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NOME XV. JAD, SCHORL, GARNETS.

A rock, which Saussure calls a mixture of jad, sparry schorl, and massive garnet. It takes a fine polish, and its large spots of red, green, and yellow, form a beautiful effect *.

NOME XVI. GRANITE AND CHALCEDONY.

Chalcedony was chiefly found in amygdalites, and by some supposed to be of volcanic origin. Saussuret discovered this curious and important rock near the city of Vienne, in Dauphiny. On examining the stones employed in building a peasant's cottage, he was astonished to find that most of them were elegant chalcedonies, more or less translucent, and mingled with leaves of a beautiful yellow pyrites. Observing that granite adhered to many of these fragments, the rock was explored, forming the adjacent bank of a rivulet called Bougelai. In some places it filled up the accidental seams of the granite, and in others formed nodules completely enveloped in that substance. The most common

[blocks in formation]

colour of the chalcedony is a bluish grey; but it also appears of a yellowish white, and often covered with ferruginous rust. Sometimes there are zones, concentric and in festoons, of a paler colour. The fracture is various, sometimes uniform, sometimes scaly, sometimes a little conchoidal; and its hardness is such that the file cannot touch it. It is coeval with the granite, for nodules of granite may be found in the chalcedony, as well as the contrary. These granitic nodules contain very little mica, but abundant felspar, yellow or reddish, and quartz, of which the aspect sometimes approaches that of the chalcedony. The pyrites is interlaced in a remarkable manner, being in plates nearly regular, a quarter of a line in thickness, and about five or six lines in length. These plates cross each other in certain places, in every

direction. Each of the plates is included in a kind of salband, of a breadth equal to that of the plate, of a deeper coloured chalcedony than the rest of the stone. The pyrites is of a pale brass colour, and granular fracture, but decomposes in the air; so that its beauty only becomes apparent on a fresh fracture *.

Saussure afterwards discovered abundance of chalcedony in the granites and gneiss of the plains, and particularly in the ancient Bourbonnois. See tome v. p. xi.

In a subsequent journey Saussure also discovered gneiss, its thin leaves alternating with thicker or thinner leaves of chalcedony.

HYPONOME 1.

Chalcedony in granite.

HYPONOME II.

Nodules of granite in chalcedony.

Micronome 1. Gneiss, alternating with chalcedony.

NOME XVII. GRANITE, WITH SCHORL

AND GARNETS.

A granite, from Bamfshire, Scotland, of red felspar, and bluish fat quartz in large grains, broad plates of micarel of a brilliant yellow, with black schorl in prisms of four lines in diameter. There are also patches of garnets.

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NOME XVIII. 'GRANITE AND LIME-STONE.

This mixture, like most of the others, appears in the Alps.

HYPONOME 1.

Granite, with lime-stone.

Micronome 1. Gneiss, with lime-stone.

NOME XIX. GRANITE AND SLATE.

Veins of granite in

slate,

Slate, by some called argillaceous schistus, is sometimes found blended with granite, though in general it rather seems to form a distinct line; and it commonly rests on granite, as being of a more recent formation. The veins of granite that run through slate have afforded matter of discussion to various theorists, who thence argue that the granite is of more recent formation, or at least that they are both coeval. It has been affirmed by some, that what is called granite, in such instances, is of an imperfect form, being either granitel of two substances, or the mica not in its usual state of crystallisation. Granites

Saussure's remarks.

of quite a new formation have been indicated by Saussure*. In describing the mountains which bound on the north-west the valley of Valorsine, he mentions that he found a mountain composed of his roche de corne, which is sometimes basalt, generally basaltin, sometimes basanite, sometimes magnesian basaltin, here called Saussurite; and sometimes a coarse slate, or argillaceous schistus, which seems here to be the caset. On observing this roche de corne in the spots where it coalesced with the granite, I saw veins of different breadths filled with a granite, which was formed and moulded in their interior.' The largest of these veins is about three feet in breadth, cutting at right angles the planes of the layers of the rock, which it traverses; and the uncovered part above the rest is about seven or eight feet in length. The sides of this vein are regular and parallel. . The granite which fills this vein is composed, like that of the mountain to which it adheres, of grey quartz, white felspar, and brilliant grey mica. This granite presents little even slits or seams, rather indicated than real, crossing each other in different directions; which seems the effect of a begin

$599. 601. † The corncus fissilis of Wallerius is hornblende slate, or slaty siderite.

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