Volcanic rocks mantle the eastern parts of these areas, which are described by Mason and Gibson (1957). They are continuous with the Maralal area (No. 085-00-011) to the east, which has been described in much fuller detail (Key, 1987). Lower and Upper volcanic units are distinguished the former comprising olivine basalts and tuffs of predominantly basaltic composition but with some ignimbrites and minor quartz trachyte. The Upper series consists of phonolite lavas that overlie both the Lower Series and the Basement rocks directly. They are feldspar-phyric rocks with a matrix of feldspar, abundant nepheline and mossy aggregates of aegirine-augite and aegirine, aenigmatite, katophorite and riebeckite.