The 20 km-diameter, approximately circular Bilet complex forms an imposing 1700 m high mountain around which is a ring-dyke of microsyenite. At the base of the volcanic pile is a conglomerate which is overlain by tuffs intercalated with basaltic flows. Ignimbrites, porphyries and peralkaline rhyolites are also represented in the sequence. On the southern side of the complex is the small, circular massif of Agadao which is of peralkaline granite. Bowden et al. (1981) give chemical analyses, with some trace element data, for ignimbrites.