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Silicesus Genus. The amorphous fort prefents thln, fireight, dif- tinft, columnar concretions; fometimes paralell, fometimes diverging or ftelliform, ftreaked, and eafily feparable from each other; very feldom ir regulär granulär concretions. Its fradture conchoidal, verging to the uneven ; and hence often refembling the ftriated. The longitudinal frafture, foliated. r Its fragments, 3. Gives a grey ftreak. Its hardnefs, 10. Its fp. gr. from 2,92 to 3,212. Whenheated to rednefs, it alters its colour in cooling, to a brownifh red; at 46°, it becomes re,dder; at 63°, it does not melt; but at 127°, it is converted into a brownifh black compaft ena- ruel; at 147 0 , it becomes more porous, and its furface fomewhat redder. I found its fp. gr. af- ter expofure to that heat 2,143, from thenunj« ber of fmall pores. The word fhorl fo frequently occurs in mine- ralogical writings, and has been fo often applied to ftones of various fpecies, notonly by different, but often even by the fameauthor, that it becomes neceffary to point out the different fenfes in which it occurs amongft the moft eminent of thofe Writers. The term itfelf is derived from the Swedifh ßorl, brittle, and was firft ufed by (Jronftedt to denote a clafs of ftones of a columnar form and conf^lerable hardnefs and denfity; their fp. gr. beingfrom 3103,4. But it unfortunately hap- per.ed, this clafs of ftones comprehended many of different fpecies, fuch as baialts, a&inolites, &c. And, as he tranflated this word into Latin 7 ty