es; for these
disintegrated rocks being washed away in the form of fine mud, or at
least of minute particles, and being deposited at the bottom of the
primeval seas, have there hardened into what are called sedimentary
rocks, which being raised above the surface by volcanic action or other
great geological forces, have been again disintegrated to yield
different soils. Thus, then, all soils are directly or indirectly
derived from the crystalline rocks, those overlying them being formed
immediately by their decomposition, while those found above the
sedimentary rocks may be traced back through them to the crystalline
rocks from which they were originally formed.

Such being the case, the composition of different soils must manifestly
depend on that of the crystalline rocks from which they have been
derived. Their number is by no means large, and they all consist of
mixtures in variable proportions of quartz, felspar, mica, hornblende,
augite, and zeolites. With the exception of quartz and augite, these
names are, however, representatives of different classes of minerals.
There are, for instance, several different minerals commonly classified
under the name of felspar, which have been distinguished by
mineralogists by the names of orthoclase, albite, oligoclase, and
labradorite; and there are at least two sorts of mica, two of
hornblende, and many varieties of zeolites.

Quartz consists of pure silica, and when in large masses is one of the
most indestructible rocks. It occurs, however, intermixed with other
minerals in small crystals, or irregular fragments, and forms the entire
mass of pure sand.

The four kinds of felspar which have been already named are compounds of
silica with alumina, and another base which is either potash, soda, or
lime. Their composition is as follows, two examples of each being
given--

----------------------------------------------------------------------------
| Orthoclase. | Albite. | Oligoclase. | Labradorite. |
------------------|--------------|-------------|-------------|--------------|
Silica | 65.72| 65.00| 67.99| 68.23| 62.70| 63.51| 54.66| 54.67|
Alumina | 18.57| 18.64| 19.61| 18.30| 23.80| 23.09| 27.87| 27.89|
Peroxide of iron | traces| 0.83| 0.70| 1.01| 0.62| -- | -- | 0.31|
Oxide of manganese| traces| 0.13| -- | -- | -- | -- | -- | -- |
Lime | 0.34| 1.23| 0.66| 1.26| 4.60| 2.44| 12.01|