The Mountain Pass district is in a block of pre-Cambrian metamorphic rocks bounded on the east and south by the alluvium of Ivanpah Valley. This block is separated from Paleozoic and Mesozoic sedimentary and volcanic rocks on the west by the Clark Mountain normal fault, and the northern boundary of the district is a prominent transverse fault. The pre-Cambrian metamorphic complex comprises a great variety of lithologic types including garnetiferous mica gneisses and schists; biotite-garnet-sillimenite gneiss; hornblende gneiss, schist, and amphibolite; biotite gneiss and schist; granitic gneisses and migmatites; pegmatites; and minor amounts of foliated mafic rocks. The rare earth-bearing carbonate rocks are related to potash-rich igneous rocks, of uncertain age, that cut the metamorphic complex. The larger potash-rich intrusive masses, 300 or more feet wide, comprise one granite, two syenite, and four composite shonkinite-syenite bodies. One of the shonkinite-syenite stocks is more than a mile long. Several hundred relatively thin dikes of these potash-rich rocks range in composition, and generally decreasing age, from biotite shonkinite through syenite to granite. A few thin fine-grained shonkinite dikes cut the granite. These potash-rich rocks are cut by east-trending andesitic dikes and by faults. Veins of carbonate rock are most abundant in and near the southwest side of the largest shonkinite-syenite body. Although most veins are less than 6 feet thick, one mass of carbonate rock near the Sulphide Queen min4e is 600 feet in maximum width and 2,400 feet long. About 200 veins have been mapped in the district; their aggregate surface area is probably less than one-tenth that of the large carbonate mass. The carbonate materials, which make up about 60 percent of the veins and the large carbonite body, are chiefly calcite, dolomite, ankerite, and siderite. The other constituents are barite, bastnaesite and perisite, quartz, and variable small quantities of crocidolite, biotite, phlogopite, chlorite, muscovite, apatite, iron oxides, fluorite, monazite, galena, allanite, sphene, pyrite, chalcopyrite, tetrahedrite, malachite, azurite, corussite, wulfenite, aragonite, and thorite. The rare earth oxide content in most of the carbonate rock is less than 13 percent, but in some local concentrations of bastnaesite the content is as high as 40 percent. The origin of the carbonate rocks and related potash-rich igneous rocks is considered in the light of similar associations of carbonate and alkalinic rocks in Sweden, Norway, Russia, South Africa, and the United States. The carbonate rock may have originated (1) as a pre-Cambrian limestone or evaporate sequence in the gneisses; (2) by reaction between magma and the Paleozoic dolomite and limestone overlying the pre-Cambrian complex; (3) by alteration of pre-Cambrian gneisses by emanations from an unknown deep-seated source; or (4) by differentiation of an alkaline magma from shonkinite to syenite to granite, leading to a final carbonate-rich fraction, containing the rare elements, which was emplaced either as a concentrated or a dilute solution. The fourth hypothesis is considered the most plausible.