The Collaborative International Dictionary of English v.0.48:

Slaty \Slat"y\, 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]