_Mountain Ranges._--If oceanic basins and continental domes constitute the greatest features of the earth’s face, and are determined by the most fundamental movements of the crust, surely next in importance come great mountain ranges. These are the glory of our earth, the culminating points of scenic beauty and grandeur. But they are so only because they are also the culminating points, the theaters of greatest activity, of all geological forces, both igneous and aqueous--igneous in their formation, and aqueous both in the preparatory sedimentation and in the final erosive sculpturing into forms of beauty. A theory of mountain ranges therefore lies at the bases of all theoretical geology. To the pre-geologic mind mountains are the type of permanence and stability. We still speak metaphorically of the _everlasting_ hills. But the first lesson taught by geology is that nothing is permanent; everything is subject to continuous change by a process of evolution. Mountains are no exception. We know them in embryo in the womb of the ocean. We know the date of their birth; we trace their growth, their maturity, their decay, their death; we even find in the folded structure of the rock, as it were, the fossil bones of extinct mountains. In a word, we are able now to trace the whole life history of mountains. Mountains, therefore, have always been a subject of deepest interest both to the popular and the scientific mind--an interest intensified by the splendors of mountain scenery and the perils of mountain exploration. The study of mountains is therefore coeval with the study of geology. As early as the beginning of the present century Constant Prevost observed that most characteristic structure of mountains--viz., their folded strata--and inferred their formation by lateral pressure. All subsequent writers have assumed lateral pressure as somehow concerned in the formation of mountains. But that the whole height of mountains is due wholly to this cause was not generally admitted or even imagined until recently. It was universally supposed that mountains were lifted by volcanic forces from beneath, that the lifted strata broke along the top of the arch, and melted matter was forced through between the parted strata, pushing them back and folding them on each side. And hence the typical form of mountain ranges is that of a granite axis along the crest and folded strata on each flank. But attention has lately been drawn to the fact that some mountains, as, for example, the Appalachian, the Uintah, etc., consist of folded strata alone, without any granite axis. In such ranges it is plain that the whole height is due not to any force acting from below, but to a lateral pressure crushing and folding the strata, and a corresponding thickening and bulging of the same along the line of crushing. Then the idea was applied to _all_ mountain ranges. So soon as the prodigious amount of erosion suffered by mountains, greater often than all that is