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soluble silicates. Should the waters contain magnesian salts, lime or soda in the spar may be replaced by the isomorphous magnesia so long as the alumina of the spar is not involved in the change, thereby giving rise to steatite 6 CERAMIC CHEMISTRY. and other pseudomorphs, a class so numerous that they cannot be touched on here. The process of weathering very seldom, perhaps never, reaches its last stage that of the formation of pure clay substance, A1 2 3 , 2SiO 2 , 2H 2 0, - but remains in some intermediate position. Thus, besides clay in all its varieties, are formed gravel, detritus, sand, loam, lias. The process may even, through the solvent action of water increased by carbon dioxide, organic substances, salts, and a higher temperature, be changed into its opposite, and the original rocks may appear again as conglomerates, brecchias, sandstones, and shales, and perhaps as crystalline rocks. Clays when indurated become mudstones, and when cleaved, slates. Shales exhibit a fissile character in the direction of lamination, while slates are fissile in parallel planes other than those of bedding. It is unknown what period of time is required for the weathering of granite, which may extend to a depth of a hundred feet in the rock, as all attempts at the artificial decomposition of felspar have met with but moderate success. But glass and pottery ware a few centuries old