Shale

Shale, a laminated sedimentary rock, typically argillaceous, but often either sandy, calcareous, carbonaceous, or bituminous. Shales split into very thin laminae parallel with the bedding of the rock. They may be the result of separate acts of intermittent deposition, as in the inundation-mud of the Nile, and are often indications of shallow waters with varying sediments, as in the paper-shales and associated beds of the Penarth (Rhaetic) series. They thus frequently mark transitions to pure sandstones or limestones, as in the Ledbury Shales (sandy) below the Old Red Sandstone and the Tuaedian, or Lower Limestone Shale below the Carboniferous Limestone. Most Palaeozoic argillaceous beds are shales, as in the Wenlock and Ludlow Shales, probably the result of the vertical pressure from the weight of superincumbent rock. The roof of most coal-seams is formed of shale (tbe "slate" of our coal-scuttles). Bituminous shales, from which paraffin is distilled, as in Lanarkshire, are recognisable by smell, by brown stains on a black surface, and by rolling up when pared.