\Gray"wacke`\, n. [G. grauwacke; grau gray + wacke
wacke. See {Gray}, and {Wacke}, and cf. {Grauwacke}.] (Geol.)
A conglomerate or grit rock, consisting of rounded pebbles
sand firmly united together.
Note: This term, derived from the grauwacke of German miners,
      was formerly applied in geology to different grits and
      slates of the Silurian series; but it is now seldom
      used.