Uses.—It forms an excellent building and paving stone, and has been extensively employed in ornamental architecture.
Topaz-Rock.
This rock, which appears to be intimately connected with granite, has the following characters. Its consti tuent minerals are quartz, tourmaline, topaz, and litho marge. It is composed of many small masses, which have the appearance of fragments, although they arc true granular concretions. Each of these masses is composed of thin layers of quartz, tourmaline, and topaz ; and these layers have different directions in the different masses or concretions. The quartz appears in granular concretions ; the topaz is also granular ; but is distin guished by its foliated aspect and hardness ; lastly, the tourmaline is in small black needles. Frequently hollow spaces occur between these concretions, which arc partly filled with of quartz and topaz, but rarely contains tourmaline. The lithomargc occurs amongst these crystals, and has generally an ochre yellow, rarely green, and seldom white colour. It is worthy of remark, that the colour of the crystals of topaz, and also its inten sity, depends on that of the lithomarge ; as if this sub stance, or at least its colouring matter, was the same as that of the topaz.
Topaz rock is very distinctly stratified, and the strata are of considerable thickness. It rests upon granite, and is covered by clay-slate.
It has hitherto been found only near to Auerbach, in Voightland, in Saxony, where it forms a rock named Sehneekenstein, which was formerly of considerable ex tent, but has been much diminished by the operations of the miners in procuring topazes.
The inconsiderable extent of this mass prevents our viewing it as a distinct species of mountain rock.
II. Gneiss.
Gneiss.—Werner.
Gneiss and Granite Veine —Saussure. Gneiss.—Kirwan.—Jameson.
I. Nume.—The name Gneiss is of Saxon origin, and was applied by miners in the vicinity of Freyherg, to the decomposed stone that forms the walls of their me• talliferoes veins. Henkel describes gneiss as an indu rated stone, mixed with a steatitical and clayey matter; hut Werner ascertained that it was a compound of fel spar, quartz, and mica.
2. Constituent Parts.—This rock, like granite, is a compound of felspar, quartz, and mica ; but it contains more mica than granite. It is granular in the smnii, and
slaty in the large ; hence it is said to be granular-slaty. The granular felspar and quartz form plates, which arc separated from each other by the mica.
Felspar, although the predominant mineral, is still in less quantity than in granite. The felspar is usually greyish, yellowish, and reddish-white ; and sometimes so much altered that it appears earthy. The mica is most commonly grey, which passes through various shades into blackish-grey. The quartz is almost always grcyish-white, and generally in smaller grains than the felspar.
3. Imbedded Minerals.—Besides felspar, quartz, and mica, it sometimes contains schorl ; more rarely garnet, and also hornblende. The schorl occurs more rarely, and in less quantity, than in granite: but the garnet is more frequent and abundant than in granite.
4. Kinds of gneiss —There are three principal kinds of gneiss, of which we shall now give short descrip tions.
(I ) In this kind the mica occurs in small quantity : the scales of mica, although separated from each other, are arranged in parallel ranges, and the rock breaks in a direction cenformable with these. It is the parallelism of the ranges of mica which distinguishes this kind of gneiss from granite, because its slaty structure• is fiery indistinct, and the quantity of felspar is nearly the same as in granite. The quartz and the mica form each sepa rate layers ; those of the felspar are thicker, and such varieties, when broken across, have a ribbon-like aspect. Sometimes the rptartz, in place of being disposed in layers or plates in the felspar, is in small parallel rods or bars ; and when the rock is cut perpendicular to their direction, it appears not unlike petrified wood.
(2.) This, which is named common gneiss, consists of small layers, or lenticular plates, composed of grains of felspar and quartz, placed over each other, and separated by layers formed of scales of mica. It is sometimes glandular, ur contains balls of quartz, or of compounds of quartz and felspar, or of mica. This variety has been confounded with conglomerate. The island of Fetlar, one of the Shetlands, affords an example of this variety.