The general geology of the Naivasha area has been described by Thompson and Dodson (1963) but the more recent description of Clarke et al. (1990) is the principal source of the following account. Both east and west margins of the Gregory Rift are encompassed by the area and, apart from some alluvial deposits within the rift, the whole area is mantled by volcanic rocks. On the rift margins the volcanic comprise parts of the widespread ‘plateau’ series but within the rift are the three major volcanic centres of Eburru, Olkaria and Longonot, which are described individually (Nos 085-00-054 – 085-00-056), and three smaller centres. Lake Naivasha dominates the central area of the rift and formerly was continuous with lakes Elmenteita and Nakuru in the Nakuru area (No. 085-00-051) to the north. The eastern rift margin is blanketed by trachytic, often ignimbritic, tuffs with some trachytic lavas, both of which contain sodic amphibole and pyroxene and aenigmatite. Much of the western margin is mantled by trachytic and pantelleritic pumice and ash fall deposits ejected from the Eburru volcano. The smaller volcanic centres within the rift comprise the Elmenteita, Ndabibi and Akira groups. The Elmenteita group lies immediately north of the Eburru volcano and extends as far as Lake Elmenteita in the Nakuru area but, unlike Eburru, is dominantly basaltic. It consists of a series of scoria cones and lavas the chemistry of which indicates that they include basalt, mugearite and benmoreite (Clarke et al., 1990). The Ndabibi group comprises basaltic cratered volcanic cones and rhyolitic domes and lava flows on the plains west of Lake Naivasha and between the Eburru and Olkaria volcanic centres. The rhyolites include comendite and pantellerite the former consisting of alkali feldspar, a sodic amphibole, aegirine-augite and quartz, while the latter are similar but also contain aenigmatite. The Akira group, described by Clarke et al. (1990), lies mostly south of latitude 1°00’ S, and consists of basalt and hawaiite lava flows, trachytic welded scoriaceous lapilli and some lavas containing pale green pyroxene, fayalite and aenigmatite. The geochemistry of the volcanic rocks is discussed and illustrated in Clarke et al. (1990) but the data are not given, although the sources are cited. The chemical changes undergone by two thick flows and a plug of comendite are described by Baker and Henage (1977).