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l ° z slrgtilaceous Genus. Sturbridge clay feems of this fort. Colour, dark grey ; does not adhere to the tongue. I found ioo parts of it in the flate of drynefs in which it comes to us to contain j 2,5 per ct. of moifture, 12 of coarfe fand, partly white and cryltalhne, and partly brown and reddifh brown and 30 per ct. of a brownifh fand, fo fine as lcarcely to be difcerned through a lens. The ar« gillaceous part was grey, and contained ftill more iand, which could not be feparated but bv boilmg in acids. 4 Water quickly adfs upon it, a white argilla- ceous part foon diffufes itfelf, and afterwards ano- ther part which is darker falls into powder* hence it fhould feem there are two forts of indu* rated clay. The fandy, and the clayey, ^d Family. Shiftofe Clay. ift Variety. Slate Clay, Shalc. Schiefer Thon of Werner. Colour, gcnerally bluifh, or yellowifli grey more rarely dark blackifh, or reddifh grey, or greyifh black, or greenifh yellow. Often found in ftrata, particularly in coal-mines, of which it is almoft a fure fore-runner; it commonly bears knobby 16 impreffions * Its furf ace general ly Luftre,
