1. C. Koeberl, Institute of Geochemistry, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria.
2. C. W. Poag, U.S. Geological Survey, Woods Hole, MA 02543, USA.
3. W. U. Reimold and D. Brandt, Department of Geology, University of the Witwatersrand, Johannesburg 2050, South Africa.