Marysvale, Utah, is an area of special geological interest. The district is a source of consistent uranium production where ores occur in veins related to igneous activity. Here is an unusual illustration of the nature of hydrothermal alteration in connection with vein formation. The region furnishes data on the relationship between centers of igneous invasion and the sedimentary accumulations of the Colorado Plateau. The area of uranium production lies in the vicinity of the Prospector, Freedom No. 2, Bullion Monarch, and near-by workings and constitutes a zone about a mile long by half a mile wide. Numerous prospects lie in a surrounding area about 7 miles across. Ores occur throughout a vertical distance of 1500 feet as indicated by drill holes and mine workings. Hypogene uranium occurs as pitchblende associated with black fluorite and fine pyrite in breccia and distributed veinlets along veins. Nearly vertical uranium veins range from a few inches to several feet in thickness. Veins pinch and swell, and ore shoots are either vertical or exhibit a steep west rake. Supergene uranium minerals occur near the surface. Umohoite is a new hydrous uranium molybdate first discovered in the Freedom No. 2 mine. The mineral occurs in the ore-bearing vein along with pitchblende. This occurrence at depth and associated with pitchblende rather than the oxidized uranium-molybdenum minerals ilsemanite and jordisite suggests hypogene origin. Earlier hydrothermal alteration produced numerous masses of alunite which occur in the rocks of the Bullion Canyon volcanic series; these are mostly pyroxene andesite and tuff. Radioactivity is low in these masses. Later hydrothermal alteration is associated with the uranium-bearing veins. Clay minerals occur along vein zones which cut quartz monzonite or granite and contain mixed aggregates of kaolinite, illite, halloysite, and montmorillonite. These are cut by broken bands of silicification with pitchblende either disseminated or in streaks. Chlorite occurs along vein borders, and adularia is a vein associate. Minor alunite is a late alteration product. Clay appears to diminish at depth. Near the surface some veins abut unconformably overlying rhyolite. Here clay masses may be found which contain oxidized ore that extends upwards into rhyolite fractures. The Potts fraction, Bullion Monarch open cut, and Lucky Strike ore bodies represent such accumulations. Clay zones extend beyond the ore-bearing portions of veins which constitute a small portion of the total length of clay veins in the area. On the other hand, ore is associated with alteration. Mount Belknap glassy rhyolite forms dikes, fills vents, and appears to be associated with diatremes. In places glassy dikes have been converted to masses of clay minerals by hydrothermal alteration. In the Bullion Monarch mine pitchblende is associated with glass, and ores occur in some of the argillitized dikes. The occurrence of uranium in late Tertiary alteration zones confirms Pb:U age determinations which place the age of Marysvale uranium at about 10,000,000 years. Marysvale lies along the western margin of the Colorado Plateau near the center of an igneous area of about 4000 square miles. Quartz monzonite and granite have invaded plateau strata and Bullion Canyon post-Wasatch Tertiary volcanics. All have been eroded. The eroded surface has been covered in places with Mount Belknap volcanic rocks. Rafted blocks of metamorphosed plateau sediments suspended in quartz monzonite intrusive plugs suggest that these strata underlie the exposed volcanic sequence. Several square miles of Paleozoic and Mesozoic sediments along the Deer Trail Mountain escarpment west of Marysvale indicate the character of the sediments which lie deeply buried beneath the volcanic rocks near the intrusives. The Antelope Range in which the uranium deposits are located lies in a graben between the Sevier fault on the east and the Tushar fault zone on the west. The graben has been disrupted north of the Tushar fault by the invasion of the Mount Belknap volcanic series.