Major element analyses, together with selected trace element analyses, of fourteen Apollo 15 mare basalt samples show that these basalts fall into two distinctive groups. On a normative basis these two groups have been designated Apollo 15 olivine basalts and Apollo 15 quartz basalts; previous identification of these groups on a modal basis led to the names porphyritic olivine basalt and porphyritic pyroxene basalt respectively . The Apollo 15 olivine basalts include genetically related flow units in which there has been near-surface fractionation controlled mainly by movement of olivine. It is also shown that these olivine basalts cannot be genetically related to Apollo 15 quartz basalts, nor to Apollo 12 olivine basalts nor to Apollo 11 basalts by any process of near-surface crystal fractionation. Evaluation of known and predictable high pressure crystallization sequences leads to a model of magma genesis in which the source rock (lunar mantle) is olivine-bearing pyroxenite and distinctive parental mare basalt magmas are formed by variable degrees of partial melting and different depths of magma segregation.