Gough Island, one of the most southerly islands on the Mid-Atlantic Ridge, consists of rocks of the alkali basalt → trachyte association. Representative rock types, ranging from picrite basalt, olivine basalt, trachybasalt, trachyandesite to trachyte have been analyzed for major and minor constituents. Crystallization trends have been determined for the olivines, pyroxenes, feldspars, and iron-titanium ores using optical, chemical, and X-ray methods. Strong evidence is given for differentiation by crystal fractionation.