The Roraima formation in British Guiana consists of quartzitic and arkosic sandstones, conglomerates, and shales of continental origin. The Waracobra noritic gabbro sill, exposed by the Kuruaparu creek, rests on the Mazaruni volcanics and apparently terminates against a wall of sandstone. The parent magma of the sill is considered to be tholeiitic. Layering by crystal fractionation and settling is absent, but zoned crystals indicating small-scale differentiation are present. A chemical analysis of a chilled margin is remarkably similar to that of the lower contact of the Palisade sill in New Jersey. Discovery of a few felsite dikes on nearby Kanaima mountain, with high-temperature quartz phenocrysts, may provide a clue to the age of the Roraima formation, now unknown.