The area appears to be underlain by Juro-Cretaceous Coast Plutonic Complex diorite. The Granite Mountain is comprised chiefly of a medium-grained greyish-white biotite granite. In hand specimen the rock displays a faint gneissic texture. Microscopic examination indicates an even distribution of orthoclase and sodic plagioclase. Several orthoclase crystals are Carlsbad twinned and some of the plagioclase crystals are polysynthetically twinned with good zonal structures. A small amount of microcline is also present. The biotite is very fresh, its pleochroism ranging from very dark brown to a light straw-yellow. The quartz frequently possesses a faint undulatory extinction. Magnetite occurs in minute well-formed crystals, or in aggregates of small irregular grains. A little epidote, a few small needles of apatite and numerous small crystals of zircon complete the mineralogical composition.