Al'um-slāte, n.
n. (Min.) A variety of shale or clay slate, containing iron pyrites, the decomposition of which leads to the formation of alum, which often effloresces on the surface of the rock. Dana.
n. (Min.) A variety of shale or clay slate, containing iron pyrites, the decomposition of which leads to the formation of alum, which often effloresces on the surface of the rock. Dana.