Journal of the Asiatic Society of Bengal . I fancy, uncommon, and I will describeone of them with its dimensions, in order to give an idea of theproportions of the crystals. The crystal is always twin or composed of two hexagonal plates(fig. 2) two and half inches in diameter between opposite angles, inch thick. Either four or the six edges of the plate arebevelled by oblique facettes, which form with the plane of the surfacean angle of about 138°, so that one surface is considerably smaller * The angles of these crystals were measured with strips of paper anda graduated half circle ; t
Journal of the Asiatic Society of Bengal . I fancy, uncommon, and I will describeone of them with its dimensions, in order to give an idea of theproportions of the crystals. The crystal is always twin or composed of two hexagonal plates(fig. 2) two and half inches in diameter between opposite angles, inch thick. Either four or the six edges of the plate arebevelled by oblique facettes, which form with the plane of the surfacean angle of about 138°, so that one surface is considerably smaller * The angles of these crystals were measured with strips of paper anda graduated half circle ; the crystals were also much weathered ; the resultsare therefore mere approximations. If I had had the means of measuring theangles with precision, I would have figured the crystals. 13 102 Mr. Verchere on tlie Geology of Kashmir. [No. 2, than the other. Two such plates are applied one against the otherby their greatest, surface, but one of the plates has (apparently) rotatedhalf a turn, so that A of one plate is opposite B of the Fig. 2. This rotation is of course only apparent, but it appears to have takenplace from the cleavage of the two plates being opposite, so that whenwe look at a section of the double crystal (fig. 2), one side presentsthe shining striped surface of a lamellar cleavage, whilst the othershows the dull rough surface of a fracture across the grain. Thisopposition of cleavage is probably due to a play of opposite electricitygenerated during crystallization, but it gives the idea of one of theplates having made half a turn before applying itself against itsfellow. The perfect crystal is rarely seen ; it is generally broken across,and the section (fig. 2) is conspicuous on the surface of the rock,so that, at first sight, one may fancy the crystals to be prisms,and a little trouble is necessary to understand the arrangement of thetwin plates. This made is therefore, to all appearance, a twin crystalof one of the numerous modifications of triclinic alb
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Keywords: ., book, bookcentury1800, booksubjectorientalphilology, bookyear1832
