- GeoRef, Copyright 2008, American Geological Institute. Reference includes data from Bibliography and Index of Geology Exclusive of North America, Geological Society of America, Boulder, CO, United States
"Mineral, chemical, and textural compositions of classical graywackes from the Harz mountains (Germany) and of classical arkoses from the Auvergne (France) are discussed. The type graywackes cannot be defined by the presence of a matrix; nor does the presence of granitic rock fragments or the absence of other rock fragments characterize the type arkoses. Both rock types may be distinguished only by their total mineral and chemical compositions."