- GeoRef, Copyright 2004, American Geological Institute. Reference includes data from Bibliography and Index of Geology Exclusive of North America, Geological Society of America, Boulder, CO, United States
"Kaolinite-mudstone bands, widely recognized in the Carboniferous of western Europe, where they are known as tonsteins, are also represented in Britain. On the basis of the probable correlation eight such bands have so far been found in the Coal Measures of the east Midlands. Laterally two of these bands are thought to pass into rocks intermediate in type between tonstein and more normal Coal Measures mudstones."