indurated by recrystallization, such as the introduction of interstitial silica. In some districts the greywackes are cleaved, but they show phenomena of this kind much less perfectly than the slates. Some varieties include feldspathic greywacke, which is rich in feldspar, and lithic greywacke, which is rich in tiny rock fragments. Although the group is so diverse that it is difficult to characterize mineralogically, it has a well-established place in petrographical classifications because these peculiar composite arenaceous deposits are very frequent among Silurian and Cambrian rocks, and are less common in Mesozoic or Cenozoic strata. Their essential features are their gritty character and their complex composition. By increasing metamorphism, greywackes frequently pass into mica-schists, chloritic schists and sedimentary gneisses. Role in the history of geology Greywacke, a common rock in Britain, was an object of study in Britain where the Geological Society was founded in 1807 and excited much public interest in geology. Greywacke was interesting because it was found in many places in Britain and its occurrence in particular places was evidence of the pattern of geological strata that had been laid down. See The Greywacke: How a Priest, a Soldier and a School Teacher Uncovered 300 Million Years of History. See also Greywacke zone Torlesse Greywacke References External links National Park Service site Presidio Franciscan Greywacke/Shales Sedimentary rocks Sandstone