Figs. I–VII give a graphic representation of the average silica percentage of a number of igneous rock types, so arranged as to show the relation between silica-percentage and rock nomenclature. All the analyses used were obtained from the 1917 edition of H. S. Washington's Chemical Analyses of Igneous Rocks”. This book contains some 8,000 rock analyses from many parts of the world. They are divided into two classes, superior and inferior. Only those of the former category (some 6,000 in all) have been utilized.