THE PROPOSED QUANTITATIVE SYSTEM.

The distinguishing characteristic of the more rigorous system designed to meet the needs of scientific petrology is its quantitative chemical character. All igneous rocks are classified primarily according to their chemical composition and only secondarily according to their mineral constituents, texture, and other characters. The rigorous application of the system requires chemical analyses of the rocks, but as these are not available in many cases, the authors of the system have devised a method of optical mineral analysis by which the nearly exact proportions of all the constituent minerals can be determined, and by knowledge of their chemical nature the results may be converted, by computation, into chemical terms. This can only be done for holocrystalline rocks whose crystals are large enough to be measured under the microscope, but aphanitic rocks may often be approximately classified by comparison with similar rocks already accurately determined. To facilitate this method of chemical analysis by measuring the minerals, the chemical composition of certain common rock-making minerals is expressed in proportional parts and tabulated, and is used somewhat as molecular weight is in ordinary chemical analysis. Certain of these are selected as standard minerals, the selection being such that the standard minerals embrace all the essential elements that enter into the composition of rocks. All other minerals are converted into their chemical equivalents in terms of these standard minerals by the use of the tables. All the mineral constituents being thus reduced to standard minerals, the classification is built up systematically on these standard (or standardized) minerals.

A new system of names is required, and these have been very skillfully formed by selecting significant letters from the names of the leading minerals or from words signifying their preponderance, so that short terms which carry their meaning in their forms, are secured, and this has been done so that these are usually euphonious, however strange they may seem to our preoccupied senses. For example, minerals composed chiefly of silica and alumina are called salic; those of ferromagnesian minerals, femic; those of aluminous ferromagnesian minerals, alferric, etc. When in a combination of salic and femic minerals, the salic are extremely abundant, the rock is persalic; if notably dominant, dosalic; if the salic and femic minerals are nearly equal, salfemic; if the femic are dominant, dofemic; if extremely abundant, perfemic, and so on, the system being mnemonic. This method of deriving names is applicable only to a portion of the necessary divisions. For the rest, a series of roots derived from geographic names, with a system of terminations, has been employed.

All standard minerals are divided into two groups of primary importance: one of minerals characterized by alumina, as the feldspars,—orthoclase, albite, anorthite,—leucite, nephelite, sodalite, noselite, and corundum, to which are added the closely associated minerals, quartz and zircon. This is called the salic group. The second group contains minerals characterized by iron and magnesia with no alumina, as hypersthene (enstatite), acmite, olivine, magnetite, hematite, and ilmenite, to which are added the closely associated minerals, titanite, perofskite, rutile, apatite, and all other rock-making minerals except those containing alumina together with iron and magnesia. The second group is called femic.

Aluminous ferromagnesian minerals, such as hornblende, augite, mica, etc.; are called alferric, and are not classed as standard minerals, because their complexity of composition makes it better to treat them as though made up of the simpler minerals of the standard list.

The composition of all igneous rocks can be expressed in terms of the relative proportions of the two groups of the standard minerals, salic and femic. By subdividing these groups successively on a mineral and chemical basis, a series of classificatory divisions of greater and greater precision has been formed. In each stage of the series, two factors only are compared, and a simple set of ratios has been selected to limit the divisions. Assuming the possibility of a continuous range of variable mixtures of the two factors (A and B) from an extreme composed wholly of one (A), and an extreme composed wholly of the other (B), five ideal cases have been chosen as types or centerpoints about which variation in mixture may take place. These are:

—

—,

—

—,

—

—,

—

—,

—

—,

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

(1)

—

—,

(2)

—

—,

(3)

—

—,

(4)

—

—,

(5)

—

—.

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

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 minerals, polmitic. M 3 5
	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:

Section	1.	P 7 — > —, extremely rich in pyroxene, perpyric. O 1
	2.	P 7 5 — < — > —, dominant pyroxene, dopyric. O 1 3
	3.	P 5 3 — < — > —, pyroxene and olivine, equal or nearly equal, pyrolic. O 3 5
	4.	P 3 1 — < — > —, dominant olivine, domolic. O 5 7
	5.	P 1 — < —, extremely rich in olivine, perolic. O 7

In the last two orders a distinction between the preponderant mitic minerals is recognized by suborders, five in number. The minerals containing Fe₂O₃ are compared with those containing TiO₂. The former, magnetite and hematite, are called hemic, mnemonic of hematite; the latter subgroup, titanite, ilmenite, perofskite, rutile, are called tilic, mnemonic of titanite and ilmenite. Of orders 4 and 5, there are

Suborder	1.	H 7 — > —, hemic minerals extreme, perhemic. T 1
	2.	H 7 5 — < — > —, dominant hemic minerals, dohemic. T 1 3
	3.	H 5 3 — < — > —, hemic and tilic minerals equal or nearly equal, tilhemic. T 3 5
	4.	H 3 1 — < — > —, dominant tilic minerals, dotilic. T 5 7
	5.	H 1 — < —, tilic minerals extreme, pertilic. T 7

Further subdivision, producing rangs and subrangs, is made on the character of the chemical bases in the standard minerals used in forming orders and is expressed in terms of the molecular proportions of certain oxides. For the salic minerals, forming orders in the first three classes, the bases are alkalies—K₂O and Na₂O—and lime, CaO. For the femic minerals, forming orders in the last two classes, the bases are MgO, FeO, CaO and alkalies, K₂O, Na₂O. In classes I, II, and III rangs are formed by comparing salic alkalies, K₂O′ + Na₂O′, with salic lime, CaO′; and subrangs are formed by comparing K₂O′ with Na₂O′.

Rang	1.	K₂O′ + Na₂O′ 7 —————— > —, alkalies extreme, peralkalic. CaO′ 1
	2.	7 5 “ < — > —, alkalies dominant, domalkalic. 1 3
	3.	5 3 “ < — > —, alkalies and lime equal or nearly so, alkalicalcic. 3 5
	4.	3 1 “ < — > —, lime dominant, docalcic. 5 7
	5.	1 “ < —, lime extreme, percalcic. 7

Subrang	1.	K₂O′ 7 ——— > —, potash extreme, perpotassic. Na₂O′ 1
	2.	7 5 “ < — > —, potash dominant, dopotassic. 1 3
	3.	5 3 “ < — > —, potash and soda equal, sodipotassic. 3 5
	4.	3 1 “ < — > —, soda dominant, dosodic. 5 7
	5.	1 “ < —, soda extreme, persodic. 7

In classes IV and V rangs are formed by comparing femic MgO + FeO + CaO″ with femic alkalies K₂O″ + Na₂O″.

Minerals containing magnesia, iron, and lime are called mirlic.

Rang	1.	MgO + FeO + CaO″ 7 ———————— > —, extremely mirlic, permirlic. K₂O″ + Na₂O″ 1
	2.	7 5 “ < — > —, dominantly mirlic, domirlic. 1 3
	3.	5 3 “ < — > —, equally mirlic and alkalic, alkalimirlic. 3 5
	4.	3 1 “ < — > —, dominantly alkalic, domalkalic. 5 7
	5.	1 “ < —, extremely alkalic, peralkalic. 7

Sections of rangs distinguish between MgO + FeO and CaO″. Minerals with MgO + FeO are called miric.

Section	1.	MgO + FeO 7 ————— > —, extremely mirlic, permirlic. CaO″ 1
	2.	7 5 “ < — > —, dominantly miric, domiric. 1 3
	3.	5 3 “ < — > —, equally miric and calcic, calcimiric. 3 5
	4.	3 1 “ < — > —, dominantly calcic, docalcic. 5 7
	5.	1 “ < —, extremely calcic, percalcic. 7

Subrangs distinguish between MgO and FeO, thus:

Subrang	1.	MgO 7 —— > —, extremely magnesic, permagnesic. FeO 1
	2.	7 5 “ < — > —, dominantly magnesic, domagnesic. 1 3
	3.	5 3 “ < — > —, equally magnesic and ferrous, magnesiferrous. 3 5
	4.	3 1 “ < — > —, dominantly ferrous, doferrous. 5 7
	5.	1 “ < —, extremely ferrous, perferrous. 7

Finally a recognition of the character of the subordinate standard minerals leads to further subdivisions known as grads and subgrads. They only occur in classes II, III, and IV, because these are the only ones in which the subordinate minerals are in notable amounts. Grads are formed in a manner similar to that employed to produce orders. Thus grads in classes II and III correspond to orders in class IV and the reverse. Subgrads are the same in form as rangs when the difference in the treatment of salic and femic minerals is borne in mind. The names given to these divisions, which in fact recognize only the character of the magma, are derived from geographical localities and embrace many of those already in use, except that the names of orders are taken from countries or nations. Specific terminations indicate the place in the series of divisions:

ane for class, one for subclass.

are for order, ore for suborder.

ase for rang, ose for subrang.

ate for grad, ote for subgrad.

This may be illustrated as follows:

Class I. persalane, all rocks extremely salic.

Order 4. britannare, feldspar dominant over quartz, quardofelic. Many rocks of granitic composition whether crystalline or glassy.

Rang 1. liparase, peralkalic, rocks in which the potential feldspars are extremely alkalic, orthoclase, or albite.

Subrang 2. Omeose, dopotassic, rocks in which the extremely alkali feldspars are dominantly potassic, orthoclase, with subordinate albite. Examples of omeose are: granite from Omeo, Victoria, Australia, and rhyolite from Silver Cliff, Colorado.

The presence of distinctive minerals not indicated in the standard mineral composition of norm is expressed by qualifying the magmatic name by the name of the distinctive mineral; as, a hornblende-monzonose.

The precise texture of the rock is expressed by qualifying the magmatic name by a textural adjective; as, a grano-monzonose, a vitro-monzonose, a phyro-monzonose, etc.