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2^8 Argillacecus Genus. It often prefents fmall or coarfe-graincd diflindt granulär concretions. Hardnefs from 8 to 9, and difficultly broken, except when in a ftate of decompofition, and then orten only 5 or 6. Sp. gr. when found lies be- tween 2,78 and 3,021 exclufively, by my trials; the fpecimen that exhibited the laft is that in Leike S. 1695, whofe colour is reddifh brown, and has feveral red grains in it. The fpecimen Le/ke 1223, which is greyifh black, has its fp. gr. 2,728, but then jt is not pure; for, at 148°, it melted only into a fomewhat porous liver-brown mafs, hetween a.porcelain ahd anenamel. The fpecimen Lefke G. 264, which is bluifh qr greenifh bläck, has its fp. gr. 3,478; but then, as Mr. Karften obferves, it contains a qnantity of mag- nette iron ftone. The fp. gr. of the ftone calltd trap by Mr. Briffon is 2,745, and of that which he calls bafaltof an irregulär form or touchftone, • 2 >4i53; buc thismoft probably is that fort of fihceous fchiftus, called- lydian ftone, of which hereafrer. 1 he fp. gr. of t'his ftone muftundoubt« ed ly vary with its ftate and degree of decompo fition, and with the quantity and fort of fbreign ingredients contained in it, as well as the propor- tion of its own conftituent parts; but we may ftate that of the pureft at a medium 2,87 or 8 8* By expofure to the influences of the atmo- fpherfe,' and often by mere moifture, this ftone is frequently decompofed, and invefted with a brown, yellovviil\, or reddifh brown rind. _ Expofod to a heat of from 120° to 130° it melts into a black compadt glafs. 4 • According
