The carnotite of Colorado and Utah occurs as an impregnation in sandstones and shaly sandstones, mostly in the McElmo and the La Plata formations, lying at the top of the Jurassic beds and below the Cretaceous sandstones and conglomerates of the region. The deposits seem to have been formed by the precipitation of carnotite from solution along certain strata of these formations, and the material occurs along bedding planes, in fissures and small cavities, in layers or irregular masses from a fraction of an inch to several inches in width, and sometimes as a general impregnation of the sandstone for several feet in thickness. It seems to be especially abundant in strata impregnated strongly with vegetable or animal matter, and is often in unusual quantities in lignitized or petrified trunks of trees. This phenomenon suggests the influence of organic matter in precipitating and segregating the carnotite. The rocks carrying the carnotite lie horizontally or dip at low angles in most parts of the Colorado region; in Utah they lie often in the same way, but occasionally dip at steep angles. Where they appear on the surface, the carnotite sometimes impregnates certain strata for several hundred feet or more along the outcrops, but more generally it occurs in spots along them, with little or no carnotite in the intervening spaces. As these outcrops are followed into the hillsides, the ore appears to be even more irregular in its distribution than on the surface, and in many or most cases it becomes much scarcer the further it is explored underground, until within 10 to 40 or 50 feet from the surface it often mostly or entirely disappears. There are exceptions to this feature, but the gradual and often rapid decrease in quantity and grade of the carnotite ore as it is followed into a hill is generally recognized. This fact suggests that the carnotite may have been redissolved in the sandstone and carried to the surface by capillary action in this arid climate, forming rich, superficial efflorescences. In many of the carnotite deposits, vanadium minerals occur independently of the vanadium in the carnotite, but this association is not always observed. They occur in sandstone and often give it a dark-gray or blackish color. In eastern Colorado several mines near Central City, Gilpin County, have produced limited quantities of pitchblende. Among these are the Kirk, the Wood, the Belcher, the Alps, the German and the Calhoun mines. The pitchblende occurs as a subordinate constituent in the gold-bearing veins of that country. The veins intersect old metamorphic rocks intruded by igneous rocks. The mines of Gilpin County are today producing little if any pitchblende, and the total production has been small, amounting in all probably to only a few tons. Much more pitchblende, however, was let go to waste in former days when the mines were worked for other ores and the value of uranium was not recognized.