VARIOUS CLASSIFICATIONS AND NOMENCLATURES.

From the foregoing sketch of the processes of rock-making it may easily be inferred that the varieties of rocks may be almost unlimited, and that they may be defined, named, and classified on many different bases; for example.

(1) If the mode of origin is chiefly in mind, rocks may be classed as igneous (lavas, tuffs, etc.); metamorphic (schists, gneisses, anthracite, magnetite, etc.); aqueous (water-laid sediments, stalactites, travertine, etc.); eolian (dunes, loess in part); glacial (till, moraines); clastic (mantle-rock, sandstone, conglomerate, etc.); organic (peat, lignite, coal, etc., and indirectly, limestone, chalk, infusorial earth, etc.); and so on.

(2) If the textural or structural characters are in mind, rocks are designated vesicular (pumice, scoria, etc.); rhyolitic (flow-structure rocks); glassy (obsidian, tachylite); porphyritic (distinct crystals in obscure matrix); granitic (well-grained); compact, porous, earthy, arenaceous (sandy), schistose, etc.

(3) If the chemical composition is chiefly regarded, they may be classed as silicious, calcareous, carbonaceous, ferruginous, etc.; or, if the chemical nature is considered, they are grouped as acidic, basic, or neutral.

(4) If the crystalline character is made the basis, they are designated phanerocrystalline (distinctly crystallized), microcrystalline (minutely crystallized), cryptocrystalline (hiddenly crystallized), and amorphous (non-crystalline).

(5) If attention is fastened on certain ingredients, rocks are characterized as quartzose, micaceous, chloritic, talcose, pyritiferous, garnetiferous, etc.

(6) When rocks are regarded as mineral aggregates, if (a) the aggregates are simple, they are named from the dominant minerals, as dolomite, hornblendite, garnetite, anorthite, etc.; and if (b) the aggregates are complex they take special names, as syenite (orthoclase and hornblende), gabbro (plagioclase feldspar and pyroxene), etc.

(7) When the point of view is structure of the mass, they are classed as massive, stratified, shaly, laminated, slaty, foliated, schistose, etc.

(8) When physical state or genesis is considered, they are grouped as clastic, fragmental, or detrital (conglomeratic, brecciated, arenaceous, argillaceous, etc.); or pyroclastic (tufaceous, scoriaceous, agglomeratic); or massive, in a sense slightly different from that above (7).

As sometimes one of these characteristics and sometimes another is most important in a given rock, or in a given study, no one classification is satisfactory in all cases, yet each has its advantages in particular cases.

New System of Classification and Nomenclature.

The present systems of classifying and naming rocks have grown up gradually out of earlier and cruder methods, many of which were inherited from popular usage. Most of the names and definitions came into use before microscopical and other modern means of study were adopted. These systems, therefore, retain many inherited crudities and inconsistencies, and lack adaptation to present needs. They are too complex and difficult for field use and for general discussions, while not sufficiently exact and systematic for the more rigorous petrological discussions. A more adaptive and consistent practice has been earnestly sought by petrologists, and a new system of classification of igneous rocks has been offered by a group of leading American petrologists, an outline of which is here given.[203] To some extent this may be extended to the metamorphic crystalline rocks with necessary modifications and additions. The classification and nomenclature of the secondary rocks must probably always remain variable and plastic to express the various points of view which it is desirable to take. During the transition to this or some other new system, which seems inevitable, the appended alphabetical reference lists of the most common minerals and rocks, with brief definitions in accordance with current usage, will be found serviceable. The proposed system includes two parts, a field system and a quantitative system, the one applicable to rocks on casual inspection, and the other, only after detailed study.

The proposed field system.

The proposed field names are based largely on texture and color. The mineral constituents are used for subdivisions when they can be determined; otherwise they are neglected.

Classifying chiefly on the basis of texture and crystalline state, there are three groups: Phanerites, in which all the leading mineral constituents can be seen megascopically; aphanites, in which all, or at least an appreciable part, of the constituent minerals cannot be distinguished megascopically; and glasses, in which the material is wholly or largely vitreous.

I. The Phanerites may be further classified by their chief mineral constituents as follows:

1. Granites (f.n.),[204] consisting largely of quartz and feldspar of any kind, with or without mica, hornblende, pyroxene, or other minerals. This differs from the present common use in not regarding mica as an essential constituent, and in not distinguishing between alkali feldspars and calcic feldspars, thus broadening the class.

2. Syenites (f.n.), consisting predominantly of feldspar of any kind, with subordinate amounts of hornblende, mica, or pyroxene, but with little or no quartz. This differs from the common use in giving hornblende a subordinate place, and in embracing rocks with calcic feldspars, thus broadening the class.

3. Diorites (f.n.), consisting predominantly of hornblende and subordinately of feldspar of any kind, with which there may be mica, pyroxene, or other minerals. This is nearly the present use except that any kind of feldspar may form the subordinate element.

4. Gabbros (f.n.), consisting predominantly of pyroxene and subordinately of feldspar of any kind, with or without other minerals. This nearly coincides with one of the various present uses of the term except that the range of the feldspar is increased.

5. Dolerites[205] (f.n.), consisting predominantly of any ferromagnesian mineral not distinguishable as hornblende or pyroxene, with subordinate elements of feldspar of any kind, and with or without other accessory minerals. A name to be used when the dominant mineral is clearly ferromagnesian, but cannot be satisfactorily identified as either hornblende or pyroxene, although it may probably be one of these. In other words, the dolerites (deceptive) embrace the whole diorite-gabbro group when too obscure for separation.

6. Peridotites, consisting predominantly of olivine and ferromagnesian minerals, without feldspar, or with very little.

7. Pyroxenite, consisting essentially of pyroxene without feldspar or olivine.

8. Hornblendite, consisting essentially of hornblende without feldspar or olivine.

II. The Aphanites may be non-porphyritic or porphyritic.

(a) Non-porphyritic aphanites when light-colored may be classed as felsites; when dark-colored, as basalts.

(b) The porphyritic aphanites or porphyries, when light-colored, are leucophyres; when dark-colored, are melaphyres (f.n.). They may be classified further, according to the kind of phenocryst imbedded in the aphanitic ground-mass, as

III. The glasses are classified, according to color and luster, into obsidians or pitchstones when dark and lustrous; perlites, when a spheroidal fracture gives them a pearly appearance; and pumice when greatly inflated by included gases.

In general discussions, it is regarded as serviceable to use the term granitoids in a broad generic sense, to include all crystalline rocks of the general granitoid type, including the granites, syenites, gneisses, etc. In a similar broad way, the term gabbroids may be used to include the dark crystalline rocks in which the ferromagnesian minerals predominate, as the diorites, gabbros, dolerites, peridotites, etc. In this convenient and comprehensive way, two contrasted groups of igneous rocks may be designated. As the granitoids are usually acidic and the gabbroids usually basic, the grouping represents a broad fact of importance.