The north-south-trending Bear Lodge Mountains are a dome with a core of Tertiary igneous rocks of 8.4x3.4 km together with more than 30 separate igneous bodies which are concentrated in the north. Sedimentary rocks around the dome include upper and lower Palaeozoic formations, and Jurassic and Cretaceous rocks, and Precambrian granite is caught up in the igneous core in the southern part of the mountains. Along the flanks of the dome the Tertiary intrusive rocks and older sediments are overlain by Tertiary and Quaternary sedimentary rocks. A subsidiary dome occurs at Hershey Creek in the northwestern part of the area. The Tertiary igneous rocks form dykes, sills, irregular intrusions and possibly laccoliths and there are some flows and pyroclastic deposits. They are commonly porphyritic rocks generally of a potassic chemistry, with phonolites and trachytes being predominant, but a few small bodies of syenite and nepheline syenite do occur, and are also encountered in boreholes. Latite, pseudoleucite porphyry, lamprophyre and carbonatite are also found. The distinction of many rock types is difficult because of the general heterogeneity and because of widespread hydrothermal alteration. Most of the core rocks and many of the separate intrusions are of trachyte and phonolite which comprise phenocrysts of sanidine and nepheline, and occasionally pseudoleucite, plagioclase, aegirine-augite, and some amphibole and melanite in a matrix of feldspar, magnetite, a little biotite and accessories. Generally, K2O>Na2O. Latite with abundant oligoclase-andesine, hornblende and aegirine-augite phenocrysts is the dominant rock type of the Hershey Creek dome. Pseudoleucite porphyry forms a sill about 6 m thick and 3.6 km long in the northwest of the area and also several smaller intrusions. The rock contains 5-20% pseudoleucites which are up to 3.8 cm in diameter. Only one carbonatite dyke has been found at the surface but they have been encountered in seven drill holes, one of which cut more than 20 dykes. They are composed principally of calcite with accessory ancylite, bastnaesite, strontianite, fluorite, sulphides and many others, including locally abundant aegirine and actinolite. Later hydrothermal activity produced disseminated and vein deposits rich in Th and REE.