The most important deposit of wulfenite is at the Mammoth and Collins mines, in Pinal County. They were originally gold mines, and when the value of molybdenum became evident, Colonel Randolph, owner of the Mammoth mine, decided that it would be worth while to run the tailings dump for wulfenite. He converted his mill to this purpose, and not only ran the Mammoth dump but also the dump at the old Yuma mine, in Pima county. The total amount of concentrates produced by Colonel Randolph and others in the vicinity during the three years 1916 to 1918 are probably represented by 1,000 to 1,200 tons of wulfenite concentrates. While these operations were going on, they represented practically the only production of molybdenum concentrates in this country except on a very small scale. One other operating company is the Rowley Copper Mines Co., Gila Bend, Arizona, the ore being wulfenite and the principal impurity barite. The company has succeeded in making satisfactory concentrates, which carry, however, a considerable amount of barite. The Golden, Colo., station of the Bureau of Mines has run some tests on this concentrate and has made a partial separation of the barite and wulfenite. Molybdenite is found in a considerable number of places in Colorado, Montana, Washington, Nevada, Utah, Texas and other western and northwestern states, but the largest occurrence is in Colorado. Generally speaking, the individual deposits in the West are not large enough to warrant the building of a mill for any one of them, and as the deposits are widely scattered, it is difficult to find a place where a custom mill could obtain a sufficient amount of ore. This is one of the chief difficulties in producing a large tonnage of molybdenite concentrates, outside of Colorado. Probably the largest deposits of molybdenite in the world are at Climax, Colorado. These deposits are on the southwestern slope of Bartlett Mountain, Summit County, about 15 miles from Leadville. Outcrops are practically continuous across the whole length of the mountain and at places are one hundred to two hundred feet thick. The ore is rather granular and not flaky molybdenite, the average grade running about 8 per cent. MoS₂.