THE 



AMERICAN JOURNAL OF SCIENCE 



[THIRD SERIES.] 



Art. XXII. — Rocky Mountain Protaxis and the Post- Creta- 

 ceous Mountain-making along its course y by J. D. Dana. 



The Rocky Mountain Protaxis, or Summit Archaean range — 

 which includes the Front Range of Colorado and Montana, 

 and is continued in British America to the parallel of 52£°, 

 and beyond this in some isolated areas — was described, in my 

 Toronto paper,* as having an eastward bend in Montana and 

 Wyoming through more than 250 miles of latitude. It was 

 stated that owing to this eastward shove of the grand line of 

 heights, the United States have a Rocky Summit area of great 

 breadth west of the protaxis, and that this summit area of the 

 protaxis is continued into British America east of the axis. 

 The latter was proved to be the true continuation of the for- 

 mer by references (1) to its having, in the main, the same rocks 

 in the same succession to the top of the Cretaceous, as is shown 

 on a colored geological map by the course of the western out- 

 line of the green-colored Cretaceous areas, and (2) by the evi- 

 dence that both areas participated alike, and together, in the 

 Rocky-summit upturning closing the Cretaceous period, this 

 being sustained by the observations of the Canadian geologists. 



It was further remarked that, aligned with the Canadian or 

 northern part of the Archaean protaxis, there was, to the south, 

 its interrupted continuation, 10,000 to 12,000 feet high, for a 

 hundred miles along the Wasatch Mountains ; and that this 

 range, which King showed to be near the eastern limit of the 

 Great Basin, was the true western limit of the "Rocky Sum- 

 mit" region. 



* Bull. Geol. Soc. of America, i, 36. 



Air. Jour. Sci — Third Series, Vol. XL, No. 237.— Sept., 1890. 

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