Recent ⁴⁰Ar/³⁹Ar ages on pumice samples from the Bapang Formation in central Java, Indonesia have constrained the ages of many of the Homo erectus fossils from the Sangiran dome to between 1.5 and 1.0 Ma. (Larick et al., 2001, PNAS, v. 98, p. 4866-4871). The stratigraphy of this formation consists of five fining-upward sedimentary cycles deposited in an aggrading fluvial system. Consistent mineralogical variations in the pumice were recognized in these cycles. The oldest pumice (ca. 1.5 Ma) lacked augite and contained bright green hornblende; ca. 1.25 Ma pumice had < 1 % augite and brownish-green hornblende, intermediate augite content and ca. 1.0 Ma pumice contained 1-5 vol. % augite and brownish-green hornblende (Larick et al., 2001). The Bapang Formation overlies the Sangiran Formation in this area, which holds the oldest Homo erectus fossils found in the dome. This formation has a basal laharic breccia that is overlain by dark-colored siltstones and mudstones with interspersed thin tuff layers containing rare pumice. The stratigraphy of this unit, including the stratigraphic context of the Homo erectus fossils is poorly understood. The goal of this study is to determine if compositions of hornblende have the potential to be used to interpret the stratigraphy of the Sangiran Formation. Hornblende crystals in pumice from the Bapang Formation will be analyzed by electron microprobe techniques to determine if their compositions vary consistently between cycles. If so, then these same techniques will be applied to samples from the Sangiran Formation to interpret the stratigraphy associated with the oldest Homo erectus fossils in Java.