The Ol Esayeiti volcano is located southwest of the Ngong Hills and extends from the southwest corner of the Nairobi map sheet (No. 085-00-062) (Saggerson, 1991) into the Kajiado area to the south (No. 085-00-071) (Matheson, 1966). It comprises flows of phonolite, tephrite and trachyte which partly overlie the products of the Ngong volcano. A phonolite at the base of the succession contains phenocrysts of nepheline, up to 1.5 cm, anorthoclase, titanaugite and titanite set in a groundmass of alkali feldspar, oligoclase, aegirine-augite, aenigmatite and katophorite. The tephrites are typically feldspar-phyric, with oligoclase phenocrysts up to 1.5 cm; ‘barkevikite’ and titanaugite lie in a trachytic matrix of oligoclase, aegirine-augite, nepheline and magnetite.