Two basalt flows and an andesite fissure eruption occur in the Rozel Hills - Black Mountain area in Box Elder County, Utah. Both basalt flows are aphanitic, and contain olivine, plagioclase, augite, and opaque oxides. They may be distinguished both chemically and on the basis of their textures. Unit 2 basalt is finer grained, and appears to be associated with a fault in the Black Mountain area. Chemically, it is similar to high-iron lavas in the Craters of the Moon, Idaho area. It has higher alkali, total iron, and titanium contents that Unit 1 basalt. Unit 1 basalt is similar to other tholeiitic basalts in the Basin and Range province. It is coarser-grained and has higher silicon, magnesium, aluminum, and calcium contents than Unit 2 basalt. While the basalts have characteristics of alkali-olivine basalts, such as absence of calcium-poor pyroxene, and a high alkali to silica ratio, both units are hypersthene- and olivine-nonnative and classified as olivine tholeiites. Subsurface basalt in the area appears to represent both lava types, but extensive alteration makes comparison difficult. Andesite from the area is aphanitic, and contains plagioclase, orthopyroxene, and opaque oxides. Temperatures of equilibration calculated from co-existing magnetite and ilmenite range f rom 1148°C to 745°C for basalt. Temperatures calculated from co-existing olivine and clinopyroxene range from 1009°C to 994°C. While the two basalt units cannot be related by fractional crystallization at surface conditions, some parent - daughter relations are likely within each unit, and the andesite is most likely a differentiation product of Unit 1 basalt. Partial melting of pyrolite and spinel lherzolite mantle compositions to produce Unit 1 and Unit 2 basalts was investigated. Since no unique temperature and pressure of equilbration for these melts and residu~l material from each mantle type was found, it is concluded that partial melting of a mantle of pyrolite or spinel lherzolite composition did not produce these lavas. Unit 2 basalt may be derived from Unit 1 basalt by fractionation of high pressure (8 kb) phases. This mechanism, similar to that proposed for the Craters of the Moon - Snake River Plain system, may account for an evolved basalt (Unit 2) with a lower silica content than a less differentiated basalt (Unit 1).