in a rock yields more information than that provided by the geochemical signature alone because it reflects the origin and history of the element in the rock. The aim of geochronology is to calibrate and standardize chronostrati­ graphic scales, to develop geological time scales that have a sensitive or at least useful resolution in order to place the geological events in the correct chronological order, and to assign their proper time spans. In practice, the application of geochronology is much wider because the data in the "natural archives" often provide information on the origin, genesis, and history of the materials. This, of course, requires an understanding of the geochemical behavior of the substances involved.