This page has been validated.
86
A TREATISE ON GEOLOGY.
CHAP. VII.
The system alluded to is, however, well worthy of consideration; and being much and usefully employed on the Continent, it appears proper to offer the following brief account of that portion which relates to our present subject.
Mixed Rocks.
I. Crystallised isomerous[1] rocks, in which the constituent parts are equally blended.
A. Felspathic rocks, the characteristic mineral being felspar.
1. Granite.—Composed of laminated felspar, quartz, and mica.
2. Protogine.—Composed of felspar, quartz, steatite, or talc, or chlorite, with little or no mica.
3. Pegmatite, or graphic granite.—Consisting of laminated felspar and quartz.
4. Mimose.—Laminated felspar and augite.
B. Hornblendic rocks, the characteristic mineral being hornblende.
1. Sienite.—Composed of laminated felspar, hornblende, and quartz, the first predominating. One of the most remarkable varieties is the zircon sienite of Norway.
2. Diabase, or greenstone.—Composed of disseminated hornblende and compact felspar. (The orbicular greenstone of Corsica is a singular variety.)
II. Crystallised anisomerous rocks, in which the constituent parts are not equally mixed.
A. Basis of serpentine with imbedded minerals.
Ophiolite. In this occur oxydulous iron, chromate of iron, diallage, garnet, &c.
B. Basis of cornean, with imbedded minerals.
1. Variolite.—It contains nodules or veins, calcareous or siliceous, not older than the base.
2. Vakite.—The base is wacké, with augite, mica, &c. imbedded.
- ↑ From ισος, equal, and μεζος, a portion.
