Catanda occupies an area of 55 km2 and is a deeply eroded volcanic edifice with a lake occupying the centre of the structure. It is apparently located at the intersection of four fault systems and much of the geology is obscured by up to 4 m of recent sediments. There appear to be a number of craters, the source of extensive pyroclastic deposits, and several satellite cones. The pyroclastic rocks are carbonatitic and according to Silva (1973), although most are fragmental, there are also carbonatite lavas. General views of the area and two photomicrographs are given by Silva (1973). The lavas are porphyritic, the phenocrysts being augite, amphibole, olivine, biotite, apatite and rare opaque minerals. In the carbonate-rich matrix apatite is abundant and pyrochlore and baddeleyite have been identified. Chromite (47% Cr2O3) is apparently plentiful in the pyroclastic rocks (Gomes, 1969) and occurs with olivine, diopside, titanomagnetite and kaersutite, analyses of which phases are given. Dykes of tinguaite, phonolite, diabase and quartz rock occur.