graywacke

GRAYWACKE, n. A rock somewhat remarkable in its structure and geological relations; a kind of sandstone, composed of grains or fragments of different minerals, chiefly of quartz, feldspar, siliceous slate and argillite. These fragments are sometimes angular, and sometimes their edges and angles are rounded, thus forming nodules or globular masses. The size is very variable, passing from grains to nodules of a foot in diameter. The several ingredients are united by an indurated argillaceous substance, or the interstices between the larger fragments are filled by the same materials which compose the larger parts of the rock, but in grains so comminuted as to resemble a homogeneous cement. The colors are some shade of gray or brown, as bluish gray, reddish brown. &c.