The study of natural radioactivity has experienced a substantial revival in recent years. One reason is the general increase in importance and popu larity of all aspects of nuclear science, with this branch partaking of its share. The practical importance as raw materials for nuclear energy of uranium and thorium, which are the parents of most of the known natural radionuclides, has stimulated a study of the properties and occurrences of the latter. It has also been realized that natural radioactivity is a phenome non of considerably greater variety and more general incidence than had been believed previously (339,340,379). .The well established and newly discovered geological ramifications of the occurrence of radioactivity in nature have induced considerable numbers of earth scientists to employ nuclear tools in their studies. The natural occur I The preparation of this paper has been assisted by the United States Atomic Energy Commission through its support of nuclear chemistry research at Carnegie Institute of Technology. ! The following abbreviations are used in this chapter: dis. (disintegration); m.y. (million years) ; B.P. (before present) .