The Puch Prasir Plateau, which lies essentially south of latitude 3°00’N, is a mountainous area surrounded by high cliffs and constititutes the southerly end of a dominantly rhyolitic group of hills that extend for nearly 100 km into the North Turkana area (No. 085-00-01) and cover about 2000 km2. The Plateau consists predominantly of rhyolites which are surrounded by andesitic rocks and these in turn, to the south and east, by further rhyolite and basalt. The total sequence is some 1500 m in thickness and Fairburn and Matheson (1970) give the upward succession as olivine basalt, Muruasigar rhyolite, andesite and Puch Prasir rhyolite. Both rhyolite sequences seem to be mainly peralkaline while peralkaline trachytes occur amongst all units and phonolites amongst the andesites and basalts. The Muruasigar rhyolites include agglomerates, tuffs and probable ignimbrites. They generally contain sanidine and quartz phenocrysts with riebeckite usually altered to iron oxides. The Puch Prasir rhyolites form the high plateau the vertical cliffs at the edge of which display vertical columnar jointing, suggesting emplacement as a single unit. As in the lower rhyolites agglomerates, tuffs, including probable welded varieties, predominate. These rhyolites are also sanidine- and quartz-phyric and riebeckite, as both phenocrysts and acicular tufts, is widespread. The andesites are essentially pyroxene and andesine-phyric lavas with amphibole, biotite and olivine in some facies. One sample of phonolite occurring amongst the andesite group, and described by Fairburn and Matheson (1970), contains numerous phenocrysts of nepheline, up to 8 mm in length, and rather fewer of aegirine-augite in a fine-grained matrix of aegirine-augite, alkali feldspar, euhedral nepheline and an opaque phase. Another example contains phenocrysts of alkali feldspar and apatite in a matrix containing abundant aegirine-augite and much analcime as angular crystals, which may have replaced nepheline. Trachytes found amongst the andesites contain alkali feldspar phenocrysts and sodic amphibole as microphenocrysts or in the matrix; aenigmatite also occurs. Aegirine-bearing trachytes are found amongst the rhyolites. Dykes of riebeckite-bearing microgranite, phonolite and aegirine trachyte are found in the area.