Thorium-bearing veins present the largest known potential resource of thorium in the United States in deposits that can be easily exploitable in the near future. Veins of this type are principally of quartz, feldspar, and iron oxides and are known from at least 13 widely scattered areas: (1) Lemhi Pass, Montana-Idaho; (2) Diamond Creek, Idaho; (3) Hall Mountain, Idaho; (4) Wet Mountains, Colorado; (5) Powderhorn, Colorado; (6) Laughlin Peak, New Mexico; (7) Capitan Mountains, New Mexico; (8) Gold Hill district, New Mexico; (9) Quartzite district, Arizona; (10) Cottonwood area, Arizona; (11) Monroe Canyon, Utah; (12) Mountain Pass, California; and (13) Wausau, Wisconsin. Veins in these areas range in length from a few feet to at least 5,000 feet and in thickness from thin seams to 50 feet. Many are 200 to 600 feet long and 0.5 to 3.0 feet thick. The veins occupy shear zones and the shearing occurred before, during, and after vein formation. Fracturing in hard, brittle rocks appears unrelated to either the contacts of units or the earlier geologic structures. Fifty-one different minerals have been identified in the veins in the 13 areas. Thorite is by far the most abundant thorium mineral. Monazite is also commonly present and in a few veins there is brockite and allanite. The rare-earth minerals, bastnaesite, xenotime, cenosite, florencite, and synchisite, are all extremely rare except in the veins at Mountain Pass, which contain substantial amounts of bastnaesite. Quartz is the most abundant gangue mineral and is commonly accompanied by microcline. Calcite is common in some areas. Other gangue minerals that are commonly found in moderate to minor amounts in some areas include muscovite, biotite, chlorite, barite, apatite, and fluorite. Iron oxide minerals, especially limonite and hematite, are characteristic of these veins and they commonly stain most of the rock. Over 20 other accessory and secondary minerals of which pyrite and rutile are the most common have been found. The thorium content of samples from various veins ranges from 0.001 to 21.0 percent ThO 2 . The rare-earth content of the veins is substantial and, depending on demand, may be economically as important as the thorium. Although rare earths commonly are present in the same minerals as thorium, there may be little correlation between thorium and rare-earth content in a sample.In eight of the 13 areas thorium veins are associated with alkalic rocks or carbonatites, or both. Carbonatite dikes are generally found near the center of an alkalic rock intrusion whereas thorium veins are at greater distances from the center. The thorium veins are believed to have been derived from a volatile late-stage phase of the magma that formed the alkalic rocks. The low-viscosity fluids traveled long distances in major fractures and slowly formed veins at low temperature, generally in minor shears.