The "siliceous reef" uraniferous veins of the Boulder batholith contain, in decreasing order of abundance, finely crystallized pyrite, sphalerite, galena, uraninite, tetrahedrite, argentite, chalcopyrite, and covellite. Microcrystalline quartz comprises over 90% of the vein material; opal and barite are minor. The color of the veins varies from light gray to black due to variation in size and concentration of the microscopic grains of opaque minerals. Microcrystalline quartz was deposited in several stages, separated by intervals of brecciation. Quartz of the later stages is generally dark gray to black. It occurs as fracture fillings in massive early microcrystalline quartz of light color, and as a cementing matrix about breccia fragments of the lighter quartz. The U content of nearly 100 vein samples was determined by alpha counting and fluorimetry. Radioactive equilibrium of the U series is indicated by agreement between measured Ra content (analyzed by New Brunswick Laboratory, AEC) and that required for equilibrium, with U determined in 2 samples, and by general agreement between fluorimetric and alpha-activity determinations of U. Semiquantitative spectrographic analysis of the same samples provided data on the content and distribution of 28 minor elements in the veins. In most cases, the darker the vein quartz, the higher is the content of those elements generally occurring in opaque vein minerals: Ag, As, Ba, Co, Cu, Fe, Mn, Mo, Ni, Pb, Sb, Sn, U and Zn. Elements that showed no significant correlation with vein color are Al, B, Be, Ca, Cr, Cs, Ga, Mg, Na, P, Sr, Ti, V, and Zr. Five elements - Fe, As, Co, Mo, and Pb - showed a marked positive correlation with U.