Certain lavas on Gardiner River in Yellowstone Park, Wyoming, have been cited (Iddings, 1899, p. 430; Fenner, 1934; 1937; 1938) as representing extensive assimilation of solid basaltic material by molten rhyolitic lava at pressures near that of the atmosphere. The writer, however, concludes that the features of the Gardiner River lavas resulted from the mixing of nearly contemporaneous rhyolitic and basaltic lavas. On this basis, the occurrence does not bear on the question of the relative importance of assimilation versus gravitational differentiation in the origin of rocks.

The proponents of assimilation base their case on the definite intrusive relationships of the rhyolite into the basalt (Iddings, 1899, p. 430–32; Fenner, 1938), the roughly V-shaped form of the rhyolite-basalt contact, and the chemical gradation from rhyolite to basalt (Fenner, 1938). However, the writer concludes that, if the two lavas had been erupted simultaneously, the solidifying effect of the normally cooler rhyolitic lava on the basaltic lava would have caused the same relationships.

Two important features point strongly to an origin of the Gardiner River lavas by mixing of molten rhyolitic and basaltic lavas: (1) A chill phase of the rhyolite against country rock but not against the basalt indicates that the basalt was not cold when contacted by the rhyolitic lava. (2) The presence of quartz and orthoclase xenocrysts (originally phenocrysts of the rhyolitic magma) within the basalt several yards from the contact indicates that the basalt was liquid or mushy when contacted by the rhyolitic lava.

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