Evidence from several volcanic regions indicates that andesite and dacite of the basalt-andesite-dacite-rhyolite association form by contamination of primary basalt and rhyolite magmas by material of the volcanic pile. This evidence includes bulk composition, phenocryst assemblage, inclusion content of lava, and the association of different lava types in time and space. The dacite of Glass Mountain, formed by contamination of rhyolite magma as it intruded the Medicine Lake highland shield volcano, illustrates this process.

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