SiUcecus Genus, 24r sbction X. Siliceous Genus. i ft Specie«. ■ Mountain Cryftal and Quartz. These form two families, the only diffe- rcnce being in thcir fra€lure and tranfparency. ift Family. Mountain or Rock Cryftal. Whcn pureft it is colourlefs, but it is alfo found. greyilh, and yellowifb white, or pale yellow, otherwife called Citrine, and impure topaz; or pale or dark brown, or brown falling into black, then called Marion, or, though veiy rarely, pale rote red, or white, or brown, and iri- dcfcent. Its luftre glafly, 3. Tranfparency, 4. It caufes a double refraction. R The