[gray.wacke] n [partial trans. of G Grauwacke] (1811): a coarse usu. dark gray sandstone or fine-grained conglomerate composed of firmly cemented fragments (as of quartz or feldspar)
GRAYWACKE
Meaning of GRAYWACKE in English
Merriam-Webster English vocab. Английский словарь Merriam Webster. 2012