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3° 2 Siliceous Genus. generally uneven and wrinkled, either fmooth or rough, orten covered vvith a rind, either calcare ous or argillaceous. Its externa! luftre, o or i. Internal, i. Tranfparency, 2.1, fometimes nearly 3* Fradture conchoidal, leldom imperfectly. Fragments, 3. Hardnefs, from 10 to xi. Sp. gr. from 2 38 to 2,63. Heated, it decrepitates,whitens, becomes brittle, and opake, is infufible at r6H°, and is barely foftened by pure air; the ufual fluxes affedt it as they do quartz. The impreffions of marine fhells, and even ofleaves, are frequently found in fiints, which leaves no doubt of their having been produced in the moifl: way, and even thatiome are of modern formation *. According to Mr. Wiegleb’s analyfis, they contam about 0,80 filex, 0,18 argill, and 0,02 calx -f\ Its tranfitions are into quartz, calcedony, car- nelian, and hornftone. It is frequently intimately mixed, not only with quartz or calcedony, but even with calcareous earth, or calcareous fpar, as may be feen in Lelke’s Catalogue, S. p. ij6. * Charpent. Min. Geograph, p. 40. 66. Lefke Reife durch Sacks. Ferb. Oryctogr. von Derbyfli. d 16 1 Naturfoi. &c. t 6 N. Afta. Natur. Curiofa, p. 408. 32d Speciei.
