Editing Greywacke (section)

=== Varieties ===
The term "greywacke" can be confusing, since it can refer to either the immature (rock fragment) aspect of the rock or its fine-grained (clay) component.

Greywackes are mostly grey, brown, yellow, or black, dull-colored sandy rocks that may occur in thick or thin beds along with [[shale]]s and [[limestone]]s. Some varieties include ''feldspathic greywacke'', rich in [[feldspar]], and ''lithic greywacke'', rich in other tiny rock fragments. They can contain a very great variety of [[mineral]]s, the principal ones being quartz, [[orthoclase]] and [[plagioclase]] feldspars, [[calcite]], [[iron oxide]]s and graphitic, carbonaceous matters, together with (in the coarser kinds) fragments of such rocks as [[felsite]], [[chert]], [[slate]], [[gneiss]], various [[schist]]s, and [[quartzite]]. Among other minerals found in them are [[biotite]], [[Chlorite group|chlorite]], [[tourmaline]], [[epidote]], [[apatite]], [[garnet]], [[hornblende]], [[augite]], [[sphene]] and [[pyrite]]s. The cementing material may be [[siliceous]] or [[argillaceous minerals|argillaceous]] and is sometimes [[calcareous]].{{sfn|Flett|1911}}