This classic paper presents Fenner's views of magmatic differentiation against those of N.L. Bowen (1928). Fenner presents the original case for iron-enrichment trends in residual basaltic liquids, contrasted with an iron depletion trend leading to rhyolite. He discusses the general crystallization process of basaltic lavas, mentions Clarence King's (1878) identification of olivine settling in Hawaii, and considers the results of the crystallization of augite and olivine from a picrite-basalt of East Maui.