Olorgesailie is the highest mountain in the Magadi section of the East African rift (No. 085-00-067) the whole volcano covering about 15x12 km. Inliers of lava similar to those of Olorgesailie occur on Shanamu to the west (Baker, 1958). The volcano is predominantly formed of lavas, with some agglomerates, that are mainly feldspar- and pyroxene-phyric trachytes with olivine basalt at the foot of the mountain on its eastern side. The summit area consists of ‘nephelinites’ which include abundant nepheline and fewer aegirine-augite phenocrysts in a trachytic groundmass of alkali feldspar, aegirine-augite, aegirine and titanite. Shanamu is built of trachytes, which differ from those of Olorgesailie in being more peralkaline, with aegirine-augite and aenigmatite present in some samples, while basalts are more abundant.