A general term for the property possessed by some rocks of splitting easily into thin layers along closely spaced, roughly planar, and approx. parallel surfaces, such as bedding planes in shale or cleavage planes in schist; its presence distinguishes shale from mudstone. The term includes such phenomena as "bedding fissility" and "fracture cleavage." Etymol. Latin fissilis, that which can be cleft or split. Adj. fissile.