The Ngong Hills, 24 km southwest of Nairobi, are the remnants of a volcano that probably had a diameter of about 11 km (Saggerson, 1991). It consists principally of lavas of basanite and tephrite but the small Ngoroi Hill on the eastern side of the complex consists of biotite-olivine melanephelinite (ankaratrite) and feldspar-bearing melanephelinite (tannbuschite). The ankaratrite contains abundant phenocrysts of olivine up to 1 cm across and some of biotite set in a matrix of pyroxene, generally zoned with green terminations of aegirine-augite, magnetite and nepheline. The tannbuschite is similar but for the presence of a little feldspar. The most widespread lavas are basanites, with some nephelinites, which contain large phenocrysts of zoned titanaugite and olivine pseudomorphs in a groundmass of augite, labradorite, magnetite and nepheline with, in some rocks, biotite. The tephrites are similar but lack olivine and contain phenocrysts of barkevikite.
SAGGERSON, E.P. 1991. Geology of the Nairobi area. Report, Geological Survey of Kenya, 98: 1-91.