 At least five mines were active at one time, and by 1919 the two ore mills at Placerville were producing 30% of the world's vanadium.Through 1940, the mines produced about 3.7 million pounds of vanadium.Although carnotite was recognized as a minor constituent of the ore since its discovery, the amount was small, and no assays were made of the uranium content of the ore until World War II.