Graywacke
Graywacke , noun
[German grauwacke; grau gray + wacke wacke. See Gray, and Wacke, and compare Grauwacke.]
(Geology) 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.