These adjacent areas contain the most westerly manifestations of volcanic rocks associated with the Gregory Rift at this latitude (Jennings, 1964; Sanders, 1963a). Both areas are extensively covered by plateau phonolite lavas but the southern half of the Kapsabet Plateau area is occupied by phonolites, nephelinites and melilite nephelinites forming the northern flanks of the Tinderet volcano, which is described separately (No. 085-00-047). Upper and lower plateau phonolites can be distinguished which are respectively older and younger than the Tinderet volcanic rocks. The phonolites derive from the area to the east (No. 085-00-031) and generally overlie tuffs and agglomerates. They are feldspar- and nepheline-phyric rocks with groundmass aegirine-augite to aegirine, katophorite and aenigmatite with biotite in some facies.