The Pleistocene to Holocene Honolulu Volcanic Series was erupted from about 37 vents scattered over the older Koolau tholeiite shield. The rocks of this series are compositionally zoned with respect to the shield; near the Koolau caldera the predominant rocks are melilitenepheline basalts, but these give way outward to nepheline basalts, and ultimately, at the apron of the shield, to alkalic olivine basalts. The xenoliths in these are likewise zoned: most of those in the caldera area consist of dunite, most of those at intermediate distances of lherzolite, and some of those in the apron of the shield consist of garnet pyroxenite and peridotite. The zoning of the xenoliths, however, does not coincide with that of the enclosing rocks. We believe that copious eruption of Koolau tholeiite produced a lateral and vertical heterogeneity in the mantle beneath Oahu, and that the zoning in both Honolulu lavas and their xenoliths is caused by that heterogeneity. The textures of the xenoliths indicate that the basalts were mainly produced by fractional melting rather than fractional crystallization. There is some evidence that the dunite xenoliths are mantle residua produced during the generation of the tholeiite, and that the Honolulu magmas were generated at greater depths than the Koolau magmas, probably as a result of elastic unloading. © 1970 Oxford University Press.