Since the end of World War II isotope geology has grown into a diversi fied and complex discipline in the earth sciences. It has progressed by the efforts of a relatively small number of specialists, many of whom are physi cists, chemists, or mathematicians who were attracted to the earth sciences by the opportunity to measure and to interpret the isotopic compositions of certain chemical elements in geological materials. The phenomenal growth of isotope geology during the last 25 years is an impressive indi cation of the success of their efforts. We have now entered into a new phase of...