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.

&fist; 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.