Macdonald recounts the history of the 1840 eruption drawn from earlier sources. He gives the petrography of some vents and an analysis of a lava sample, and olivine separated from it, collected from the lowermost vent. He calculates the composition of the groundmass and compares it to analyses of 1894 and 1920 Kilauea lava, stating that the petrogenesis can be inferred without analyzing samples from the other 1840 vents. Olivine basalt, nearly devoid of olivine phenocrysts, was erupted from vents near Alae and Makaopuhi craters, followed shortly by eruption of picrite-basalt with abundant olivine phenocrysts from lower vents in eastern Puna. Olivine phenocrysts were inferred to have settled out of the upper part of the reservoir, which fed the upper vents, and accumulated in the lower part of the reservoir, which fed the lower vents.