1. Department of Geology, University of Vermont, Burlington, Vermont 05405, U.S.A.
2. Department of Geology and Environmental Earth Sciences, Miami University, Oxford, Ohio 45056, U.S.A.
3. GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany
4. Department of Geology, University of Johannesburg, P.O. Box 524, Auckland Park 2006, South Africa