​GREYWACKE, orGrauwacke (a German word signifyinga grey earthy rock), the designation, formerly more generallyused by English geologists than at the present day, for impure,highly composite, gritty rocks belonging to the Palaeozoicsystems. They correspond to the sandstones, grits and fineconglomerates of the later periods. Greywackes are mostlygrey, brown, yellow or black, dull-coloured, sandy rocks whichmay occur in thick or thin beds along with slates, limestones, &c.,and are abundant in Wales, the south of Scotland and the Lakedistrict of England. They contain a very great variety ofminerals, of which the principal are quartz, orthoclase andplagioclase, calcite, iron oxides and graphitic carbonaceousmatters, together with (in the coarser kinds) fragments of suchrocks as felsite, chert, slate, gneiss, various schists, quartzite.Among other minerals found in them are biotite and chlorite,tourmaline, epidote, apatite, garnet, hornblende and augite,sphene, pyrites. The cementing material may be siliceous orargillaceous, and is sometimes calcareous. As a rule greywackesare not fossiliferous, but organic remains may be common inthe finer beds associated with them. Their component particlesare usually not much rounded by attrition, and the rocks haveoften been considerably indurated by pressure and mineralchanges, such as the introduction of interstitial silica. In somedistricts the greywackes are cleaved, but they show phenomenaof this kind much less perfectly than the slates. Although thegroup is so diverse that it is difficult to characterize mineralogically,it has a well-established place in petrographicalclassifications, because these peculiar composite arenaceousdeposits are very frequent among Silurian and Cambrian rocks,and rarely occur in Secondary or Tertiary systems. Theiressential features are their gritty character and their complexcomposition. By increasing metamorphism greywackes frequentlypass into mica-schists, chloritic schists and sedimentarygneisses. (J. S. F.)