SLATE (OF. esclat, Fr. eclat, splinter, frag ment, from OHG. slizan, Ger. sohleissen, AS. slitan, Eng. slit). A hard, fissile rock which has been produced from shale or clay by metamorph ism. This process by means of heat and pressure consolidates the original rock and obliterates the original stratification, developing new lines of parting or cleavage planes along which slate splits easily and in thin layers. Many fine grained shales which split readily along the bed ding planes are erroneously called slate, but true slate is a very hard and compact rock, little likely to be acted on by the weathering agencies. Owing to its origin, slate is found only in regions of metamorphic rocks, and therefore its geo graphical distribution is somewhat restricted. Its geological range extends from the Cambrian to the Slate is commonly bluish black or gray black in color, but red, green, purple, and variegated varieties are known. In the *United States the most important slate quarries are in Maine. Vermont, New York, Pennsylvania, Maryland, and Georgia. A small production is also made in California and Minnesota. The sup ply of slate in Europe is derived mostly from Wales and France.
Slate was formerly quarried by blasting, hut at present it is generally extracted in large blocks by means of a. channeling machine, simi lar to that used in quarrying marble. The rock splits best when it is green or freshly taken from the quarry. Slate is extensively employed as a
roofing material and for sinks, washtubs, bil liard-table tops, electrical switchboards, floor ing, mantels, blackboards, school slates, pencils, and in acid towers. In roofing it is necessary to lay the slates in two thicknesses, so that the slop ing joints may be covered by the overlap of the course above, and the third course must also cover the first by an inch or two, to prevent rain from penetrating. Slate for interior decoration is subjected to a process called marbling, which consists in coating it with materials which give the surface a veined appearance like marble.
The value of the slate produced in the United States in 1901 was $4,7S7,525. Most of this product was used for roofing purposes. The ex ports amounted to $898,262, a large part being shipped to Great Britain.
BIBLIOGRAPHY. Merrill, Stones for Building Bibliography. Merrill, Stones for Building and Decoration (New York) and "The Strength and Weathering Qualities of Roofing Slates," Transactions of American Society of Civil Engi neers, September, 1392, and December, 1894; Dale, "The Slate Belt of Eastern New York and Western Vermont," in Nineteenth Annual Report of United States Geological Survey, pt. iii. For statistics, see volumes on Mineral Resom•ces is sued annually by United States Geological Survey.
Sec METAMORPHISM; SHALE; BUILDING STONE.