Granite_, which is so abundant in certain parts of the north of
Scotland, and which gives rise to the soils in the neighbourhood of
Aberdeen, is made up of a mixture of quartz, felspar, and mica. It
depends on the felspar present--_i.e._, whether it is orthoclase,
oligoclase, or albite--whether the soil will be rich in potash or not.
Granite containing orthoclase felspar