This paper presents a detailed petrographic and mineralogic study of the rocks of the Nipissing diabase sill near Cobalt, Ontario, Canada. The sill has an undulatory form, and the petrology of the sections in the basins differs from that of the arches. Olivine-bearing hypersthene diabase is confined to the basins. Augite, pigeonite (in part inverted), and hypersthene occur in the sill; the systematic manner in which they vary and are related to each other is detailed herein. The diversity of major facies is reasonably explained by gravity control of differentiation accompanied by some lateral movements. Continuous exposures of rock at several mines have facilitated detailed studies.