Page:Popular Science Monthly Volume 39.djvu/691

Rh many ranges in various parts of the world have been formed in this way, their sites being now marked by profound faults which are clearly traceable, though the tilted mountains of the upheaved side of-the faults have long since passed through youth, maturity, and old age, leaving no topographic evidence of their former existence.

Many stages of mountain-building have left their impress on the great Cordilleran belt of the western part of the United States and the Dominion of Canada. The Gold and Selkirk ranges of British Columbia, according to Dr. George M. Dawson, consist of Archa3an, Cambrian, and Silurian formations, which were pushed up into mountain folds before the close of these very ancient divisions of geologic time. The auriferous slates of the Sierra Nevada, as Becker has shown, were similarly built up in a folded mountain range at the close of the Gault epoch in the Cretaceous period. During the ensuing long lapse of time to the end of the Tertiary era, this precursor of the Sierra Nevada range had been worn down to only a moderate elevation by the gnawing frosts, heat, rains, and running streams; but the beginning of the Quaternary era, according to Le Conte and Diller, brought revolutionary changes. The previously base-leveled region which now forms the Great Basin was then upheaved as a high plateau; intense volcanic activity was manifested in many parts of this area, and especially from the vicinity of Lassen Peak and Mount Shasta northward to the Columbia River and eastward along the Snake River to the Yellowstone National Park; and long faults, running mostly from north to south, divided the distended region into a multitude of orographic blocks, which, being soon allowed to sink, became tilted in their subsidence and form the present Basin ranges.

If we attempt to correlate these events with the Quaternary glaciation of the northern part of our continent, they seem to have been contemporaneous with the maximum extension of the ice-sheet of the first Glacial epoch. The ice accumulation I have attributed, on evidence derived from fiords and from river-channels now deeply submerged by the sea, to former great elevation of the glaciated areas, probably three thousand to four thousand feet higher than now. But the glacial and modified drift show that toward the end of each of our two principal Glacial epochs the land on which the ice lay was depressed nearly to its present level or in part lower. This depression of the earth's crust I believe to have been caused by the vast weight of the ice-sheets; and, in the first Glacial epoch, we have the correlative somewhat sudden elevation of a contiguous area, with outpouring of lava and formation of tilted mountain ranges, in the Great Basin and north to the Columbia. During the long interglacial epoch very