1139. Igneous unstratified rocks. The principal of these is granite, which is composed of quartz, feldspar, and mica, with the occasional addition of hornblende, and some other mineral substances. This rock generally forms the basis of all other rocks in mountain chains ; though it is also occasionally found on the surface, having been either washed bare by the violent action of water, or forced up by some internal fire through the strata deposited upon it. There are various kinds of granite, and the constituent parts of granite are occasionally found either separately or two combined together. Thus hornblende is sometimes found mixed with feldspar only, forming what is called syenite ; and sometimes crystals of feldspar are found imbedded in massive feldspar, forming what is called porphyry. Serpentine is another rock formed from the materials of granite; and talc sometimes replaces mica. Granite is found in great abundance in various parts of England, particularly Devonshire and Cornwall, and it is also found abundantly in Scotland. The other igneous rocks are apparently of volcanic origin.
