a. [ From Slate. ] Resembling slate; having the nature, appearance, or properties, of slate; composed of thin parallel plates, capable of being separated by splitting; as, a slaty color or texture. [ 1913 Webster ]

Slaty cleavage (Min.), cleavage, as of rocks, into thin leaves or plates, like those of slate; -- applied especially to those cases in which the planes of cleavage are not parallel to the planes of stratification. It is now believed to be caused by the compression which the strata have undergone. --
Slaty gneiss (Min.), a variety of gneiss in which the scales of mica or crystals of hornblende, which are usually minute, form thin laminae, rendering the rock easily cleavable. [ 1913 Webster ]