1U.S. Geological Survey, Box 25046, MS 913, Denver Federal Center, Denver, Colo 80225 USA
2U.S. Geological Survey, MS 981, National Center, Peston, Va. 22092 USA
3U.S. Geological Survey, MS 971, National Center, Reston, Va. 22092 USA
4U.S. Geological Survey, MS 937, Menlo Park, Calif. 94025 USA
5Department of Geology, Grand Valley State College, Allendale, Mi. 49401 USA