Page:Popular Science Monthly Volume 39.djvu/695

Rh upward, which, has produced the four other types of constructive mountain ranges and masses by diverse phases of its manifestation—namely, the slow arching of limited areas, as the Uinta, Junction, and Yampa Mountains; the sudden volcanic lifting of the laccolite mountains; the upheaval and subsidence, with faulting and tilting, of the Basin ranges; and the outpouring of lava, as in the Cascade Range. Each of these four phases of vertically acting energy depends upon a viscous and plastic (neither solid nor perfectly liquid) condition of the earth's interior. Greater pressure of some portions of the crust than of others upon the plastic interior would induce each phase of upward energy in mountain-building. Where isolated blocks of the crust yielded slowly to the resulting quasi-hydrostatic pressure of the interior, mountains of the Uinta type were formed; but large areas, as the Great Basin, being swelled upward and anon subsiding, as the interior pressure increased and diminished, have become marked by tilted mountain ranges. Where the relations of intense heat, immense pressure, and chemical influences, with presence of water or its further ingress, have allowed portions of the interior, often of great extent, to become liquid lava, its extravasation by the same pressure has formed laccolite mountains and erupted mountain masses, while many volcanic cones have been mainly built up of fragments of solidified lava, much of it so fine as to be called ashes, explosively ejected.

In an appendix of Wright's Ice Age in North America, I have pointed out the source of the relationship by which these two kinds of mountain-building energy are united, both being caused by the earth's contraction in cooling, and the second or upwardly acting kind of energy being dependent on the first in the intermittent and occasional relief of stress of the earth's crust by its folding along the great orographic belts. Between the epochs of mountain-building by plication, the diminution of the earth's mass produces epirogenic distortion of the crust, by the elevation of certain large areas and the depression of others, with resulting inequalities of pressure upon different portions of the interior; and these effects have been greatest immediately before relief has been given by the formation of folded mountain ranges. There have been two epochs pre-eminently distinguished by extensive mountain-plication, one occurring at the close of the Palæozoic era and another progressing through the Tertiary and culminating at the beginning of the Quaternary era, introducing the Ice age. During the last, besides plication of the Coast Range, of 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