Dictionary.net

A rock of a grayish green colour nearly allied to basalt, of which it may be regarded as a softer and earthy variety See Graywacke.

A German miner's term for a soft earthy variety of trap-rock of a greyish-green colour, resembling indurated clay, and readily crumbling down when exposed to the weather.

n. [German]A rock allied to basalt, of which it may be regarded as a more soft and earthy variety. Graywacke, a kind of conglomerate gritrock.