This is a zoned intrusion of about 5 km diameter forming a hill and emplaced in Devonian Furnas sandstones. There is a central core of dunite surrounded by peridotites and pyroxenites and these in turn by a border group of alkaline gabbros and nepheline syenite. The dunites are partially serpentinized. The pyroxenites, as well as 'augite', contain olivine and a little plagioclase. These rocks pass gradually into biotite-bearing gabbros. The nepheline syenites are coarse rocks with nepheline phenocrysts up to 1. 5 cm, perthite, 'sanidine' phenocrysts and Ti-augite zoned to aegirine. A small area of rock composed essentially of pyroxene and magnetite and described as jacupirangite occurs, while small veins of carbonatite with phlogopite cut the dunite.