The eastern half of this huge area has not been investigated in detail but, as indicated on the 1:1,000,000 Geological Map of Kenya (1987), much of the area is covered by basaltic rocks. The western half is described in reconnaissance reports by Charsley (1987a) and (Key, 1987a), each covering a one degree square, who indicate that the extensive volcanic rocks are principally basalt, but that there are some alkaline rocks. More than half of the northwestern quarter (North Horr area) (Charsley, 1987a) is occupied by the Huri volcanic shield of 6100 km2 with the smaller Asie volcanic shield extending south into the southwestern quarter (Marsabit area); much of the rest of the North Horr area is covered by plains of basaltic lava; there are many well preserved volcanic land forms. The plains lavas are tholeiitic with the central shields alkaline. Analysis of a lava from a composite cone on the Huri shield proved to contain 23% normative ne and Charsley (1987a) considered the rock to be a phonolitic nephelinite. Similarly, an analysed rock from a composite cone on the Asie shield yielded 25% ne and was identified as a basanite, while chemical data from the southern half of Asie also indicate its generally alkaline nature (Key, 1987a). The eastern half of the Marsabit area is dominated by the Marsabit volcano which extends northeastwards for over 100 km (Key, 1987). The summit area contains numerous pyroclastic cones and maars but the bulk of the shield is built of basaltic flows. The basalts typically contain olivine but in the summit area analcime is widespread which is reflected in up to 17% ne in analysed rocks, which are described as basanite and tephrite (Key, 1987a). The eastern part of the area is dominated by volcanic rocks, as is apparent from the 1:1,000,000 ‘Geological Map of Kenya (1987), of which the southern half comprises part of the Marsabit volcano. Although basalts are apparently dominant (e.g. Williams, 1978) no detailed work appears to have been done which might indicate whether more alkaline facies are present. Pyroxenitic and peridotitic xenoliths from pyroclastic deposits of maar-type volcanoes north and west of the town of Marsabit have been described in detail by Henjes-Kunst and Altherr (1992).