The Fourmile area, Custer County, S. Dak., is underlain by pre-Cambrian metamorphic rocks that surround the granitic core of the Black Hills. The main structure in the area is the upright limb of an overturned anticline that plunges about 30 ? S. 10 ? E.
Three units of metamorphic rocks are described that have a total thickness of at least 7, 700 feet. The oldest of these units, a quartz-mica schist, is more than 6, 500 feet thick. The overlying unit, about 200 feet thick, is composed of thin beds of amphibolite and hornblende schist, lime-silicate rock, cordierite-biotite schist, microcline-biotite schist, and other types of rocks. The youngest unit, a quartz-mica-feldspar schist, is more than 1,000 feet thick. The presence of kyanite, staurolite, cordierite, and sillimanite in the rocks indicates that they have been subjected to high-grade metamorphism.
About 420 pegmatites were mapped in the quartzmica-schist and the quartz-mica-feldspar schist. A few thin pegmatites in the third unit were not mapped. Most of these are concordant with the schistosity and relict (?) bedding of the enclosing metamorphic rocks. They are as much as 250 feet thick and range from 10 to 2, 600 feet in length. Nine peqmatites are zoned and classified as heterogeneous. The remainder are homogeneous and are poorly zoned. The major constituents are plaqioclase, quartz, perthite, and muscovite. The accessory minerals are tourmaline, apatite, garnet, and biotite. Beryl was observed in 15 peqmatites. The heterogeneous pegmatites contain commercial deposits of potash feldspar, mica (sheet and scrap), and beryl.