The Bir Um Hibal complex is an elliptical intrusion covering 15x10 km which is overlain in places by Nubian sandstone. Most of the complex consists of peralkaline quartz syenite but in the south a broad band of peralkaline granite intrudes the syenite (Mohamed, 1996). The circular, 2 km diameter Agib complex comprises a core of leucocratic syenite surrounded by rhyolite lavas that cross-cut the quartz syenite. The quartz syenite is a coarse-grained rock of perthite, minor quartz, a calcic amphibole, aegirine, which is generally rimmed and replaced by arfvedsonite, prisms of arfvedsonite, relict fayalite and biotite. The granite is a medium-grained rock of perthite, albite, quartz, riebeckite and arfvedsonite, aenigmatite and biotite, which forms fringes around arfvedsonite. Eight rock analyses, including an extensive suite of trace elements, is given by Mohamed (1996).