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216 QUARTZ FAMILY. very seldom kidney-shaped. The surface of the rolled pieces is rough, and reddish-brown. Ihe fracture is perfect conchoidal; in the reniform varieties it is fibrous. Ihe lustre is glistening, sometimes passing into shin ing, and is vitreous. 1 he fibrous varieties occur in concentric lamellar con cretions. It is generally semi-transparent; seldom translucent. It is hard, but in a lower degree than common calce- dony and flint. Specific gravity, 2.320, 2.581, Karsten. 2.606, 2.624, Brisson. Chemical Characters. It is infusible without addition. Constituent Parts. Silica, - 94 Alumina, - 3.50 Iron, - . ().75 100 Bindheim, Geognoslic and Geographic Situations. It frequently occurs as a constituent part of agate, and in general has the same geognostic situation as common calcedony. The floetz-trap rocks so abundant in Scot land, often contain carndian, either alone, or in agate. The most beautiful carnelians, viz. those having an uni form blood-red colour, are found in rolled pieces, and are brought to this country from Arabia, India, Surinam, Siberia, and Sardinia: less beautiful varieties arc found in