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OPAL. 251 green with red or yellow spots. Chalcedony in alternate layers of different colours is called onyx. In irregular layers, or con taining dendritic markings, it is called agate. Flint is a variety of chalcedony found in nodules in the upper chalk. Jasper, flinty slate, and homstone, are variously-coloured mixtures of quartz with alumina, lime, carbon, oxides of iron, manga nese, &c. Quartz occurs as an essential constituent of various rocks, as granite, gneiss, mica slate, topaz rock. The finest crystals are found in the mountains of Switzerland, the Tyrol and Salzburg, Dauphine, Madagascar, Ceylon and the Brazils ; smaller crystals at Quebec; implanted in drusy cavities in granular limestone at Carrara in Italy; isolated in the county of Marmarosch in Hungary; of a brown colour in many places in Bohemia, in the Morne mountains in Ireland and in Siberia; of a pale violet-blue m Hungary, the Tyrol and Siberia; the purple variety called amethyst in Hungary, Porkura in Transylvania, Siberia, the ■Brazils, in many parts of India and Persia, in pebbles in Ceylon; in the Ilarz, Saxony, Silesia, Scotland, Spain, &c., in veins, and in agate balls. A rose-red variety is found at Rabenstein near ihviesel in Bavaria, and in Siberia ; of paler colour at Konigs- werth in Bohemia; milk-white in Norway, Greenland, Spain, France. Chalcedony is found in Trevascus mine in Cornwall, Hay tor in Devonshire (pseudomorphous after datholite), Scot land, Greenland, Iceland, the Faroe islands. Agate in the cavities of amygdnloidal rocks, at Oberstein, near Vicenza, in Hungary and Transylvania, Chemnitz, Freyberg, Schneeberg in Saxony, and in various places as the substance of petrifactions. Carnelian in Arabia, India, Surinam, Siberia. Flint in nodules and small beds, enclosing sponges, alcyonia, echinites and other fossils, is found in the chalk formation in the islands of Hiigen, Moen, Jutland, England, the north of Ireland, Cham pagne in France, the south of Russia, &c. The forms d, ??, ?, 0, £ were observed by Mr. Brooke in crystals in his collection. For the description of the other forms, the editors are indebted to G. Rose’s Memoir on the Crystallization of Quartz, in the ‘ Berlin Transactions for 1S44,’ communicated to them by the author. 128. OPAL.—Opal; Phillips. Quarz rdsinite; Hauy. Un- theilbarer Quarz ; Molis. Opal; Hausmann, Haidinger. Amorphous. Fracture conchoidal. Transparent...trans lucent. Lustre vitreous, in some varieties inclining to resinous. Colourless, white, yellow, red, brown, green, grey, black. Streak ii C
