H. Kombrink, B.J.H. van Os, C.J. van der Zwan, and Th.E. Wong. “Geochemistry of Marine and Lacustrine Bands in the Upper Carboniferous of the Netherlands”. Netherlands Journal of Geosciences, vol. 87, Dec. 2008, pp. 309 -22, doi:10.1017/S0016774600023374.