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| 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 | -----------------------------------------------------------------------------