Introduction. ____________________ _ _______________ Acknowledgments __ ___________ _ _________________ What is "black shale"? ____________________ What is "uraniferous black shale"? __ _______________ Historical review of investigations for uraniferous black shales. ______________________________ _ _________ Investigations, 1944-47 _ _______________________ Investigations, 1947-52 _ ______ _ __________ _ _ Investigations, 1952-57 _ _______________________ Summary.-._ __________________________________ Sources of uranium and mechanisms of its enrichment. __ Examples of uranium-bearing shale and mud_______ Sources _ _____________________________________ Sources and chemical form of uranium in sea water. __________________________________ Uranium content of modern sea water_________ Uranium content of water in ancient seas______ Influence of source rock on uranium content of water in bordering seas______-_-____-____ Mechanisms of enrichment _ ________________ _ __ Organic matter.... _________________________ Amounts and types in uranium-bearing shale and mud_ ____ ______ _ Solid organic matter________________ __ Humic acids. __________________________ Uranium incorporated by living plants or animals __ _________________________ Hydrogen sulfide __________________ ______ Precipitation of uranium __ _____________