Copyright 1996 Elsevier. Permission to reproduce these collected papers here granted by Femke Wallien on 5 September 2003. Further reproduction or electronic distribution is not permitted. For further details please visit the Elsevier web page or contact Dr. Wallien, Senior Publishing Editor, Earth Sciences Dept., Elsevier Science BV, Molenwerf 1, 1014 AG Amsterdam, The Netherlands, Tel. +31 20 4852702