Greywacke

Greywacke (Ger. Grauwacke), a partially translated German word, used as the name of an indurated sedimentary rock, which occurs extensively among the Palæozoic systems, where it is associated with similarly indurated shales and conglomerates. It is an aggregate of rounded sub-angular and angular grains and splinters of quartz, felspar, and slate, sometimes with mica and grains of other minerals and rocks, embedded in a hard paste or matrix, which may consist of siliceous, calcareous, argillaceous, or felspathic matter. The rock is generally harder than most sandstones, and is usually gray or dark blue in colour, but green, red, brown, yellow, and even black varieties are met with. It varies in texture from fine-grained and compact up to conglomeratic and brecciform, and occurs in thick massive beds like liver-rock (see SANDSTONE), and in thinner beds and layers like ordinary sandstones and flagstones. It represents the muddy sediments of the Palæozoic seas, and often retains ripple-marks, sun-cracks, worm burrows and castings, and other superficial markings.