This is an elliptically-shaped intrusion of the Ipora Group covering some 50 km2 and intruded into Devonian sandstones of the Furnas Formation, which are metamorphosed to pyroxene hornfelses. The northeastern and southwestern parts are occupied by ultramafic rocks which are separated by a nepheline syenite intrusion. Dunites are the most abundant rocks comprising a magnesian olivine, which is variably serpentinized, and a little pyroxene and opaques. Pyroxenites are concentrated in the southwest and contain both clino- and orthopyroxene; they are cut by fine veins of plagioclase, quartz and alkali feldspar. The syenites are extremely variable and consist of orthoclase, anorthoclase, albite/oligoclase, nepheline, ferrohastingsite, augite/aegirine-augite, melanite, sphene, biotite and opaques. Theralitic essexites and nordmarkites also occur and dykes, including monchiquites, are very abundant both within the intrusion and cutting the country rocks.