Subgraywacke

Dark-coloured sedimentary rock that contains from 65 to 95 percent free quartz, in grains 0.06 to 2 mm in diameter, held together by a matrix with a low mud content and often a high carbonate content. Some geologists favour a definition of graywacke (q.v.) that permits no more than 75 percent free quartz in the rock, and they thus would class as subgraywackes those rocks with (1) over 25 percent