Slate

a sort of flaky rock formed from clay by heat and tremendous pressure in the making. Experiments have demonstrated that if a cube of ordinary clay be squeezed in a press, it will separate into flakes lying parallel to the surface on which pressure is applied. The black color of slate must be attributed to the stain, presumably of iron, and to heat. Slate rocks are found in most mountain regions. The best known slates of this country are quarried in eastern New York, Vermont, Pennsylvania, and other localities of the northern Alleghany region. Ordinary roofing slate is made up of sand, aluminum, iron, potash, magnesia, carbon, and water. Split thin and trimmed to uniform dimensions, slate forms an excellent substitute for shingles. If laid carefully a slate roof lasts practically forever. Blackboards are made from a soft quality of slate free from grit. It is quarried in large sheets, ground by machinery to a smooth surface, then cut by a power saw into squares of the required size. Many substitutes for slate blackboards have been offered. Some of them show an excellent surface to begin with, but all lack durability and prove in the end poor economy. Slate-pencils are made in two ways, either by sawing the pencil out of slabs of the stone, or else by grinding fragments into a powder and molding the pencil in a press. See Talc