Originally discovered from its aeromagnetic signature, the St- Honore intrusion covers 10 km2 but is blanketed by a thin layer of Trenton limestone which is locally overlain by black shales. The intrusion has been extensively drilled to evaluate the extent of Nb mineralization. The core of the complex is highly variable carbonatite which is surrounded by a ring of syenitic, nepheline syenitic and ijolitic rocks with, in the southeast, a triangular- shaped mass of cancrinite and melanite-nepheline syenite. There is some fenitization of the granitic and dioritic country rocks. The central carbonatite passes outwards from dolomitic and ankeritic rocks with up to 4.5% REE to ring dykes or cone sheets of Nb- and REE-poor dolomite. These are followed by Nb- rich dolomites and sovites with massive, red, altered dolomites in the south, which are succeeded by pyroxene sovites. To the north the outermost carbonatite is a partial ring dyke of phlogopite sovite. A conical form is defined by an inward-dipping foliation. Rare earths are concentrated in bastnaesite and minor monazite, while pyrochlore, pyrite, monticellite and sphene with a little sphalerite, chalcopyrite, pyrrhotine and baryte are present; apatite averages about 15%. Carbonatite dykes cut the country rocks. Analyses of a range of rocks are given by Vallee and Dubuc (1970, Table 4) and the evolution of the complex is discussed by Thivierge et al. (1983).