182 J. D. Dana — Rocky Mountain Protaxis and tin 



I return to the subject to illustrate further the characters of 

 the Rocky Mountain protaxis and the results of post-Cretaceous 

 mountain-making along it, deriving the facts presented from 

 the U. S. Government Geological Survey and that of the 

 Canadian Dominion. 



1. Comparative Features of the Eastern and Western Protaxes. 



1. The bend in the Western Protaxis related in origin to 

 that in the Eastern. — In my paper on the Eastern Archaean 

 axis, in the last volume of this Journal, I point out the fact 

 that the Green Mountain protaxis had a landward bend oppo- 

 site the southern extremity of the Archaean continental nucleus, 

 the nucleal V ; and, in a note to page 379, the great bend in 

 the Rocky Mountain protaxis is referred to as similarly situated 

 abreast of the termination of the V. This correspondence in 

 the two suggests similarity of origin ; and we can hardly doubt 

 that the bends were there made because the V there termin- 

 ates ; that the lateral thrust landward in direction which out- 

 lined the Y, and later determined the existence and position 

 of the protaxis, encountered diminished resistance where the 

 nucleus loses its emergence, and that it hence shoved the line 

 of uplift farther inland. 



As regards the eastern protaxis this origin of the bend is 

 recognized in the first edition of my Geological Manual (1863, 

 p. 737), where I say " The Azoic [Archaean] nucleus of North 

 America, spreading southward, formed a peninsula in northern 

 New York. Even this bend in the nucleus continues in the 

 finished continent ; for New England has the same outline. 

 Its east and south outlines are but a repetition of the east and 

 south coast-lines of the old Azoic peninsula. This exact copy- 

 ing of the nucleus by the growing continent proves, better 

 than all other evidence, the grand fact that the progress has 

 been through oscillating forces acting against the stable Azoic 

 nucleus." The dependence of continental mountain-making 

 on Archaean features was thus fully recognized. 



Differences between the two protaxes. — The eastern or Green 

 Mountain protaxis is essentially simple in its course, notwith- 

 standing the bend ; the western was made a divided chain in 

 consequence of the bend. The eastern protaxis is the eastern 

 or sea-ward limit of the geological formations of the Conti- 

 nental Interior from the close of the Lower Silurian onward. 

 But the main branch of the western protaxis, south of the 

 bend, that of the Front Range and its continuation southward 

 to Mexico, is not the western or sea-ward limit of any of the 

 geological formations, and even the Cretaceous, the Jast of the 

 Mesozoic series, extends west to the Wasatch line. This Wasatch 

 line may hence be well designated the western of two summit 



