1. Ashley G. H. (1928) Bituminous Coalfields of Pennsylvania, Pennsylvania Topographic and Geologic Survey, Series 4, Bull. M–6, Part 1. 24.
2. Kaolin tonsteins in the Westphalian of North Staffordshire;Barnsley;Geological Magazine,1966
3. Bohor B. F. Triplehorn D. M. (1981) Guidebook, Geological Society of America, Annual Meeting, Coal Division Field Trip, Coal and Coal-Bearing Rocks of Eastern Kentucky Volcanic origin of the flint clay parting in the Hazard No. 4 (Fire Clay) coal bed of the Breathitt Formation in Eastern Kentucky (Kentucky Geological Survey, Lexington), pp 49–54.
4. Bohor B. F. Triplehorn D. M. (1993) Tonsteins: Altered Volcanic Ash Layers in Coal-bearing Sequences, Geological Society of America, Special Paper, 285. 44.
5. Fréquence des manifestations volcaniques au Carbonifère supérieur en France;Bouroz;Comptes Rendus Sér. D Académie des Sciences, Paris,1966