Source : Webster's Revised Unabridged Dictionary (1913)

Graywacke \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.