The upper critical zone in the central sector of the eastern Bushveld Complex consists predominantly of norites, although bronzitites, anorthosites, chromitites, and other chromitic rocks appear at intervals in the sequence. Gabbronorites appear near the middle of the zone. The rock sequence is divided into 14 principal units, plus the series from the base of the Merensky Reef to the top of the Bastard Reef.Mineralogical compositions of successive units have been determined from modal analyses, and compositions of bronzite and plagioclase in the various rock units have been determined by microprobe analysis. The results indicate that, although parts of the zone show normal fractionation, the zone as a whole is not the product of progressive fractionation of a magma. Crystal settling appears to have played a relatively minor role in evolution of the zone. Successive pulses of parent magma into the chamber may account for some of the mineralogical and chemical variations, but features of certain thick norite units suggest stratification of the magma in the chamber.The study suggests that the Merensky Reef registers a sudden event in the Bushveld magma chamber, an event that altered the chemistry of the system and triggered precipitation of sulfides and platinoid minerals. The alternative hypothesis, that the Reef is the culmination of a long period of magma fractionation, is not supported by the evidence presented.Chromite occurs in a variety of associations with plagioclase and bronzite. The associations are consistent with phase relations in the system Mg 2 SiO 4 -iron oxide-CaAl 2 Si 2 O 8 -SiO 2 at elevated pressure.