This is an elliptical complex of 3.2x2.7 km much of which lies beneath Lake Nipissing, the outer fenite ring of which crops out on five islands. Extensive drilling and tunnelling have allowed some definition of the extent of the complex. Ordovician sedimentary rocks overlie the complex unconformably on all five islands. Outer quartz and inner syenitic fenite zones can be distinguished. The syenitic fenite consists essentially of alkali feldspar and sodic pyroxene with pegmatitic veins having cockscomb pyroxenes; other minerals present include sodic amphibole, calcite, occasional quartz, brown biotite, apatite and minor pyrite, magnetite, monazite and uranium pyrochlore. The fenites are cut by carbonatite veins and drilling indicates that the central area contains thick screens of carbonatite. The carbonatites are variably coarse to fine grained calcite rocks with streaks of sodic pyroxene together with biotite, magnetite, apatite and locally uranium pyrochlore and pyrite. Between the syenitic fenites and central carbonatite area are alkali pyroxenites and syenites, presumably fenite variants, in which some altered nepheline occurs. Rocks described as 'altered feldspathic rocks' occur in this zone to the south, and could be potassic fenites. Lamprophyric and syenitic dykes cut the fenites.