Scores of volcanic necks, dikes, and lava-capped mesas rise from the high plateau of northeast Arizona and the adjacent parts of Utah and New Mexico. These are the remnants of a volcanic field that formerly covered many thousands of square miles. Erosion has so far dissected this field that the original cones have disappeared, the sheets of lava have been dismembered, and the old volcanic conduits now rise as giant towers, revealing their inner structure with singular clarity. The objects of this paper are to discuss the features of these denuded volcanoes, the character of their ejecta, and the age of their eruptions. To petrologists, the region is one of unusual interest, because of the similarity of its igneous rocks to certain of those in the classic regions of alkaline rocks in Montana and Wyoming. So extensive is the Navajo-Hopi volcanic province, that the present survey is to be . . .