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Uarnßone Porphyry. 351 Hornftone Porphyry. Thefe are diftinguilhed by their hardnefs, and a conchoidal fine fplintery frafture, their ilight tranfparency, and want of luftre; they are aifo not difficultly fufible; of this fortis the porphyry. 1 Sauff. §. 150, 156, and 157. And manymore in Lelke G. p. 25» & c ’ Sometimes the felfpar is decayed, and fome- times the hornftone alfo, while the quartz and hornblende remain entire; this gives the whole the appearance of indurated volcanic alhes, though the quartz might prove the contrary; if the felfpar alone be decayed, the hornftones will appear porous, and may be taken for a lava. See Lelke G. p. 26, Scc. Its tranfitions are into granite and fandftone. Pitchllone Porphyry. Lelke S. 1779. Its colour, black, grey, green, red, or yel low, of various ftiades, generally many colours at once in the fame fpecimen. Luftre, greafy 2.1. Tranfparency, 2.1. Fradure, imperfedly con choidal. Hardnefs, 8, 9, 10. The felfpar ofcen blue. See Liebenroth Beobacht 4 2. Of tlns fee a remarkable fpecimen, Lelke S. 950. Its hardnefs, 10. Luftre, 1. _ Of fome the fradure is flaty, as for mftance, that in Lelke S. 1779* Ics col ° ur ’ y ellowlfh grey. Luftre, fcarcely t. Tranfparency, 1. Hardnefs, fcarcely 9. Sp. gr. 2,452.