Sutumi is a volcanic plug in which most of the plug materials have been displaced by later intrusions of granite. Rhyolites and agglomerates form screens between the granite and the basement rocks at several places around the plug. Coarser, recrystallised areas of rhyolite contain small phenocrysts of quartz and perthite with mafic streaks of poikilitic amphibole and biotite and accessory fluorite. Xenoliths of rhyolite occur in the granites which contain euhedral hornblende, sometimes with a rim of a blue-green amphibole, and biotite. Two minor intrusions of biotite granite complete the igneous events.