The 3.5 km diameter northern Tarbtie structure is emplaced in a high plateau of Nubian sandstone, although the contacts are usually faulted. Ring-like features, particularly in the southern half of the intrusion, are expressed by wadis and fault and dyke patterns. Remnants of volcanic rocks including syenitic porphyries with sodic amphibole, lavas, agglomerates and other pyroclastic rocks are preserved in a few places. West of the intrusion a volcanic cone 360 m in diameter and rising 30-40 m above the wadi floor consists of a central stock surrounded by agglomerates and tuffs and vesicular rhyolite and trachyte. The rocks of the main intrusion are peralkaline syenites with quartz and sodic amphibole, referred to as nordmarkite by El Ramly et al. (1971), who also give some rock analyses; further trace element data are available in Soliman (1987).