An elliptical intrusion of 8x3.5 km Numbi is emplaced in Precambrian Burundian schists and quartzites. The principal rock type is a nepheline syenite of microcline, which is generally aligned, albite, abundant nepheline, cancrinite, sodalite, green biotite, melanite, titanite, opaque phases, fluorite and apatite; chevkinite and lavenite are accessories. A variant, which is usually found near the margins of the complex, contains aegirine and aegirine-augite, but is free of biotite; this rock type is particularly rich in chevkinite and fluorite. Chemical analyses are available. A small area of ankaratrite with leucite is described by Denaeyer (1958) from 2 km southeast of the Numbi occurrence. This is undoubtedly younger and related to the Virunga volcanic field (No. 6).