Keyword "VOLCANIC INTERLAYER" in:
Volcanogenic interlayer (tonstein) in the Main coal seam of the Nýřany group of seams (Asturian) in the Plzeň region
Pešek,J., Drábková,J., Martinec,P., Sýkorová,I., volume 44, 2011 (GRR for 2010), pages 36–38
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