Kahusi comprises both intrusive and extrusive suites which occupy an area of about 9x9 km. There is, however, a considerable disparity in the ages of the suites (see below) which inevitably makes their petrogenetic relationship problematical. The intrusive part is an annular structure of syenites and granites which are overlain in the south and east by an arc of volcanic hills, of which Kahusi itself is the highest. Only parts of both suites are peralkaline. The plutonic units are as follows (Biayi-Kalala, 1983; Kampunzu et al., 1985; Lubala et al., 1985): annite-lepidomelane syenite, quartz syenite with ferrohastingsite and annite, biotite microgranite and granites, including varieties with amphibole + biotite, arfvedsonite, arfvedsonite + aenigmatite and arfvedsonite + aegirine. Details of the mineralogy are tabulated by Kampunzu et al. (1985). The volcanic rocks include rhyolitic flows and tuffs with some trachytes. A number of individual flows and units have been mapped by Boutakoff (1956) and Kampunzu et al. (1985) consider their distribution to be typical of a cauldron, the curved line of volcanoes possibly standing above a ring fault. The rhyolites are quartz- and feldspar-phyric with sodic amphibole in some varieties. The trachytes contain feldspar, and sometimes quartz, phenocrysts, with biotite in most varieties (Biayi-Kalala, 1981). All the rhyolites are highly potassic and the trachytes particularly so (K2O 12.68%; Na2O 0.55%) by which they differ markedly from the intrusive suite in which the alkalis are approximately equal. Analyses for major elements of both suites are given by Kampunzu et al. (1985), for the intrusive rocks by Biayi-Kalala (1983) and for the trachytes by Biayi-Kalala (1981).