1. Y. Chen, P. E. Smith, N. M. Evensen, D. York, Department of Physics, University of Toronto, 60 Saint George Street, Toronto, Ontario M5S 1A7, Canada.
2. K. R. Lajoie, U.S. Geological Survey, 345 Middlefield Road, MS-977, Menlo Park, CA 94025-3591 USA.