The postglacial Mount Edgecumbe volcanic field contains at least 14 rock units ranging in composition from olivine-augite basalt to augite-bearing quartz latite. Mesozoic graywacke and slate and Tertiary granitic intrusions underlie the gently dipping basalt which forms the base of the pile. Andesite and basaltic andesite overlie the basalt near Mount Edgecumbe and, in turn, they are probably overlain by the dacitic rocks which make up the composite cone of Mount Edgecumbe proper and by dacite flows and cinder cones on its southwest flank. Mount Edgecumbe and a nearby remnant of a similar cone are cut by latite domes. The remnant is now the site of a caldera 1.6 kilometers in diameter and 240 meters deep. Widespread dacite(?) lapilli and ash probably resulted from explosive eruptions during the formation of the composite cones. Nine chemical analyses define a smooth compositional trend that correlates with the relative age of the map units. The magma series is calc-alkaline and has a close relationship to the high-alumina basalt series.