Shale (Ger. schälen, to peel, to split), a rock composed of clayey sediments consolidated in layers which are fissile like the original clay, but not often divisible into smooth sheets like the argillaceous slates. Beds of shale are common throughout the range of the secondary rocks, and constitute a large portion of some of the formations. They alternate with the sandstone and other strata of these formations, and in the coal measures are abundant in beds blackened by the carbonaceous matters intermixed with the clayey sediments. In the red sandstone groups they are commonly also red from oxide of iron, and in other formations they are olive, and sometimes green and variegated. They are soft and earthy, and are easily worn down into a muddy powder. By intermixture of carbonate of lime they become calcareous, and as the proportion of this increases they pass from calcareous shale to argillaceous limestone. Sand renders them arenaceous, and with excess of it they become sandstones. Carbonaceous matter renders them bituminous, and when the proportion is large the material is used for some of the purposes of coal.
Shales of this character yield oil. (See Coal, and Peteoleum).