The petrography of the olivine-rich basaltic rocks forming the lower part of the Karroo basalt sequence in the Nuanetsi province is briefly reviewed. These rocks contain phenocrysts of olivine, microphenocrysts of clinopyroxene, and phenocrysts of orthopyroxene jacketed by clinopyroxene. Mineral analyses and bulk rock analyses are presented and lead to the conclusion that most of the olivine and all of the clinopyroxene crystallized at low pressures shortly before solidification of the lavas. The orthopyroxene and some of the olivine, however, crystallized at a higher pressure and equilibria involving these phases controlled the bulk compositions of the liquids. The results of experimental determinations of phase relations of selected samples at 7 and 10 kb are reported and suggest that equilibration with olivine and orthopyroxene took place through a pressure range of approximately 6–12 kb (18–36 km depth). It is possible that the process involved was one of polybaric crystal-fractionation but it is perhaps more likely that equilibration of liquids with excess harzburgite wall-rock over the same pressure range accounts for the trend of bulk compositions. If this is so, a wedge of upper mantle peridotite must have penetrated through the continental crust to within 18 km (or less) of the surface. In view of the position of the province on the line along which Gondwanaland underwent its first profound disruption, this conclusion is plausible on general tectonic grounds.