- Class 2. Earths.
- Order 2. Aggregate.
- Gen. Petuntse.
- Spec. Granite.
- Syn. Roche Feldspathique. Haüy, 4. 430.
This Granite is considered as of the primitive formation; it was once brought from Siberia as a great rarity, and cut for seals, &c., at a handsome price, though not perhaps extravagant, being brought so far: it however happens in this instance, as in many others, that where curious minerals are sought for abroad, the same are to be found at home.
Monymusk in Scotland affords many beautiful varieties. One of the characters of Primitive Granites is the crystallization of its component substances; the two of which this is composed are, as it were, coeval in the act of crystallizing, and each in part prevented the other, which gives it the particular appearance in these varieties, said to resemble the letters of the Greek alphabet, though to me they rather represent in form the Hebrew characters:—they have, however, been termed Graphite and Siberian Greek Stones most commonly.
The crystals of the Quartz occasionally terminate in such a manner as to show their facets, and they often present themselves remarkably flat, with a perfectly concordant structure, and even new facets:—thus the shaded face in the upper left hand outline is at present new to this substance, more particularly as being placed on the middle of one of the pyramidal faces:—there is some indication of it in some of the Crystals of tab. 319.
The position and angles of these Crystals caught in the fracture of the Petuntse give the curious appearance. The Petuntse has rather an amorphous appearance in general, but is all crystallized; and when the large primitive sides are reflected to the eye, they show its beautiful and peculiar lustre. It sometimes shows the terminations. The Quartz varies in colour from transparent to gray brown and nearly black—the Feldspar, from white to reddish and brown. Mica is found in moderate-sized specimens. Two of the ingredients would constitute it a Granitel; three, a Granitine of Kirwan: but these distinctions are not really of consequence; and I am afraid the term Sienite is not much better unless perhaps as an original term used by Pliny. It is understood chiefly to consist of Feldspar inclining to red, and Hornblende, and sometimes are added Quartz and Mica, as I and others have it from Egypt; my specimen by favour of W. Morel, Esq. was detached from one of the pillars at the entrance of the temple of Isis in the ancient Canopus near the fort of Aboukir. Its structure is that of Granite with the addition of Hornblende in a small proportion.