H.J. Zwart, P. Hartman and A. C. Tobi 1984. To Professor Dr. Emile den Tex - on the occasion of his retirement from the chair of Petrology, Mineralogy and Crystallography, which he held at the State Universities of Leiden (since 1959) and Utrecht (1978-1983). Netherlands Journal of Geosciences. (Jan. 1984), 129-130.