Songo

Exposures of ijolite over an area of about 800x300 m appear to be part of a plug, the marginal contacts of which are not exposed. The area forms a low ridge which is surrounded by sediments, probably of Tertiary age, and recent alluvium. The ijolite is partly obscured by bauxite. Magnetometer readings indicate that ijolite probably lies at no great depth to the south of the outcrop and there may be a separate body, obscured by swamp, 3-4 km to the south. The ijolite is a banded rock of darker and lighter layers representing varying concentrations of pyroxene and nepheline. Nepheline varies from about 47 to 62% but in some urtite schlieren may be over 90%; a little cancrinite is sometimes associated with it. The pyroxene is diopside-hedenbergite but there are also occasional minute clusters of acicular aegirine. An opaque phase and apatite are plentiful and analcime, or possibly sodalite, also occur, as does titanite and possible perovskite. Biotite forms rims to an opaque phase; zeolite is also present and calcite veins several millimetres thick occur. A dyke, probably between 3 and 10 m wide and at least 500 m long, is marked by a train of boulders. The rock is readily weathered but fresh material indicates it to consist overwhelmingly of amphibole, with dark brown melanite in some samples, abundant ilmenite and magnetite, with a little zeolite along fissures. The amphibole consists of densely packed, sub-prismatic crystals which Baker et al. (1956), based on optics, considered to be katophorite. Vorma (1961) also describes the mineralogy of the dyke in considerable detail, but he finds it to consist essentially of phosphates. He estimates a modal composition of 70-80% rockbridgeite, 20% goethite, 5-10% Al-strengite and various other secondary phases including cacoxenite and wardite. The radical difference in the descriptions of Baker et al. and Vorma indicate that quite different material was sampled, with the relationship between them unclear.