Exploration in the lignite basin in western North Dakota and eastern Montana followed discovery of trace amounts of uranium in Paleocene lignites. Proved and probable reserves of more than 200,000 tons of lignite containing more than 0.10 percent uranium oxide have been developed.Bedded and lenticular deposits in both lignite and sandstone beds were concentrated by circulating ground water.Most of the uranium is in complex carbon-uranium compounds. Metatyuyamunite and autunite are common secondary minerals. Ore deposition is controlled by carrier bed permeability, by host-rock petrography, and by structure.The milling of lignite to produce a metal is a difficult process to perfect. Progress in milling is necessary before this potential uranium reserve can become an economic ore.