Page:Popular Science Monthly Volume 39.djvu/693

Rh masses and plateaus, while the crust segments have been only slightly tilted and sometimes lie wholly beneath the sea-level. Such eruptions form the Cascade Range, the mountainous plateaus of Iceland and the much-eroded Färoë Islands, the Deccan plateau in India, the volcanic chains of the Sunda, Kurile, and Aleutian Islands, and the Hawaiian Island belt.

The Cascade Range is a typical example of this class, having an extent of more than 500 miles from south to north across Oregon and Washington, showing a thickness of nearly 4,000 feet of lava where it is cut through by the Columbia, and bearing here and there volcanic peaks which rise to altitudes 10,000 to 14,000 feet above the sea. The eruptions producing this range took place during late Tertiary and early Quaternary time, being contemporaneous with the faulting and tilting of the Basin ranges, the Wahsatch, and the Sierra Nevada, and with the folding and upbuilding of the Coast Range. As Jamieson and Alexander Winchell have well suggested, the outpouring of the vast lava floods of the Cordilleran belt in the United States, a portion of which forms the Cascade Range, was probably in large part or wholly dependent on movements of elevation and subsidence of the adjacent glaciated area. Another erupted range, on a smaller scale, but very interesting in its details as described by Russell, belonging to the Quaternary era, and partly to the recent epoch, lies close south of Lake Mono, in California.

—The form and contour of nearly all mountains, excepting volcanic cones, have been given to them by the sculpturing agencies of subaërial denudation. This is true of each of the foregoing classes, where mountain building energy has supplied the mass, but erosion has shaped the slopes, ridges, and peaks, the ravines and valleys. These five classes of mountain ranges have been sculptured by erosion, where previous mountain-building has raised limited areas to an exceptional altitude. But besides these orogenic upheavals, there have been broader uplifts of the whole or large parts of continents, which Gilbert and White have called epirogenic movements. The sixth class of mountain ranges, here to be noticed, is distinguished from all the preceding by its comprising mountains which owe their origin to no definite mountain-building process, being simply remnants of extensive areas which have been uplifted by epirogenic energy as great plains and since have been deeply eroded.

The plains which slowly rise from the Mississippi Valley and Manitoba westward to the foot of the Rocky Mountains afford examples of this type of mountain structure. Perhaps the most striking is the range of the Crazy Mountains in Montana, which lies immediately north of the Yellowstone River near Livingston, and