1. Department of Geosciences, Wellesley College, Wellesley, Massachusetts 02481 U.S.A.
2. Department of Earth Sciences, Brock University, St. Catharines, Ontario, Canada L2S 3A1
3. Department of Geological Sciences, Salem State University, Salem, Massachusetts 01970 U.S.A.
4. Weston Observatory, Boston College, Weston, Massachusetts 02493 U.S.A.
5. Now at Department of Geosciences, University of Rhode Island, Kingston, Rhode Island 02881 U.S.A.