The compilation of data on uranium and thorium deposits in various western states has shown that many of these deposits are closely associated with zones of secondary radioactive iron minerals. A study of these radioactive "limonites" was therefore undertaken in an attempt to find out in what form the radioactive material is present in the limonite, how it was introduced, and whether a field study of leached ferruginous cappings over uranium and thorium deposits might yield information about the depth, distribution, size, and grade of underlying ore bodies. This investigation consisted largely of a detailed study and analysis of many samples generously contributed by others--samples containing from 0.001 to 0.7 percent uranium, from 0.003 to 5.3 percent equivalent uranium, and from 0.001 to 4.16 percent ThO 2 .It is concluded that uranium minerals in an oxidizing sulfide environment go into solution in acid sulfate waters as uranyl sulfate in the presence of ferric sulfate. When these acid waters are neutralized, ferric sulfate hydrolyzes to form colloidal ferric oxide hydrate. This adsorbs the uranyl ion and thus removes most of the uranium from solution. As the colloidal ferric oxide hydrate ages, it crystallizes to form goethite and in this process most of the uranium is expelled to form particles of secondary uranium minerals in the resulting limonite. Most thorium minerals are resistant to weathering and remain in their original form in thorian limonites.