The Tin Mountain rare element pegmatite of the Black Hills, South Dakota, has become an important source of geochemical data and interpretations, but its structure and bulk chemical composition have been unclear because of irregularities in the shape of the pegmatite and because most of it is concealed beneath the surface. Construction of structure contour and isopach maps based on surface and underground maps, three drill holes, and revised versions of previous cross sections has yielded a rather simple and orderly interpretation of the structure and has led to calculations of the tonnage of each of the internal units and of the bulk composition of the pegmatite.The outcropping part of the pegmatite trends S 75 degrees E for 90 m and then connects with an arm of the pegmatite that plunges gently north-northwest for approximately another 90 m. An albite-quartz-muscovite wall zone completely surrounds four inner units. Though these four inner units are termed zones, in accordance with standard practice, differentiation is more in a vertical direction than in a concentric arrangement. The upper two of these units are perthite rich: the uppermost is a perthite-quartz-albite hood, and beneath it is a body consisting almost entirely of perthite. Both perthite units are chiefly in the easterly trending arm of the pegmatite. These are succeeded by a quartz-spodumene-lithian muscovite core and an underlying body of albite-quartz-spodumene pegmatite, which are almost entirely in the north-northwest-trending arm of the pegmatite.The pegmatite has about 250,000 metric tons of rock. The wall zone makes up about 37 percent of this tonnage. The percentages of the other four units are, from the top down, about 22, 13, 21, and 7, respectively. The normative bulk composition is quartz, 34.5 percent; albite, 34.1 percent; orthoclase, 22.3 percent; spodumene, 6.3 percent, and corundum, 2.8 percent.