674 THE POPULAR SCIENCE MONTHLY. 



thick subaerial 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 subse- 

 quent 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 Ap- 

 palachian system, was at or near the close of the Palaeozoic 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 Plio- 

 cene or early Quaternary rocks, were formed by very late mount- 

 ain-building, perhaps correlative, like the faulting and tilting of 

 the Basin ranges, with great movements of the earth's crust pro- 

 ducing 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. 



5. Erupted Mountain Ranges. 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 mount- 



