The core of Campania Island consists of clean, massive medium to coarse-grained biotite quartz monzonite of the Coast Plutonic Complex. To the west of a northwest trending fault is granodiorite.

Mica, resembling coarse muscovite crystals, occurs in 15 to 60 centimetre wide bands of coarse pegmatite within the quartz monzo- nite. These bands are irregular and discontinuous and are 7 to 30 metres in extent. Belts and streaky zones of fine crystalline mica, up to 100 metres length, are widely distributed in finer-textured pegmatites.

The coarser-grained mica constitutes about 10 to 25 per cent of the bands and the finer mica composes 25 to 50 per cent of the zones.