Re-examination of petrochemical data for high-silica ash-flow tuffs suggests that crystal-liquid fractionation plays a major role in the differentiation of high-silica magmas. For the high-silica Bishop Tuff, the enrichment and depletion of 27 major and trace elements correlates with the bulk crystal-liquid partition coefficients (calculated from the data of Hildreth) in ways that are consistent with crystal fractionation. Comparison of the Bishop Tuff with other high-silica tuffs and rhyolites with different compositional variations and different phenocryst assemblages shows the correspondence of chemical gradients with mineralogy. Crystal zoning studies in the Cordillera Paine granite in southern Chile suggest that the evolution of differentiated liquids similar to those in the Bishop Tuff results from crystal fractionation. The physical mechanisms of separation of liquid and crystals for the Paine granite and Bishop Tuff were quite different, however. Differentiation of the high-silica ash flows might result from advection of a less dense liquid from a crystallizing boundary into the top of the magma chamber.