This is by far the largest intrusive complex in Libya covering 430 km2. The occurrence extends eastwards into Egypt and Sudan, but the greater part is located within Libya. The complex comprises a large, sub-circular ring intrusion up to 25 km in diameter flanked on the northern side by three overlapping subsidiary rings. Parts of the eastern side of the complex are capped by Lower Palaeozoic sediments. The main complex has a high, circular outer rim of coarse-grained quartz syenite inside which granite has weathered to an annular depression; both rock types contain aegirine. An inner circular ridge, up to 8 km wide, consists of a series of both inward and outward dipping cone sheets, and slopes inwards to a central syenite. The central syenite, which is cut locally by trachyte dykes, contains up to 12% quartz, but quartz is not present in the surrounding cone sheets. The preceding account is based on a report of Hunting Geology and Geophysics (1974) but Klerkx (1985) says that foidal syenites occur in the inner part of the structure, while the centre is composed of trachytic breccias and phonolites. A lens of distinctive riebeckite microgranite, which is cut by aplitic and granophyric dykes, is located 8 km southwest of the centre of the complex. Exposure of the three rings north of the main complex decreases northwards. The more southerly consists of a central porphyritic syenite which is surrounded by a coarse-grained quartz syenite and this in turn by a trachytic intrusion breccia. The central ring comprises medium to coarse syenites cut by trachytic cone sheets, while the poorly exposed most northerly ring is of quartz syenite. Extensive mineralogical data are given by Flinn et al. (1991) and Atherton et al. (1991) give modal and chemical analyses of 14 rocks including a full set of REE for three rocks.