The immediately preceding paper in this issue of the Bulletin deals with a rhyolite-basalt complex on Gardiner River, Yellowstone Park, previously described by me (Fenner, 1938). Wilcox interprets relations in a very different manner, attributing the contact effects to the interaction of two simultaneous liquid lava flows. The present discussion adheres to the original view that the basalt is much older and that the rhyolite followed a canyon that had been erroded in the basalt. It is contended that all the contact effects can be explained logically on this basis, whereas Wilcox’s view requires extraordinary and seemingly impossible behavior of the two magmas. Subsidiary phenomena likewise are out of accord with his explanation.

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