[Illustration: FRONTISPIECE.—Half-stereogram of Mount Ellsworth, drawn to illustrate the form of the displacement and the progress of the erosion. The base of the figure represents the sea-level. The remote half shows the result of uplift alone; the near half, the result of uplift and erosion, or the actual condition. (See page 95.) ] DEPARTMENT OF THE INTERIOR. U. S. GEOGRAPHICAL AND GEOLOGICAL SURVEY OF THE ROCKY MOUNTAIN REGION. J. W. POWELL, IN CHARGE. REPORT ON THE GEOLOGY OF THE HENRY MOUNTAINS. By G. K. GILBERT. [Illustration: [Logo]] WASHINGTON: GOVERNMENT PRINTING OFFICE. 1877. DEPARTMENT OF THE INTERIOR, U. S. GEOGRAPHICAL AND GEOLOGICAL SURVEY OF THE ROCKY MOUNTAIN REGION, _Washington, D. C., March 5, 1877_. SIR: I have the honor to transmit herewith a report on the Geology of the Henry Mountains, by Mr. G. K. Gilbert. I am, with great respect, your obedient servant, J. W. POWELL, _In charge_. The Hon. SECRETARY OF THE INTERIOR, _Washington, D. C._ DEPARTMENT OF THE INTERIOR, U. S. GEOGRAPHICAL AND GEOLOGICAL SURVEY OF THE ROCKY MOUNTAIN REGION, _Washington, D. C., March 1, 1877_. DEAR SIR: I submit herewith my report on the Geology of the Henry Mountains, prepared from material gathered under your direction in the years 1875 and 1876. I am, with great respect, your obedient servant, G. K. GILBERT. Prof. J. W. POWELL, _In charge_. PREFACE.