An almost circular intrusion nearly 10 km in diameter, the Otto Stock cuts metamorphosed Archaean volcanic and sedimentary rocks, and in the south granodiorites of the Round Lake batholith. Numerous inclusions of country rocks are found in the stock, particularly near the margins, and there is a contact metamorphic aureole up to 0.4 km wide on the north side. Most of the stock is composed of granite, quartz syenite and syenite porphyries, with a general decrease in quartz towards the margins, nepheline syenite occurring locally along the northern side. Apart from microperthite and quartz, most of the rocks contain amphibole and up to 14% of aegirine-augite. Foliated nepheline syenite occurs as sills, thin pegmatitic dykes and as gneiss. In some rocks nepheline reaches 50%, the other minerals generally being microcline, often perthitic, aegirine-augite, biotite, scapolite, apatite and sphene.