Cutting Upper Cretaceous sediments in the Upper Benue Valley in an arc from west to south of Garoua (Koch, 1959a) are 25 trachytic plugs ranging from less than 1 km to more than 4 km in diameter. There are rather fewer small flows of basaltic rocks (Deruelle et al., 1991) and a single occurrence of monchiquite. This field is probably an eastward continuation of the Filiya area of Nigeria (Nigeria No. 45). The monchiquite contains phenocrysts of olivine, augite and barkevikitic amphibole in an indeterminate groundmass. The basalts are olivine-phyric. The trachytes comprise fluidally textured sanidine and aegirine/aegirine-augite, a blue amphibole, which may be arfvedsonite, and a little biotite. One variety contains fluorite and large biotite phenocrysts.
DERUELLE, B., MOREAU, C., NKOUMBOU, C., KAMBOU, R., LISSOM, J., NJONFANG, E., GHOGOMU, R.T. and NONO, A. 1991. The Cameroon Line: a review. In A.B. Kampunzu and R.T. Lubala (eds), Magmatism in extensional structural settings. 274-327. Springer-Verlag, Berlin.GRANT, N.K., REX, D.C. and FREETH, S.J. 1972. Potassium-argon ages and strontium isotope ratio measurements from volcanic rocks in northeastern Nigeria. Contributions to Mineralogy and Petrology, 35: 277-92.KOCH, P. 1959a. Notice explicative sur la feuille Garoua-Ouest. Carte Geologique de Reconnaissance à l'échelle du 1/500,000. (O52) République du Cameroun. Paris.