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.
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The voluminous extrusive rocks of Cameroon together with a series of intrusions, which are principally granitic and syenitic, lie along a northeast-trending line which is known as the Cameroon Line. This alignment continues southwestwards into the Gulf of Guinea in the form of the islands of Bioko (Fernando Po), Principe, Sao Tome and Pagalu (Annobon) and northeastwards as far as Lake Chad. There are many papers on the Cameroon Line and useful reviews will be found in Fitton (1987), who is particularly concerned with the extrusive rocks and their chemistry, and Derruelle et al. (1991), who also cover the intrusive rocks, discuss the tectonic context, and present the various interpretations of the line. Four large volcanic complexes lie along the line within Cameroon and extend into Nigeria. They range from basaltic and nephelinitic to rhyolitic and trachytic rocks and are generally alkaline, the most extreme alkalinity being manifested by the lavas of Mount Etinde, some of which have unique compositions. Gèze (1943) describes in considerable detail the geography and geology of the southernmost volcanoes. There are about 60 intrusions lying along the Cameroon Line within Cameroon itself (Déruelle et al., 1991), but many of these are not alkaline. Apart from the reviews cited above a useful account of the intrusive complexes is that of Lasserre (1978) while many age data are given in papers by Cantagrel et al. (1978) and Fitton and Dunlop (1985). Halliday et al. (1988) give Nd, Sr, Pb and O isotopic data for a number of Cameroon Line centres and discuss the genesis of the line in some detail.
Braun et al. (1993) describe a lateritic profile overlying ‘the Akongo syenite’ with aegirine-augite being present in the fresh syenite. This locality is not on the ‘Cameroon Line’. No further details have been found but Braun et al.
An area of microcline-amphibole gneisses lies along the coast north of Beio and these are described by Champetier de Ribes and Reyre (1959) as being of the same type as the gneisses of Nkonglong (No. 34). However, no petrographic details are given.
Located 80 km north of Mt Cameroon, the Rumpi Hills extend over some 1500 km2 and culminate in Mount Rata at 1768 m.
Mount Cameroon rises from the sea coast along its southern margin to the 4095 m summit of Fako and is the highest mountain in West Africa.
Etinde is a steep-sided volcano rising to 1715 m the lower flanks of which are covered by lavas that emanated from Mount Cameroon. Etinde is built principally of lava flows, most of which have flowed southwards, with two areas of tuffs.
Eight kilometres south-southwest of Nkonglong, at a locality referred to by Champetier de Ribes and Reyre (1959) as 'Mont de l'Elephant', is a conformable, 5 km long lens of nepheline-bearing gneisses depicted on the 1:500,000 geological map (Champetier de Ribes, 1959) as ' 'syenite microcliniqu
Grea is a 3x3.5 km intrusion of pyrochlore-bearing riebeckite-aegirine granite surrounded by alluvium. It is cut by small dykes of trachyte.
Mindif is a syenitic intrusion containing sodic amphibole; few other details have been traced.
Guider is reported by Tempier et al. (1981) as being a syenite containing sodic amphibole, and is thus similar to Mindif (No. 5).