Biega, like Kahusi just to the north, consists of an intrusive suite of granites and syenites which are partly overlain by volcanic rocks. The complex is intruded into Precambrian metamorphic rocks of the Kibaran and Burundian systems. It is about 9x4 km, elongated north-south and with a line of volcanic hills along the full length of the eastern margin. The intrusive rocks are principally biotite and hornblende granites and syenites with some ferroaugite-katophorite-fayalite syenites. The extrusive rocks are principally rhyolite with some trachyte, the latter being exceptionally potassic like those of Kahusi (No. 15); these rocks are not peralkaline. The mineralogy is tabulated and rock analyses given by Kampunzu et al. (1985), and Lubala et al. (1986) give analyses of rocks as well as feldspar, amphibole, clinopyroxene, biotite and Fe-Ti oxides.