Over a large area of the Plateau Province in western Colorado, Mesozoic sedimentary rocks are locally stained bright yellow, orange, or green. Such staining suggests mineralization, and radioactive compounds were recognized here before 1900. At that time, however, there was little or no market for them or for the vanadium frequently associated with them. When Marie Curie required radium for experiments with her newly discovered element, the raw materials were sent from western Colorado; by and large, though, production of radium from these ores was prohibitively expensive. In 1905, vanadium was found to be effective in toughening steel. The Vanadium Corporation of America was formed to mine the Colorado ore. This company mines a rich zone in the Jurassic Entrada Sandstone, where vanadinite occurs with carnotite and other uranium ores. In the early days of vanadium mining, the uranium ores were discarded with other gangue materials; now, of course, uranium is produced from them. Since 1945, uranium production has been an important Colorado industry; in 1969 about $17,500,000 worth was produced. Uranium occurs in the state in two very different situations. In the Plateau Province, where it was first discovered, it occurs in sedimentary rocks as patches of pitchblende, carnotite, and a greenish yellow mineral called schroekingerite. It is most abundant in the Triassic Chinle Formation and the Jurassic Entrada and Morrison Formations, where it was probably deposited by downward movement of rainwater from overlying uranium-rich Tertiary volcanic rocks. Concentrations of uranium often occur in or near organic matter such as coal, fossil bone, or petrified wood, so mines tend to be located along rock layers carrying abundant organic material. Another type of uranium ore is found in the Mountain Province. Veins in Precambrian rocks of the Front Range and several other ranges contain pitchblende which seems to have been deposited by hot groundwater rising through broken and fissured Precambrian rocks. Often exceedingly rich, such ore is mined in the manner of most of Colorado’s metals. The Schwartzwalder Mine, a few miles northwest of Golden, has produced more ore of this type than any other mine in Colorado. OIL, NATURAL GAS, AND OIL SHALE Petroleum and natural gas have been found in large quantities in the Prairie and Plateau Provinces in Colorado, as well as in smaller quantities in North Park in the Mountain Province. They generally occur in porous sandstone and limestone layers, where they have been trapped by overlying finer-grained, less permeable layers in or near folds and faults. Several oil and gas seeps were found along the mountain front shortly after the arrival of the earliest settlers. Near Canon City, on Oil Creek, a plaque commemorates the first production: