5
7
B
7
These ratios are used throughout the system. In (1) A is extreme; in (5) B is extreme; in (2) A dominates over B; in (4) B dominates over A; in (3) A and B are equal or nearly equal.
All igneous rocks are grouped in five (5) primary divisions called Classes on a basis of the proportions of the salic and femic minerals, thus:
| Class | I. | Sal 7 —— > —, extremely rich in salic minerals, called persalane. Fem 1 |
| II. | Sal 7 5 —— < — > —, with dominant salic minerals, called dosalane. Fem 1 3 | |
| III. | Sal 5 3 —— < — > —, salic and femic minerals, equal or nearly equal, called salfemane. Fem 3 5 | |
| IV. | Sal 3 1 —— < — > —, with dominant femic minerals, called dofemane. Fem 5 7 | |
| V. | Sal 1 —— < —, extremely rich in femic minerals, called perfemane. Fem 7 |
Each of these classes is divided into two subclasses according to the proportions of two subgroups of the preponderant group of standard minerals. Of salic minerals one subgroup includes quartz, feldspars, and the feldspathoids; the other includes corundum and zircon. Of femic minerals one subgroup includes the silicates with magnetite, ilmenite, hematite, and rutile; the other contains apatite and the remaining minerals of this group. Most known igneous rocks fall into the first subclass of each class.
The classes are further divided into orders according to the proportions of certain minerals in the preponderant subgroups. Thus Classes I, II, and III are each divided into nine orders on a basis of the proportions of quartz and the feldspars, and of the feldspars and the feldspathoids, quartz and feldspathoids not occurring together. The orders may be described in the same terms for each of the first three classes as follows:
| Order | I. | Q 7 — > —, extremely rich in quartz, perquaric. F 1 |
| II. | Q 7 5 — < — > —, quartz dominant over feldspar, doquaric. F 1 3 | |
| III. | Q 5 3 — < — > —, quartz and feldspar equal or nearly equal, quarfelic. F 3 5 | |
| IV. | Q 3 1 — < — > —, feldspar dominant over quartz, quardofelic. F 5 7 | |
| V. | Q or L 1 ——— < —, extremely rich in feldspar, perfelic. F 7 | |
| VII. | L 5 3 — < — > —, feldspar and lenads equal or nearly equal, lenfelic. F 3 5 | |
| VIII. | L 7 5 — < — > —, lenads dominant over feldspars, dolenic. F 1 3 | |
| IX. | L 7 — > —, extremely rich in lenads, perlenic. F 1 |