Page:The Journal of geology (IA journalofgeology21894univers).pdf/411

remains to be briefly noticed. Between the epochs of mountain-building by plication, the diminution of the earth's mass produces epeirogenic distortion of the crust, by the elevation of certain large areas and the depression of others; and these effects have been greatest just before relief has been given by the formation of folded mountain ranges. Two epochs have been preëminently distinguished by extensive mountain plication, one occurring at the close of the Paleozoic era, and the other progressing through the Tertiary and culminating in the Quaternary era, introducing the Ice age. During the last, besides plication and overthrust faulting of the Coast range, the St. Elias range, the Alps, and the Himalayas, a very extraordinary development of tilted mountain ranges, and outpouring of lavas on an almost unprecedented scale, have taken place in the Great Basin and the region crossed by the Snake and Columbia rivers. With the culminations of both of these great epochs of mountain-building, so widely separated by the Mesozoic and Tertiary eras, glaciation has been remarkably associated, and indeed the ice accumulation appears to have been caused by the epeirogenic and orogenic uplifts of continental plateaus and mountain ranges. These processes are well consistent with Dana's doctrine of the general permanence of the continents and oceanic basins; for upheaval of an ocean bed would not diminish but increase the earth's volume. The late glacial and postglacial uplift of North America from its Champlain depression, by the wave-like movement which has been here described, seems an effort of the earth to regain the state of isostasy, or flotation of the crust on the heavier mobile interior, which is capable of flow, whether it be solid or molten.