Mirror site volunteer: Rodney G.K. Johnston Tel: +44 (0)131 650 8533 (work) Department of Geology & Geophysics, Tel: +44 (0)131 667 8376 (home) The University of Edinburgh, Fax: +44 (0)131 668 3184 Grant Institute, West Mains Rd, Telex: 727442 (UNIVED G) Edinburgh, EH9 3JW, UK E-mail: rodney (AT) glg.ed.ac.uk