Differences in bulk-rock composition between granite plutons and a nearby sequence of rhyodacite ash flows that are similar in age, mineralogy, and Sr- and O-isotopic composition are considered to result from selective separation of pumice fragments during the ash-flow eruption rather than initial differences between the parent magmas. By comparison with modern ash flows it is argued that preferential loss of light vitric pumice fragments in the flow and the complementary concentration of the phenocrystal components during eruption has resulted in the ash flow being of rhyodacitic (rather than rhyolitic) composition. Quantitative estimates based on: (a) this model of selective glass loss; (b) the extremely regular variation trends of the plutons; and (c) the bulk-rock and groundmass composition of the ash flow show that a 36% loss of the vitric fraction during eruption of magma of the average composition of the plutons would explain the observed differences in composition. A loss of this amount of the vitric fraction is quite consistent with data from modern ash flows. It is suggested that some of the rhyolitic volcanic units associated with the ash flows are the complementary glass-rich ash-fall tuffs from the same series of ash-flow eruptions. Based on the chemical differences between the ash flow (rhyodacite, SiO2 = 60%) units and the inferred source magma (granite, SiO2 = 70%) it is argued that laboratory melting experiments on ash flows might be difficult to interpret if the order of mineral crystallization and/or the mineral chemistry is sensitive to bulk-rock composition.