The 120 km2 Forchhammer complex occupies the greater part of the Cape Simpson peninsular at the southern end of Traill 0. It comprises principally peralkaline syenites which domed and metamorphosed Jurassic and Cretaceous sediments containing basalt sills. The principal rocks of the complex are alkali amphibole-and pyroxene-bearing quartz syenites with minor granite and quartz-free syenite. Dyke rocks include rockallite porphyry, aegirine granophyre and spherulitic quartz syenite porphyry. Extensive areas of 'tuff-breccias' are also shown on the large scale coloured geological map of Schaub (1942).