GEOCHEMISTRY OF THE EARTH SURFACE - ABSTRACTS FOR THE 3RD INTERNATIONAL-SYMPOSIUM ON THE GEOCHEMISTRY OF THE EARTH SURFACE SPONSORED BY THEAL-ASSOCIATION-OF-GEOCHEMISTRY-AND-COSMOCHEMISTRY, THE UNITED-STATES NATIONAL-SCIENCE-FOUNDATION AND THE PENNSYLVANIA-STATE-UNIVERSITY (EARTH-SYSTEM-SCIENCE-CENTER) WHICH WILL BE HELD AT THE PENNSYLVANIA-STATE-UNIVERSITY, UNIVERSITY PARK, PENNSYLVANIA, USA ON AUGUST 1-6, 1993 - INTRODUCTION