Chapter VII: The Origin and Descent of Rocks (3)

_A_ = 1 _A_ = 3 _A_ = 1 _A_ = 1 _A_ = 0
——— —, ——— —, ——— —, ——— —, ——— —.
_B_ = 0 _B_ = 1 _B_ = 1 _B_ = 3 _B_ = 1

Division lines half-way between these points occur as the following ratios:

_A_ 7 _A_ 7 > 5 _A_ 5 3 _A_ 3 1 _A_ 1
(1) ——— > —, (2) ——— < — > —, (3) ——— < — > —, (4) ——— < — > —, (5) ——— < —.
_B_ > 1 _B_ < 1 > 3 _B_ 3 5 _B_ 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 7

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,
Fem 3 5
called _salfemane_.

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

VI. L 3 1
— < — > —, feldspar dominant over feldspathoids (lenads), _lendofelic_.
F 5 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

In classes IV and V the preponderant minerals are femic, and in subclass 1 they are silicates, titanates, and ferrates, with hematite and rutile. These are subdivided as follows:

Silicates—pyroxenes and olivine with akermanite in one subgroup; the other minerals, magnetite, hematite, ilmenite, titanite, perofskite, rutile, in the second subgroup. This first group is called _polic_, mnemonic of pyroxene and olivine; the second group is called _mitic_, mnemonic of magnetite, ilmenite, titanite.

There are five orders in each of these classes, as follows:

Order I. PO 7
—— > —, extremely rich in pyroxene or olivine, _perpolic_.
M 1

II. PO 7 5
—— < — > —, dominant pyroxene or olivine, _dopolic_.
M 1 3

III. PO 5 3
—— < — > —, pyroxene or olivine, equal or nearly equal to the mitic
M 3 5
minerals, _polmitic_.

IV. PO 3 1
—— < — > —, dominant mitic minerals, _domitic_.
M 5 7

V. PO 1
—— < —, extremely rich in mitic minerals, _permitic_.
M 7

In the first three orders a distinction between pyroxene and olivine is recognized by sections, five in number: