To the fifth class we refer those rocks which have a crystalline, granular, or slaty texture. The mutual adhesion of the heterogeneous parts, of which they consist, being, in general, inconsiderable, they are easily broken down by mechanical means, and thus contribute in a high degree to the formation of productive soil. The felspar contained in these rocks, on account of the chemical decomposition which it readily undergoes, has a great effect not only upon the quantity, but also the fertility of the soil produced. The quartz, on the contrary, as well as the mica and hornblende, long resist chemical decomposition; they are, however, useful in this respect, that the argillaceous soil arising from the felspar, has its tenacity diminished; and is consequently rendered better adapted for vegetation, by being intermixed with them. _Granite_ and _gneiss_, of all truly granular crystalline rocks, afford the deepest and most fertile soil, aptly compounded of different substances, sufficiently loose in its aggregation, and capable of retaining the necessary moisture. Soil arising from the disintegration of granite is unfavourable to vegetation only, where the rock abounds much in quartz, and where the superfluous water cannot run off, and so gives rise to marshes, which produce only vegetables of inferior quality; of which we have examples in the granite districts of Aberdeen. In such places as these, peat is easily generated, which, although of great use, is yet much less advantageous than wood. _Syenite_, which abounds much in hornblende, is inferior to granite, with respect to the production of fertile soil; and primitive _greenstone_, which resists disintegration and decomposition in the highest degree, occupies the last place in this class. In the series of slaty crystalline rocks, _mica-slate_ is next to gneiss: but on account of the small proportion of felspar which enters into its composition, it does not afford so productive a soil. In the sixth class may be placed the slaty rocks, whether simple, or intimately compounded, which do not readily undergo chemical decomposition, but which easily separate at their natural fissures, and are mechanically resolved into an earthy mass, forming a paste with water, circumstances which are observed chiefly in _clay-slate_, a rock of much importance in the formation of productive soil, usually passing into a clayey sort of earth.