What was said above with regard to the difficulty of classifying rocks, applies more especially to the igneous group, because of the way in which the various kinds shade into one another, since even the same molten mass may differentiate into several species, showing not only differences of texture, but marked changes of chemical and mineralogical composition. In an elementary work, like the present, only a meagre outline of the subject can be attempted, for the microscopic study of rocks, or petrography, has now become an independent science of great scope and interest and cannot be compressed into a few pages.
The classification of the igneous rocks now most generally adopted is made upon a threefold method, according to texture, and chemical and mineralogical composition. In the following table (modified from Kemp's) the textures are given in vertical order, while transversely the arrangement is mineralogical, chiefly in accordance with the principal felspar. In this manner the acidic rocks come at the left side of the table and the basic at the right side. The percentages of silica are given on a lower line of the table.
The acid rocks are so called because they are rich in silica, but they have only small quantities of lime, magnesia, and iron; hence they are very infusible, of low specific gravity, and generally of light colours. The basic rocks, thus named because of the predominance of the bases, have much smaller percentages of silica and higher ones of lime, magnesia, and iron; the latter substances act as fluxes, making the basic rocks much more fusible, as well as giving them a higher specific gravity and darker colour. The distinction between acid and basic rocks cannot be very sharply drawn, because the two kinds are connected by every variety of intermediate gradation. The same is true, however, of all the divisions given in the table, which is apt to produce a false impression of sharply distinguished groups of rocks, such as do not occur in nature.
Obsidian, Perlite, Pumice,
Basic Glasses, ScoriAe,
Chief Felspar Orthoclase
Chief Felspar Plagioclase
Biotite and Hornblende and
OR OR AUGITE
Biotite and Horn-
Augite AND Horn-
OR blende aND Biotite
Glassy, Compact, or Porphyritic with few Phenocrysts
Rhyolite (Felsite, Quartz-Porphyry) Rhyolite-Porphyry
Olivine-Basalt, Olivine-(Diabase) Basalt-Porphyry
A series of
Nepheline and Leucite
Dykes, Sills, Laccoliths
Porphyritic with abundant Phenocrysts
Rocks, very rare in
Nepheline-Syenite,Leu-cite-Syenite (Very rare)
50 - 40%
As a general rule, the glassy and porphyritic textures characterize those rocks which have solidified at the surface of the ground, or not very far below it, while the granitoid types have cooled slowly and at great depths; but there are exceptions to both statements. Between the glassy and porphyritic textures at one end of the series and the granitoid at the other comes the felsitic which represents an intermediate rate of cooling and intermediate depths within the earth as the place of solidification (hypabyssal rocks).
The division of the igneous rocks into families is made primarily in accordance with the mineralogical composition, with subdivisions according to texture. This method gives us five principal groups.