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ing this singular substance, which has never been carefully examined, and concerning which so many errors have been propagated both at home and abroad.

NOME IX. TOPAZ ROCK.

This beautiful anomaly is hitherto only known to exist in Saxony; and Mr. Jameson's description shall be copied, as it is probably the most authentic.

"1. The remaining primitive rocks we have now to describe, are less important than those we have already described, because they occur less abundantly, and not so widely extended. One of the most remarkable of these is the topaz rock, which is not only remarkable on account of its constituent parts, but also its structure. It is composed of quartz, topaz, schorl, and a small portion of lithomarge. The quartz is fine granular; the schorl thin prismatic; the topaz usually coarse and fine granular, and has commonly a grey colour, which is to be attended to in its discrimination. These three fossils are disposed in layers, and thus form a slaty structure; but this slaty structure occurs only in the small; for these layers are collected into parti

cular large granular masses, so that the topaz rock appears large granular in the great: a kind of structure which is termed slaty granular. The drusy cavities, that sometimes occur between these concretions, frequently contain regular crystallised topaz and quartz; sometimes also schorl and lithomarge, of the same colour as the topaz.

"2. Its stratification is uncommonly distinct. "3. Its geognostic position has not been hitherto satisfactorily ascertained. It appears to lie on gneiss, and under clay slate.

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"4. It is a very rare rock, having been hitherto found only in one place in Germany, near the town of Auerbach, in the Saxon part of Voigtland, where it forms a mountain mass of considerable extent, and is there known by the name of Schneckenstein. A rock, composed of topaz, beryl, quartz, and lithomarge, occurs in the mountain of Odontschelon, and in the neighbourhood of Mursinsk, in Siberia, which resembles topaz rock, and is suspected to be the same with that of Auerbach. The schorl-rock of Cornwall is probably very intimately connected with topaz rock."*

It is truly surprising, that what are called the geognostic relations of so remarkable a rock

• Jameson's Min. iii. 141.

should not have been explained, especially as it stands in Saxony, the very focus of mineralogic knowledge. Henkel, as quoted by Patrin, says that the mountain or hill called Schneckenberg is near the valley of Tanneberg. The slope of the mountain is gentle; but from the summit rises, like a tower, the topaz rock, being about eighty feet in height, and three times as broad. But we are still to learn the composition of the adjacent hills*.

NOME X. JACINT ROCK.

A rock, which contains jacints, and which is itself composed of large white, greenish, and yellowish grains, consisting of quartz and of jacint, so that it may be called jacint rock†.

Among the ejections of Vesuvius there occurs what may be called Chrysolite rock, that gem even sometimes serving as a base; but these fragments, placed by Gmelin among the rocks, may perhaps be mere vein-stones, or may occur in small quantities. Perhaps rocks of Corindon may be discovered. It was known to Woodward by the name of Tella Corivindum, and Nello Corivendum.

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NOME XI. BERYL ROCK.

This was discovered in France, near Limoges, by le Lievre. It had been used in paving the highway, and is seldom of a good colour, being generally of a greyish white, though some specimens offer a tint of green. It is however rather a vein-stone, though found in large masses, as it runs through the middle of a vein of quartz in a granitic region*.

NOME XII. GARNET ROCK.

The red garnet, of which this beautiful rock is chiefly composed, contains from 20 to 41 parts of iron, according to analyses of Klaproth and Vauquelin. The green garnet is even some. times fused as an ore of iron.

In his System of Mineralogy, Cronstedt regarded the garnet as entitled to a peculiar place, in the rank of earths; a singularity which would seem to show that he had a distant view of the

Faujas, Geologie, Paris 1809, vol. ii. part i. p. 208. See particularly Journal des Mines, v. 641. The analysis of Vauquelin found the same ingredients as in the emerald.

necessity of introducing the ferruginous or siderous among the other earths.

This curious rock seems unknown in any system of mineralogy, except Mr. Kirwan's, who says, "Garnet-rock of Karsten, found by him near Winneburg: it consists of amorphous garnet, in which trap, quartz, calcareous spar, and a very small quantity of blackish brown mica are found."*

But the garnet rock, recently discovered in Scotland, seems to consist of that matter minutely interspersed among siderite and felspar, with larger or smaller globules, or imperfect crystals of garnet. In some parts it seems to approach to slaty siderite, penetrated with garnet; as it is common for that schistus to contain garnets.

The surface is brown from the decomposition of iron; and the garnets are of a coarse texture, and irregular form.

STRUCTURE I

Amorphous garnet rock, containing trap, quartz, calcareous spar, and mica, from Winneburg.

Min. i. 368. The Scotish may be the rock with grains of garnet from Sweden, Norway, &c. Linn. à Gmelin, 223. The Saxum Molare Granaticum, colore rubente, of Wallerius, from Norberke in Sweden.

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