1. K. A. Farley, Division of Geological and Planetary Sciences, California Institute of Technology, MS 170-25, Pasadena, CA 91125, USA.
2. A. Montanari, Osservatorio Geologico di Coldigioco, 62020 Frontale di Apiro, Apiro, Italy, and Ecole des Mines de Paris, Paris, France.
3. E. M. Shoemaker and C. S. Shoemaker, U.S. Geological Survey, 2255 North Gemini Drive, Flagstaff, AZ 86001, USA.