While Professor Powell’s boat party was exploring the cañons of the Colorado, Mr. John F. Steward a geologist and member of the party climbed the cliff near the mouth of the Dirty Devil River and approached the eastern base of the mountains. He reported that the strata had in the mountains a quaquaversal dip, rising upon the flanks from all sides. The following year Prof A. H. Thompson then as now in charge of the geographic work of Professor Powell’s survey crossed the mountains by the Penellen Pass and ascended some of the principal peaks. He noted the uprising of the strata about the bases and the presence of igneous rocks. In 1873 Mr. E. E. Howell at that time the geologist of a division of the Wheeler Survey traveled within twelve miles of the western base of the mountains, and observed the uprising of the strata. My own observations were begun in 1875, at which time a week was spent among the mountains. They proved so attractive a field for investigation that in the following year a period of nearly two mouths was devoted to their study. _Other Igneous Mountains._—The Henry Mountains are not the only igneous group which the Plateau province comprises. They are scattered here and there throughout its whole extent. From the summits of the Henry Mountains one can see the Sierra La Sal ninety miles to the northeastward, and the Sierra Abajo seventy miles to the eastward. Beyond them and two hundred miles away are the Elk Mountains of Colorado. Fifty miles to the southwestward stands the Navajo Mountain on the brink of the Colorado; and one hundred and twenty miles to the southeast the Sierra La Lata and the Sierra Carriso are outlined against the horizon. Westward it is less than thirty miles to the Aquarius Plateau, the nearest member of the great system of volcanic tables among which the Sevier and Dirty Devil Rivers rise. Beyond the horizon at the south and southwest and southeast are a series of extinct volcanoes; Mount Taylor and the Marcou Buttes in New Mexico; the Sierra Blanca, the Sierra Mogollon, the San Francisco Group and the Uinkarets in Arizona; and the Panguitch Lake Buttes in Utah. Of the groups which are visible, all but that of the Aquarius Plateau are allied in character to the Henry Mountains.