Nosy Komba is an approximately circular island 5 km in diameter lying immediately south of Nosy-Be. It comprises an outer ring of schists of Liassic age that are cut and metamorphosed by a centrally located intrusion of amphibole-bearing gabbro and nepheline syenite and smaller intrusions and dykes of nepheline syenite, phonolite and microsyenite. The nepheline syenites are characteristically heterogeneous varying from leucocratic to mesocratic and in many places are choked with inclusions of essexite or theralite, which are probably hybrids resulting from reaction of gabbro with nepheline syenite. The nepheline syenite is described petrographically in some detail by Lacroix (1922b, p. 614) and consists of microperthite, nepheline, analcime, aegirine and a little zircon, fluorite, biotite and what Lacroix terms rinkite (mosandrite). Various textural and mineralogical types of nepheline syenite and more basic dykes occur (tinguaite, berondrite, mafraite etc.) and variously include arfvedsonite, biotite, lavenite, aenigmatite and nepheline. Analyses of many of these rock types are given by Lacroix (1922b).