A 300 metre wide band of limestone and dolomite enclosed in granitic rocks of the Tertiary-Cretaceous Coastal Plutonic Complex extends northwestward from the west shore of Frederick Arm up the side of Treble Mountain for at least 800 metres. The carbonates strike 125 degrees and dip vertically. The band is cut by fine grained diabase dykes.

The carbonate mass is composed of bluish grey, fine grained limestone containing a few beds of white to yellowish white dolomite. In places dolomite and pyrite grains are disseminated in the limestone. The limestone is occasionally contaminated with blebs of silicates. A chip sample across a 30 metre section of limestone contained 48.52 per cent CaO, 2.77 per cent MgO, 5.92 per cent SiO2, 1.16 per cent Al2O3, 0.50 per cent Fe2O3, and 0.46 per cent sulphur (Canada Bureau of Mines Report 811, p. 175, Sample 24). A chip sample across a 3.7 metre thick dolomite bed on the west side of the carbonate deposit contained 32.78 per cent CaO, 17.94 per cent MgO, 2.60 per cent SiO2, 0.55 per cent Al2O3, 0.46 per cent Fe2O3 and 0.38 per cent sulphur (Canada Bureau of Mines Report 811, p. 175, Sample 24A).