A small, circular, magnetic anomaly detected 4 km west of Mt Johnson has proved to be a Monteregian intrusion, probably some 900 m in diameter. Unlike other Monteregian intrusions it produces very little topographic expression, only rising about 20 m above the surrounding plain. Outcrop is confined to two small areas in both of which the main rock type consists of 70% plagioclase, hastingsitic amphibole, biotite and accessories. This is cut by dykes up to 5 cm thick of porphyritic nepheline syenite. A possible outcrop and numerous boulders of rougemontite occur on the western side of the intrusion.
KUMARAPELI, P.S., COATES, M.E. and GRAY, N.H. 1968. The Grand Bois anomaly: the magnetic expression of another Monteregian pluton. Canadian Journal of Earth Sciences, 5: 550-3.