Shale

Shale, common name applied to fine-grained varieties of sedimentary rock formed by the consolidation of beds of clay or mud. Most shales exhibit fine laminations that are parallel to the bedding plane and along which the rock breaks in an irregular, curving fracture. Shales are usually composed of mica and clay minerals, but the grains are so fine that the rock seems to have a homogeneous appearance, and individual minerals cannot be identified without the aid of a microscope. Most varieties of shale are colored in various shades of gray, but other colors, such as red, pink, green, brown, and black, are often present. Shales are soft enough to be scratched with a knife and feel smooth and almost greasy to the touch. All gradations in consistency exist between shales and clay; true shales differ from clays in their lack of plasticity in water. Many shales yield oil when distilled by heat, and the sedimentary rocks containing larger quantities of oil are called oil shales. Widely distributed throughout the world, oil shales are a source of oil for countries lacking petroleum.