This is a 20 km diameter, circular granite pluton rising to over 2500 m, which is surrounded by a collar of Karoo sediments and lavas that are folded downwards at the contact and metamorphosed and tourmalinised. Sheet-like remnants of the roof, considered to be fragments of a large caldera, are preserved at high topographic levels. The granite is, for the most part, hornblende-bearing with a little biotite and tourmaline, but in places a finer grained variety of granite contains microcline and stellate clusters of arfvedsonite. Dyke rocks include aplite and arfvedsonite-biotite aplite, which were called brandbergite by Chudoba (1930), who gives a rock analysis. Granite from an area described by von Knorring (1986) consists of microcline perthite, quartz, prismatic riebeckite and uraniferous pyrochlore. A highly radioactive variety of granite contains rosettes of aegirine with riebeckite partly replaced by astrophyllite. Pegmatitic varieties contain abundant pyrochlore and zircon together with monazite and fluorite. An area of arfvedsonite-aegirine granite at the southwestern margin of the complex, known as the Amis intrusion, is described by Schmitt et al. (2000), who give feldspar, pyroxene, amphibole, biotite and rock analyses, including Sr and Nd isotope data. Earlier work on this complex is summarised by Martin et al. (1960). Five rock analyses and oxygen isotope data are given by Harris (1995).