Department of Earth Sciences, University of Bristol. Wills Memorial Building, Queens Road, Clifton. BRISTOL BS8 1RJ, ENGLAND. UK Department of Cell Biology and Anatomy, School of Medicine, Jonh Hopkins University. 725 North Wolfe Street, BALTIMORE MD 21205-2196. USA E-mail: dhende@jhmi.edu artigos/papers GAIA Nº 15, LISBOA/LISBON, DEZEMBRO/DECEMBER 1998, pp. 219-226 (ISSN: 0871-5424)