With the growing recognition during the last two centuries that the Earth has an immense age and processes over long periods of time have changed the morphology and composition of the Earth's crust, geologists have become increasingly interested in determination of absolute ages. A rela tive geochronology was established on the basis of the lithostratigraphic and biostratigraphic principles developed during the last century. With the discovery of radioactivity, the basis for a new geoscientific discipline - geochronology - was established (Rutherford 1906). It is the study of geological time,...