This area represents the most southwesterly manifestation within Kenya of volcanic rocks directly associated with the Gregory Rift Valley. The area encompasses the Mara River (Williams, 1964) and Narok (Wright, 1967) quarter degree sheets with the most westerly rocks being briefly referred to in a report by Shackleton (1946a). About half of the Mara River area is blanketed by poorly exposed phonolite lava flows of uniform composition and generally not more than 60 m thick with only minor tuffs. The lava contains abundant phenocrysts of alkali feldspar up to 2.5 cm and rather smaller nephelines in a groundmass of alkali feldspar, katophorite, sodic pyroxene, aenigmatite, nepheline, apatite and rare biotite. The Narok area has an even more extensive coverage of volcanic rocks than the Mara River area but the greater part of the former is mapped as “volcanic soils derived from ashes” (map accompanying Wright, 1967) so that correlation of the two areas is not clear. Over the eastern part of the Narok area, however, is a greater variety of volcanic rocks including melanephelinite, phonolite, basalt and trachyte. The lowermost ‘Kishalduga melanephelinites’ (Wright, 1967) are extensive and continuous with similar rocks that occur in the vicinity of the Loita Hills (No. 085-00-067) while melanephelinite, nephelinite, oligoclase tephrite and anorthoclase-bearing lavas are also distinguished. All rocks are rich in augite, olivine is generally present, although may be pseudomorphed, analcime is abundant, nepheline generally identifiable and biotite rare. The pyroxene in the feldspar-bearing lavas and the nephelinites is sodic. A number of nephelinite intrusions occur in the southeast of the Narok area and contain nepheline phenocrysts up to 2 cm long, while in the immediate vicinity of Narok are found four olivine melanephelinite intrusions one of which, Mutunyi, is nearly 2 km in diameter. The phonolite flows are up to 300 m thick and overlie nephelinites while the plateau trachytes are confined to the southeast of the area, are up to 150 m thick and continuous with those extensively developed over the Magadi area (No. 085-00-067); they contain aegirine, katophorite and aenigmatite. Tuffs are present throughout the area the latest of which are probably the youngest volcanic rocks in the area.