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.

Acidic Classification Basic

Surface Flows

GLASSY

Acid Glasses

Obsidian, Perlite, Pumice,

Pitchstone

Andesite Obsidian

Basic Glasses, ScoriAe,

Tachylite, Basalt

Obsidian

Chief Felspar Orthoclase

Chief Felspar Plagioclase

Nepheline Leucite

No Felspar

Biotite and Hornblende and

OR OR AUGITE

Biotite and Horn-

OR blende

Pyroxenes

Augite AND Horn-

OR blende aND Biotite

OR

+ Quartz

- Quartz

Leucite Nepheline

+Quartz

- Quartz

- Olivine

+Olivine

-Olivine +Olivine

- Olivine

+Olivine

Surface Flows, Thin Sills, Dykes, Laccoliths

Glassy, Compact, or Porphyritic with few Phenocrysts

Rhyolite (Felsite, Quartz-Porphyry) Rhyolite-Porphyry

Trachyte (Felsite)

Trachyte-Porphyry

Phonolite

(rare)

Leucite

Rocks

(Very rare)

Phonolite-

Porphyry

Dacite (Felsite)

Dacite-Porphyry

Andesite (Felsite)

Andesite-Porphyry

Augite-Andesite

Basalt (Diabase)

Augite-Andesite-Porphyry

Olivine-Basalt, Olivine-(Diabase) Basalt-Porphyry

A series of

Nepheline and Leucite

Basaltic

Augitite

Augitite-Porphyry

Limburgite

Limburgite-Porphyry

Dykes, Sills, Laccoliths

Porphyritic with abundant Phenocrysts

Granite-Porphyry

Syenite-Porphyry

Nepheline-

Syenite-

Porphyry

Quartz-Diorite-Porphyry

Diorite-Porphyry

Gabbro-Porphyry

Olivine-

Gabbro-

Porphyry

Rocks, very rare in

America

Pyroxenite-Porphyry

Peridotite-Porphyry

Batholiths, Laccoliths

Granitoid

Granite

Syenite

Nepheline-Syenite,Leu-cite-Syenite (Very rare)

Quartz-Diorite

Diorite

(Diabase) Gabbro

(Olivine-

Diabase)

Olivine-

Gabbro

Theralite

(exceedingly rare)

Pyroxenite

Peridotite

Si02

80-65%

65-55%

60-50%

70-60%

65-50%

50-45%

50 - 40%

55-30%

Granite fam.

Syenite family

Diorite family

Gabbro family

Peridotite family

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.