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Struvian Clay. 203 Another ftone, feemingly of this kind, though differingin Tome refpedts, was annalyzed by Mr. Klaproth, its colour was whitilh grey; its frac- ture, from a number of horizontal rifts, feemed flaty; to the touch it was meagre, and adhered ftrongly to the tongue. Its fp. gr. 2,080; it im- bibed water ftrongly. He found it to contain 0,665 filex, 0,07 argill, 0,025 calxof iron, 0,015^ magnefia, 0,0125 calx, an ^ air and^ water. - 2 Chy. An. 1790, 302. It is often of pfeudo-volcanic origin, as appears from the Paris Memoirs, of 1769, 276; and per- haps alfo fometimes really volcanic; but it alfo frequently arifes from the decoinpofition, or dif- integration, of other ftones. 2d Family. Pouzzolana, orTerras. Of this I fliall treat underthe head of Volcanic Produdts. Struvian Clay *. This is a fort of clay lately noticed, and de. fcribed, by Profeffor Struvius, and found in thofe elcvations that afford falt fprings, and therefore worthy of particular attention. Colour, dark bluifh grey, inclining to black. Found in confidcrable mafles. * 2 Bergm. Journ. 179t» 244« Luftre
