644 ELIOT BLACKW ELDER 



and probably South, America was affected at about the same time. 

 Smith^ has called this the "Cordilleran revolution," but since it 

 affected less than half of the great mountain tract of the West, to 

 which we usually apply the name "Cordilleras," the designation 

 seems likely to mislead and will perhaps tend to perpetuate a wide- 

 spread popular misconception of the western mountains as all 

 belonging to one great system instead of two or three systems. 

 The term "Nevadian" has therefore been substituted in this paper, 

 in allusion to the Sierra Nevada. 



The system of folds made at this time trends in a north- 

 northwesterly direction more or less parallel to the Pacific shore of 

 today, but by no means coinciding with it. South of the typical 

 area in the Sierra Nevada, the Jurassic folds may be traced into 

 western Mexico and probably Lower California. Northward 

 along the strike it may be traced through Oregon, the Cascade 

 and Olympic ranges of Washington, the Coast Range of British 

 Columbia, and the St. Elias and Alaskan ranges beyond. There 

 are even suggestions of its extension into the Aleutian chain. Its 

 western limit is concealed everywhere by the Pacific Ocean or by 

 unconformably overlapping younger deposits which have themselves 

 been folded subsequently. To the east and northeast, the Nevadian 

 system of folds is involved with the overlapping Laramide system 

 and it therefore becomes a difficult task to discriminate the two. 

 The eastern limit of the Nevadian folds seems to be reached approxi- 

 mately near Bisbee, Arizona, where Comanchean sediments overlie 

 unconformably the deformed Paleozoic strata, and the ill-defined 

 boundary may be traced northwestward through eastern Nevada,^ 

 western Idaho, central British Columbia, the upper Yukon 

 valley in Canada, and central Alaska in general. There is good 

 evidence that it did not affect Alaska north of the Yukon nor the 

 eastern ranges of Mexico. 



' J. Perrin Smith, Science, XXX, 346-51. 



^ It should be noted that much of this region has been so modified by igneous 

 intrusions and faulting of much younger date that the results of the Nevadian 

 orogeny have been in large measure lost to view. It is these later disturbances 

 which have made the present Basin Range province of Nevada distinct from the 

 Sierra Nevada province of California, though both were once parts of the Jurassic 

 belt of folding. 



