Graywacke

(-wāk) (Synop., § 130), n. [Ger. grauwacke; grau, gray, and wacke, wacke, q. v.] (Geol.) A conglomerate or grit-rock, consisting of rounded pebbles and sand firmly united together.

☞ 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.

(Geol.), a kind of conglomerate grit-rock, composed of rounded pebbles and sand;—applied to certain hard sandstones, or grit-rocks, and the associated strata, occurring below the coal formation. Dana.