Mountains constitute the grandest relief features of the earth, and some of the most profound lessons of earth changes may be learned by studying them. To the layman who views great mountains in all their grandeur and massiveness, the expression "everlasting hills" seems appropriate. But the geologist knows that even the loftiest mountains are only temporary features on the face of the earth. Like organisms, they come and go. For example, where the great Rocky Mountains now stand was only a few million years ago (in late Mesozoic time) the bottom of an interior sea. Where the Appalachians now stand there were no mountains late in the Paleozoic era (not less than ten or twelve million years ago), but instead sea water covered the district. Then the Appalachians were formed, lifting their heads much higher than at present, after which they were cut down almost to sea level, and then once more upraised. The Coast Range Mountains of our Pacific Coast have come into existence since the middle of the present (Cenozoic) geologic era. Every mountain, like every organism, has a life history, in some cases simple, and other cases complex. All pass through stages of birth, youth, maturity, old age, and death. Some rear their heads and disappear after a short (geological) existence. Others continue their growth and persist much longer, while still others undergo periods of profound rejuvenation. Among the various processes by which mountain ranges have been formed, the folding and accompanying general uplift of strata are the most important. In fact, in most of the great mountain ranges of the world the folded structure is conspicuously developed, so much so that they may well be called "folded mountains." Very commonly, however, mountains of this type have also been subjected to more or less fracturing of the rocks (faulting), and not uncommonly they have also been subjected to igneous activity, including both intrusion and extrusion of molten material. It is among the folded mountains of greater or less degree of complexity that the "greatest exhibitions of geologic phenomena are seen and the lessons which geology as a sciences teaches may be learned. If one desires to know the history of a region, one turns naturally to its mountain ranges, for here may be found the upturned and dissected strata, a study of whose kinds, thickness, and fossils throws light upon past events, while their foldings and dislocations show the nature and results of those great dynamic agencies which, from time to time, have operated upon the outer portion of the earth, and given to it the broad distinctive features which characterize it to-day." (L. V. Pirsson.) Among the great mountains we may also see wonderful exhibitions of the results of weathering and erosion, especially the work of rivers and glaciers.