Page:Popular Science Monthly Volume 39.djvu/692

674 thick subaërial deposits, called by Russell adobe, were supplied by denudation of the mountains and spread on the lower parts of the Great Basin and in the San Joaquin Valley; and the subsequent two flooded stages of Lakes Bonneville and Lahontan belong apparently to the second Glacial epoch and to a later or third epoch of glaciation in the northern part of the Cordilleran region.

Extending our view to embrace the entire belt of which the Rocky Mountains, Sierra Nevada, and Coast Range are parts within the United States, we see that it forms the western side of both South and North America. Its length from Cape Horn to Alaska is about ten thousand miles of a great circle, from which the irregular course of the chain is nowhere widely distant. These complex mountain systems, including the., Andes, the mountains of Central America and Mexico, the Rocky Mountains and parallel ranges west to the Pacific, and the Alaskan mountains, may be together named the Andes-Cordilleran belt. In Bolivia and Peru the highest portions of the Andes are found by David Forbes to be folded Silurian strata, which are so associated with Devonian, Carboniferous, and Permian formations as to imply that the principal epoch of mountain plication there, as of the Appalachian system, was at or near the close of the Palæozoic era. But later epochs of plication are also recognized in portions of the Andes, as likewise in the rocks of the Sierra Nevada, the Wahsatch, and the Coast Ranges, in the western United States. Indeed, the last-named range, and the range which culminates in Mount St. Elias, the former stated by Whitney to contain infolded Pliocene beds, and the latter found by Russell to consist of Pliocene or early Quaternary rocks, were formed by very late mountain-building, perhaps correlative, like the faulting and tilting of the Basin ranges, with great movements of the earth's crust producing and accompanying glaciation. The present height of the Andes, as of the Appalachian, Atlantic, and Laurentian mountain systems, and the Cordilleran ranges of the west part of this country and Canada, must be ascribed to Tertiary and Quaternary upheavals of this belt, portions of which had long before and at different times been folded and raised to mountain heights, but afterward had suffered erosion almost or quite to a base-level.

—Volcanic action has often been developed on a grand scale along the deep fissures and fault planes which border and intersect tilted mountains and plateaus, as notably in the Andes and in Mexico, where it has built up very conspicuous volcanic cones of outpoured lavas and ejected blocks, bombs, lapilli, and ashes. Often, too, prolonged fissures, which may intersect each other (as in the Hawaiian Islands), reach down through the earth's crust to lavas that well up and build