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From 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]