Barth describes rocks from several central Pacific regions, including the island of Hawaii, gives modal and normative compositions, and describes individual mineral phases in detail. The theory of crystal settling is considered adequate to explain the differentiation of Pacific lavas. Part I gives mineral optics, petrographic descriptions, and normative pyroxene components calculated from the bulk rocks; Part II gives norms from analyses in Washington (1923a-c). Modal data are quoted from Stone (1926b; Kilauea) and given by the author for lavas from Kohala, Mauna Kea, Mauna Loa (prehistoric and 1859, 1887, 1881, and 1919 eruptions), Haleakala (prehistoric and 1750), Hualalai (prehistoric, 1801, and trachyte from Puu Anahulu), and Kilauea (dike).