Quaternary basalts of Gem Valley, Idaho, are present as valley fill (Group 1) and as well defined flows in the Bear River Range (Group 2) east of Gem Valley. Minerals present in both groups of basalts include olivine (Fo73 -Fo39), augite (Wo41 En39 Fs20), plagioclase (An75 -An40), and Fe-Ti oxides. Coexisting pairs of magnetite and ilmenite, and olivine and clinopyroxene in several samples indicate temperatures of crystallization from 958°c to 1167°c. The Group 2 basalts exhibit a cumulate texture with abundant large (2 cm) phenocrysts of plagioclase. Chemically, the Gem Valley basalts are similar to the basalts of the Snake River Plain with respect to SiO2, total Fe, P2O5 and Na2O but differ in the amounts of Al2O3 and MgO present. The Al2O3 is generally higher and the MgO is generally lower in the Gem Valley basalts. Comparing Group 1 with Group 2 basalts, the Group 2 basalts generally have more alumina and alkalis than the Group 1 basalts. Chemically both groups of basalt exhibit characteristics of the tholeiitic basalt suite, because they are hypersthene normative. Mineralogically, both groups of basalt contain but one pyroxene, augite, which is characteristic of the alkali-olivine basalt suite. This apparent contrast in classifications may be resolved by referring to these basalts as transitional between the alkali-olivine and tholeiitic basalt .suites, with the restriction that no genetic relationship to either suite is implied. Within the limits of this study, it is proposed that the Group 1 basalts may have formed by the partial melting of mantle material with a pyrolite composition. Furthermore, the Group 2 basalts appear to have originated as a result of the accumulation of plagioclase in a fractionating magma of Group 1 composition.