Two irregular bodies of fenitic breccia are described from the Kusk and Nemag Lakes area, 19 km southwest of Sudbury, Ontario. In each case, the host rock — feldspathic quartzite of the Mississagi Formation (Huronian Supergroup) — is brecciated and extensively replaced by aegirine, riebeckite, and alkali feldspar. Crosscutting relationships have led to a subdivision into mappable stages. The fenitic assemblages are relatively simple, but they are unusually sodic and deficient in calcium, potassium, and carbonate. Only in the most thoroughly fenitized rocks at Kusk Lake is microcline found. Desilication is a major process in the progressive conversion of the sandstone to a rock in equilibrium with the fluid phase. The scarcity of carbonate suggests that the igneous source material was essexitic or ijolitic rather than carbonatitic. The ultrasodic assemblages are interpreted as having formed near the heat source but clearly at subsolidus temperatures. Late introduction of potassium is attributed to changes in the geometry of the thermal gradient as the intrusive body cooled. Fenitization should not be so restrictive a term as to exclude cases where the resultant bulk compositions are ultrasodic or where the source is an alkaline silicate body rather than a carbonatite.