10.60|
Magnesia | 0.10| 1.03| -- | 0.51| 0.02| 0.77| -- | 0.18|
Potash | 14.02| 9.12| -- | 2.53| 1.05| 2.19| -- | 0.49|
Soda | 1.25| 3.49| 11.12| 7.99| 8.00| 9.37| 5.46| 5.05|
----------------------------------------------------------------------------|
| 100.00| 99.47|100.08| 99.83|100.79|101.37|100.00| 99.19 |
-----------------------------------------------------------------------------

It is obvious that soils produced by the disintegration of these
minerals must differ materially in quality. Those yielded by orthoclase
must generally abound in potash, while albite and labradorite,
containing little or none of that element, must produce soils in which
it is deficient. The quality of the soil they yield is not however
entirely dependent on the nature of the particular felspar which yields
it, but is also intimately connected with the extent to which the
decomposition has advanced. It is observed that different felspars
undergo decomposition with different degrees of rapidity but after a
certain time they all begin to lose their peculiar lustre, acquire a
dull and earthy appearance, and at length fall into a more or less white
and soft powder. During this change water is absorbed, and, by the
decomposing action of the air, the alkaline silicate is gradually
rendered soluble, and at length entirely washed away, leaving a
substance which, when mixed with water, becomes plastic, and has all the
characters of common clay. The nature of this change will be best seen
by the following analysis of the clay produced during this composition,
which is employed in the manufacture of porcelain under the name of
kaolin, or china clay--

Silica 46.80
Alumina 36.83
Peroxide of iron 3.11
Carbonate of lime 0.55
Potash 0.27
Water 12.44
----
100.00

In this instance the decomposition of the felspar had reached its limit,
a mere trace of potash being left, but if taken at different stages of
the process, variable proportions of that alkali are met with. This
decomposition of felspar is the source of the great deposits of clay
which are so abundantly distributed over the globe, and it takes place
with nearly equal rapidity with potash and soda felspar. It is rarely
complete, and the soils produced from it frequently conta