The Collaborative International Dictionary of English v.0.48:

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
   and sand firmly united together.
   [1913 Webster]

   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.
         [1913 Webster]