Much of the Cape Cross Peninsula is occupied by Karoo lavas and intrusive rocks or obscured by recent deposits, including evaporites, but Gevers (1933) described a small area of alkaline rocks intruding the Karoo that is probably part of a large complex extending beneath the sea. The alkaline rocks form a small hill, which is partly covered by wind-blown sand, and include syenites and tinguaite. The most abundant rock type, and the youngest, is a sodalite microsyenite consisting of perthite, a greenish amphibole, some of which has cores of a colourless pyroxene, a little blue amphibole (?arfvedsonite) and aegirine-augite, biotite, abundant interstitial sodalite, cancrinite and occasional euhedra of nepheline, apatite and titanite. A syenite which forms a small dyke comprises alkali feldspar, some displaying microcline cross-hatching, abundant biotite, a greenish amphibole with occasional patches of blue amphibole, a few needles of aegirine, plentiful titanite and apatite and minor sodalite. These two rock types intrude a tinguaite of similar mineralogy in which the sodalite forms large irregular aggregates. Gevers (1933) gives an analysis of the sodalite microsyenite. There is an oxygen isotope analysis in Harris (1995).