Uraniferous reef-type quartz pebble conglomerates of Lower Huronian age, north of Lake Huron, and disseminated uraninite-bearing pegmatites in the Greenville sub-province of the Canadian Shield have been under investigation in Ontario during the past two years. Very large tonnages of low-grade ores have been disclosed, particularly in the reef-type conglomerates. Owing to the fine state of division of the uranium minerals, the discovery of such deposits is dependent, in a large measure, on radio-metric prospecting methods. These recent developments lead to the conclusion that increasing numbers of commercially important deposits will be found under analogous geological conditions elsewhere in eastern Canada and in corresponding Precambrian formations in the United States. The development of such deposits for production is dependent upon integrated mining and metallurgical extraction operations requiring high capital costs, and the feasibility of commercial production, in many instances, will be determined by Government purchasing policy.