During the preparation of my report every part of the discussion has been submitted to Professor Powell for criticism, and many of his suggestions are embodied in the text. Similar and valuable aid was received from Capt. C. E. Dutton and Mr. William B. Taylor in the study of the physical problems to which the discussion of the intrusive phenomena gave rise. Captain Dutton rendered an important service also by the study of the collection of igneous rocks, and his report, included in the fourth chapter, testifies to the thoroughness of his work. The supervision of the publication has fallen in large share upon Mr. J. C. Pilling, and the text has had the advantage of his literary criticism, as well as of his watchful care. G. K. G. CONTENTS. Page. CHAPTER I. INTRODUCTORY 1 The rock series 3 Unconformities 8 The Great Folds 11 Cliffs and plateaus 13 How to reach the Henry Mountains 14 CHAPTER II. STRUCTURE OF THE HENRY MOUNTAINS 18 CHAPTER III. DETAILED DESCRIPTION OF THE MOUNTAINS 22 Mount Ellsworth 22 Mount Holmes 27 Mount Hillers 30 Mount Pennell 35 Mount Ellen 38 Stereogram of the Henry Mountains 49 CHAPTER IV. THE LACCOLITE 51 The Henry Mountains intrusives, by _Captain C. E. Dutton_ 61 The question of cause 72 The stretching of strata 80 The conditions of rock flexure 83 The question of cover and the question of age 84 The history of the laccolite 95 Laccolites of other regions 97 Possible analogues of the laccolite 98