Lewis Island is underlain mainly by Lower Mesozoic greenstone, and chlorite schist with minor limestone and impure layered quartzite.

White, coarse-grained calcium limestone forms a series of lenticular masses, 9 to 15 metres thick, separated by schist. Analysis of a sample gave 94.61 per cent CaCO3, 2.52 per cent SiO2, 0.42 per cent Fe2O3, 0.39 per cent Al2O3, 0.36 per cent MgCO3, and 0.02 per cent Ca3(PO4)2 - (Pub. 811, 1944).