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
- Quartz-porphyries, or quartzophyres;
- Feldspar-porphyries, or feldspaphyres (not felsophyres);
- Hornblende-porphyries, or hornblendophyres; and so on.
- These may be subclassed by color, as
- Quartz-leucophyres, light-colored quartz-porphyries;
- Quartz-melaphyres, dark-colored quartz-porphyries;
- Feldspar-leucophyres;
- Feldspar-melaphyres; and so on.
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.