subgraywacke

Noun

 * 1)  The most common type of sandstone, intermediate in composition between orthoquartzite and graywacke, typically composed of 30% to 65% quartz and chert, less than 15% clay matrix, more than 25% unstable materials such as feldspar grains and rock fragments, and voids or mineral cement exceeding the amount of clay matrix, and being lighter colored and better sorted, and with less matrix, than graywacke.