3*6 Siliceous Genus. Its hardnefs io._ The fp. gr . of different fpecimens vvhich I tried extended from 2.04c to 2,675. It is commonly (not always) the fubftance of petrified wood. A ftump of a tree fix feet in length, and as many in diameter, with roots and branches thus petrified, was lately found near Chemnitz, in Saxony. It often withers by expofure to the atmo- fphere. Its tranfitions are änto quartz, calcedony, and, as fome fay, into pitchftone or opal. 37th Species. Elaftic Quartz. Its colour is greyifh white, covercd with thin fcales of grey, or brownilh mica. Its luftre, o, or 1, from a few micaceous par- ticles difperfed throngh it. Tranlparency, 1. Its frafture I could not well dilcern, as I was rot at liberty to break the fpecimen ; but from a fmail accidental fradture, it feems earthy. Its hardnefs, 9. Krittle. Specific gravity, ac- cording to Mr. Gerhard, Mem. Berl. 17X3, p . 10 7> * s 3)75°» buthis ftone muH be vpry diffe rent from that poffeffed by the Royal Irifh Aca demy; the fpecific gravity of this being only 2,624. Flexible and elaftic in a moderate degree in every diredtion. Phofphorefces when fcraped with a knifc in the dark. According