The Mason showing is located 4 kilometres northwest of Armstrong.

In this area, sedimentary and volcanic rocks of the Upper Triassic to Lower Jurassic Nicola Group are in fault contact to the north with Cambrian-Ordovician volcanic (Tsalkom Formation) and sedimentary (Sicamous Formation) rocks. To the south the Nicola Group is in probable unconformable contact with Devonian to Triassic sedimentary and volcanic rocks of the Harper Ranch Group. Cretaceous granodiorite plugs of the Salmon Arm Intrusions intrude the Nicola, Sicamous and Tsalkom rocks. Outliers of Eocene Kamloops Group volcanic rocks are present in the area.

A marble unit in the Tsalkom Formation has been quarried for calcite for agricultural use and evaluated for other industrial potential. The unit comprises 5 to 15 centimetre beds of white to pink, bluish grey, and white and grey fine-to coarse-grained high-calcium marble, within a unit at least 55 metres thick. The beds strike 115 degrees and dip 30 degrees south. Thin sheets of intercalated schist and occasional vertical igneous dikes are present. The marble is highly fractured. Muscovite, quartz, pyrite and limonite are occasionally present. Sampling of pure marble returned values of 98.3 per cent calcite, 0.10 per cent Fe2O3 and 1.1 per cent silica.

Earlier this century, the marble was quarried and burnt on site in a lime kiln. In 1970, Mount Rose Mining Co. Ltd. drilled 11 holes, followed in 1971 by an evaluation of the industrial potential of the deposit. Measured geological reserves are 998,000 tonnes grading 55.3 per cent CaO with a cutoff grade of 0.10 per cent Fe2O3 (Industrial Minerals File - Kerr, J. 1971).