SHALE (Ger. Sehale, OHG. seala, AS. secalu, shell, husk, scale; connected with (1Church Slay. skolika, mussel. Lith. skclti, to split). An in durated clay consolidated chiefly by the pressure of overlying sediments. It often forms heavy beds in many geological formations. In the Carboniferous formation shale beds of slaty ap pearance are frequently associated with the coal and are erroneously termed slate by the miners. Shale varies considerably in composition and color, and this variation exerts an important in fluence on its uses. When ground and mixed with water many shales become as plastic as ordinary surface clays. Some approach kaolinite in composition. and are very refractory, being used in the manufacture of fire-brick. Others contain an abundance of impurities such as iron oxide and lime carbonate. The former are most ly employed in the manufacture of common brick, unless the percentage of iron oxide is high, when they lend themselves more readily to the manu facture of mineral paint. Calcareous shales are often valuable as an ingredient of Portland ce ment. The gray or black color of shale is usually caused by the presence of carbonaceous matter. and there may he a notable quantity of bitumen.
When there is sufficient bitumen present so that the mineral crackles and blazes in the fire, emit ting a black smoke and bituminous odor, it is known as bituminous shale. This variety some times passes into coal. When shale is metamor phosed it changes to slate, or by more intense metamorphism into schist. The slate splits along its cleavage planes, and not along the planes of stratification as in the ease of the shale. By an increase in sandiness shale may pass into sand stone, or (by an increase of lime carbonate) into limestone.
The value of certain decomposed shales, through which iron sulphide is disseminated for the manufacture of alum, has been long recog nized. Such shales are known as alum shales. Shales of this kind are worked in Great Britain, France, and Germany.
Bituminous shales have, in recent years, at tracted much notice as sources of oil for il luminating purposes. Such shales, which com monly occur in beds of Carboniferous age, have been found upon trial to yield from 30 to 50 gallons of crude oil per ton. A large industry based upon the distillation of shales has been established ill Scotland. See PETROLEUM; CLAY.