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may be as ancient as the massive granite; that substance sometimes rising into natural walls, as in Cornwall : or, in the great antiquity of the earth, the veins may have been formed in a softer granitic substance (more compact veins and nodules being observable on a small scale), which afterwards wasted away, and its place was supplied by the clay-slate.
Granite in slate.
Micronome 1. Slate in granite.
NOME XX. GNEISS, WITH BLUE SIDE
Near Breuil, Saussure observed a gneiss full of garnets, the surface being incrustated with little crystals of a beautiful steel blue, oblong, irregular, opake, very brilliant, striated in the longest direction, frequently porous in that direction, and with difficulty scratched by a knife when the streak is grey. The fracture laminar, equally blue and brilliant; and they are easily fusible under the blow-pipe into a shining black amel, attractable by the magnet, although the original substance be not. He adds, that all these properties characterise some kinds of hornblende, the only singularity of this being its blue and brilliant colour*.
NOME XXI. CLAY, SPATHOSE IRON.
A composite rock of clay, spathose iron, and another spart.
NOME XXII. SERPENTINE, WITH LIME
Some of the most singular compounds with lime-stone occur in the Pyrenees, where that substance forms the chief summits. The intermixture of lime-stone, or of calcareous spar, with serpentine, is there not uncommon.
Some of the noblest marbles, as the verdantique, and that lately discovered in Anglesea, consist of serpentine mingled with carbonate of lime; but the magnesia is so preponderant, and its nature so predominant and characteristic, that such are arranged in the Talcous Domain; not to mention that the union is too intimate to class them among the Composite Rocks, which are mostly only coherent, the substances forming in distinct accretions.
Dark green serpentine, with grey lime-stone, from the Pyrenees.
Micronome 1. The same, with red calcareous spar, from the same.
NOME XXIII. LIME-STONE, WITH GAR
This curious mixture also chiefly occurs in the Pyrenees.
Light brown lime-stone, with red garnets, from the Pyrenees.
NOME XXIV. LIME-STONE, WITH STEA
Tirey, one of the western isles of Scotland, presents a white marble with yellow spots, supposed to be steatite.
In the same interesting isle marble and steatite are reciprocally interveined.
Marble, with veins of steatite.
NOME XXV. LIME-STONE, WITH OLIVINE.
Olivine, before chiefly observed in lava and basalt, is also found in the micaceous lime-stone of Mount Somma, of which Vesuvius may be regarded as only a portion. Breislak has, on this occasion, given some useful information concerning olivine and chrysolite*,
. i. 150.
1. The soft chrysolite, or asparagus-stone of Olivine and
chrysolite, Werner, is a mere phophate of lime, analysed by Vauquelin.
2. The chrysolite of the jewellers is a greenish oriental topaz.
3. The common chrysolite, or peridot of the French, analysed by Vauquelin, contains—magnesia 50, silex 38, oxyd of iron 9. This is also the chrysolite analysed by Klaproth.
4. Olivine, called by some volcanic chrysolite, has also been analysed by Klaproth, and though it contained rather more silex and iron, as the proportions will even vary in different specimens of the same identic substance, it must be regarded as the same with the peridot. There is also found a tincture of lime in olivine, which may proceed from the gangart. These gems are remarkable as alone belonging to the Magnesian Domain.
The jacint of Vesuvius, the Vesuvian of Werner, is also found in the lime-stone of Somma; and it has been discovered in Siberia, and in the mountains of the Grisons. Melanite has also been found in the calcareous rocks of Somma. But the latter substance is only to be regarded as imbedded in the rock, and strictly belongs to gemmology.