Greywacke

Greywacke, a half-translation of the German grauwacke, the name of a rock, generally grey, though sometimes black or brownish, compact, with rounded or subangular grains and fragments of quartz, felspar, slate, etc., in a cement, usually siliceous, but sometimes argillaceous, calcareous, or anthracitic; though, when fine-grained, it resembles crystalline rocks, greywacke is of aqueous origin. It is sometimes ripple-marked or sun-cracked, being, in fact, a Paleeozoic near-shore deposit; but it is much indurated by pressure and often cleaved or otherwise altered. It forms so large a part of the Cambrian, Ordovian, and Silurian systems (q.v.) that at one time they were known as the Greywacke series.