Igneous rock is those which have been ejected in a melted state either from volcanoes or through fissures in the earth's crust. As the crystallizing of superficial deposits may produce rocks like those that are of true igneous origin, the same species in a few cases occur in both divisions. Thus, there are metamorphic diorite and porphyry, as well as igneous diorite and porphyry. The igneous rocks differ from most of the metamorphic series in the absence or very sparing occurrence of quartz.
There are two series of igneous rocks,—the feldspathic, having light colors and being of low specific gravity, and the augitic, having dark colors with high specific gravity.
Feldspathic series: Consisting mainly of a feldspathic base, with, often, disseminated crystals of some kind of feldspar, or of hornblende or pyroxene. Feldspathic series colors are white, gray, bluish gray, grayish brown, occasionally, as in the porphyries, dark red and brown. The light colors and low specific gravity are owing to the absence or sparing dissemination of iron.
Augitic series: Consisting of feldspar and hornblende or augite (greenish-black or black pyroxene). Augitic series Colors are, dark gray, dark grayish brown, dark greenish brown, greenish-black to black. The high specific gravity and dark colors are owing to the presence of iron as magnetic or titanic iron, or as a constituent of the augite or hornblende.
Virtually all the igneous rocks are composed largely of silicate minerals, and any rock type consists of a definite assemblage of minerals, mainly determined by temperature, which can be used to classify the rock. Although through, for example, re-melting, magmas of quite different compositions exist, it is possible to obtain virtually a full range of rock types from a single magma derived from the mantle. Although six types of igneous rock (basalt, gabbro, andesite, diorite, rhyolite, and granite) account for about 90% of all igneous rocks, there is a continuous gradation in the chemical composition of igneous rocks.