The complex, 4 km in diameter, forms a horseshoe-shaped ridge up to 500 m elevation open to the south, the opening being partly blocked by a circular peak. It comprises an outer ring of syenites and quartz syenites which dip steeply outwards, but with a faulted inward-dipping inner contact against an inner ring of highly cataclastic quartz syenites and gabbroic rocks. The central stock, which is surrounded by wadi alluvium, is predominantly of essexitic gabbros. Relics of a volcanic cone in the form of trachybasaltic flows and pyroclastic rocks are preserved on the eastern side. These rocks are cut by small bodies of alkaline and nepheline syenite and numerous and varied dykes. The so-called essexites consist of andesine-labradorite which is partly replaced by albite-oligoclase and orthoclase, biotite, barkevikite replaced by sodic amphibole, titanaugite partly replaced by aegirine, opaques, apatite and sometimes analcime; altered nepheline occurs in some leucocratic varieties. The alkaline syenites of the outer ring are leucocratic rocks of perthite, aegirine-augite and biotite. Varieties of syenite in the central stock that contain a little nepheline and cancrinite are called umptekites by El Ramly et al. (1969). Dykes include solvsbergite and grorudite, which cut mainly the inner and outer rings, and essexite, diabase, tinguaite and nepheline syenite porphyry, which are very numerous in the central stock; all these dyke types are found in the country rocks. Modes and chemical analyses of a range of rocks will be found in El Ramly et al. (1969), and U and Th data for 10 samples in El Reedy (1985). M.S. Garson (unpublished report) indicates that there are carbonate veinlets of probable carbonatitic origin at El Kahfa.