Ore conglomerates of the Blind River [Ontario, Canada], area lie at or near the base of the Huronian sedimentary section in discontinuous, sheetlike, partly overlapping, SE.-trending zones within a major NNE. belt. They are part of a sequence of detrital sediments laid down in a beach and delta environment by a northward transgressing sea. The Huronian rocks are cut by basic dikes and by quartz veins, both of which have been dated. The ore conglomerates are older than 1,200 m.y. and probably older than 1,700 m.y. The U minerals, brannerite, uraninite, and a "monazite" complex, occur with a typical detrital assemblage and appear themselves to be detrital. The ore minerals are considered to be syngenetic and of the age of the conglomerates. Age determinations on the ores suggest that the radioactive minerals have been modified, or that material has been introduced, at 1,300 m.y. and at 600 m.y. Differences between the Witwatersrand and Blind River are discussed briefly.