The 20x70 km graben of Tombel extends between Mount Cameroon and Mount Manengouba and across it are distributed more than 70 tuff cones, generally less than 100 m high but reaching 150 m. Mont Koupe (No. 28) lies in the centre of the plain. The tuff cones often form chains parallel to the northeasterly trend of the graben. The form of the cones and layering of the constituent pyroclastics are illustrated with photographs by Tchoua (1971). The rocks are basaltic consisting essentially of olivine, pyroxene, plagioclase, magnetite and glass. Of ten rock analyses given by Tchou (1971) five are ne and five hy normative, three of the ne normative rocks having >10% ne and one of which has >20%. All plot in the middle of the tephrite-basanite field of the SiO2 - total alkalis plot of the IUGS classification (Le Maitre et al., 1989) and are thus undoubtedly basanites. It appears that the basanites co-exist with tholeiites in this field.
LE MAITRE, R.W., BATEMAN, P., DUDEK, A., KELLER, J., LAMEYRE, J., LE BAS, M.J., SABINE, P.A., SCHMID, R., SORENSEN, H., STRECKEISEN, A., WOOLLEY, A.R. and ZANETTIN, B. 1989. A classification of igneous rocks and glossary of terms. Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks. Blackwell, Oxford. 193 pp.TCHOUA, F. 1971. Le volcanisme trombolien de la plaine de Tombel (Cameroun). Annales de la Faculte des Sciences, Université Fédérale du Cameroun. Yaoundé, 7-8: 53-78.