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3°8 Siliceous Genus. becaufc it is flaty, fchiftofe porphyry, However, the felfpar is ofen fo minute, that it can fcarce be diftindtly perceived ; and in fuch cafes the ftone may be placed here, and not among aggregate ßones, and may be called hornflate. In the bluifo the diilindtion of blue and white is moft perceptible. Generally amorphous, and in large mafles, fometimes columnar. Its luftre, moftly o; yet in a ftrong light a few fbining particles may fometimes be perceived. Its tranfparency, i.o. Its fradture, on the whole, flaty, but of the fingle laminar, which are often very thick; the fracture js uneven, fplintery, and fometimes ap- proaching to the conchoidal. Its fragments, 2.3. Hardnefs, nearly 10. Sp. gr. from 2,31210 2,700, by my trials. The fpecimen in Voight’s Catalogue, marked 55, and in Lefke’s S. 645, which is afh grey, and whofe fp. gr. is 2,5122, melted at 145 0 , into a dark grey, almoft compadt enamel. The fpe cimen, Lelke S. 973, which is bluifh grey, and in the form of a quadrangular prifm, its fp. gr. 2,693, melted at 166°, intb a greenifh yellow Ipongy mafs. ßy Mr. Wiegleb’s analyfis, the alh grey, Voight 55, contains 0,73 filex, 0,239 argill, and 0,03 5 iron *. As the term hornflate, or hornfchiefer, has been applied to various forts of ftones, Mr. Hoepfner, of Switzerland, generoufly offered a premium to the perfon who fliould clearly afcertain its pre- lent fignitication: fatisfaäory anfwers were accor- * 1 Chym. Ann, 1787, p. 302. dingly