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.