Most of Burritt Island is underlain by fenitized granitic rocks which grade southwards through quartz fenites into syenitic fenites of K-feldspar and aegirine. The fenites on the eastern side of the island are cut by aegirine-rich dykes with some carbonate and altered nepheline. Carbonatite extends for some 60 m along the eastern shore and up to 15 m inland. It contains abundant aegirine defining crude layers, and minor hematite, magnetite, apatite, sodic amphibole, pyrite and pyrochlore. Lamprophyre dykes cut the fenites at the northern end of the island. The extent of the complex beneath the waters of Lake Nipissing is not known.