A microcrystalline, metamorphic rock commonly derived from shale, composed mostly of micas, chlorite, and quartz. (ASTM C1528) The micaceous minerals have a subparallel orientation and thus impart strong cleavage to the rock which allows the latter to be split into thin but tough sheets. (ASTM C119) Includes shale, a laminated, indurated rock which is over two-thirds clay-sized minerals. Shales progressively grade into slate. (ASTM C119)