If these pages fail to give a correct account of the structure of the Henry Mountains the fault is mine and I have no excuse. In all the earlier exploration of the Rocky Mountain Region, as well as in much of the more recent survey, the geologist has merely accompanied the geographer and has had no voice in the determination of either the route or the rate of travel. When the structure of a mountain was in doubt he was rarely able to visit the points which should resolve the doubt, but was compelled to turn regretfully away. Not so in the survey of the Henry Mountains. Geological exploration had shown that they were well disposed for examination, and that they promised to give the key to a type of structure which was at best obscurely known; and I was sent by Professor Powell to make a study of them, without restriction as to my order or method. I was limited only in time, the snow stopping my work two months after it was begun. Two months would be far too short a period in which to survey a thousand square miles in Pennsylvania or Illinois, but among the Colorado Plateaus it proved sufficient. A few comprehensive views from mountain tops gave the general distribution of the formations, and the remainder of the time was spent in the examination of the localities which best displayed the peculiar features of the structure. So thorough was the display and so satisfactory the examination, that in preparing my report I have felt less than ever before the desire to revisit the field and prove my conclusions by more extended observation. In the description of the details of the structure a demand arose for a greater number of geographic titles than were readily suggested by natural forms or other accidents, and recourse was had to the names of geologists. Except that the present members of my own corps are not included, the names chosen are of those whose cognate studies have given me most aid. Mr. Steward and Mr. Howell saw the Henry Mountains before I did, and gleaned something of their structure from a distance; Dr. Newberry, Mr. Marvine, Dr. Peale, and Mr. Holmes have described allied phenomena in Colorado and New Mexico; and the works of Messrs. Jukes, Geikie, Scrope, and Dana have been among my chief sources of information in regard to igneous mountains in general. If any of these gentlemen feel offended that their names have been attached to natural features so insignificant, I can assure them that the affront will never be repeated by the future denizens of the region. The herders who build their hut at the base of the Newberry Arch are sure to call it “the Cedar Knoll”; the Jukes Butte will be dubbed “Pilot Knob”, and the Scrope, “Rocky Point”.