238 ROLLIN T. CHAMBERLIN 



considerable that it is scarcely worth while to attempt to evaluate 

 these subordinate factors. 



Recognizing the limitations of the method, but believing that 

 the probable errors from these complicating factors are not great 

 enough quantitatively to affect seriously the general conclusions, 

 and that in any case they are probably less than the errors which 

 will always stand in the way of a dynamic study of these ranges, 

 owing to scarcity of data upon the granite areas, this investigation 

 will proceed on the hypothesis that the peneplain level today 

 represents with fair approximation the original level from which 

 the folds arose. A hypothetical picture of this situation might be 

 as follows: The folding of the Rockies was accompanied in all 

 probability by some general uplift of the continent as a whole. 

 Suppose that this uplift brought the Great Plains belt adjoining 

 the Rockies to an elevation of 2,000 feet above the sea. Let us 

 assume that the base-leveling process in developing the peneplain 

 caused a rise of the sea to the extent of 500 feet. Then the pene- 

 plain, in order to constitute the correct datum-plane, should have 

 been formed at an elevation of 1,500 feet above sea-level. Whether 

 this imperfectly reduced Colorado peneplain, featured by many 

 large monadnocks and nearly a thousand miles by the existent 

 drainage routes from either the Gulf of Mexico or the Pacific 

 Ocean, may reasonably have developed at this elevation is left to 

 the reader to judge. 



The average height of the reconstructed folded surface above 

 this natural datum-plane was found to be as follows: 



Miles 



No. I. Lyons sheet (westernmost 7 miles) . . . . 1.51 



No. 2. St. Vrain sheet 3.13 



No. 3. Allen's Park sheet 3- 9° 



No. 4. Continental Divide sheet . . . . . . . 4.03 



No. 5. Granby sheet 2.18 



No. 6. Sulphur Springs sheet 1.68 



No. 7. Troublesome sheet 2.42 



No. 8. Gore Range sheet 2.40 



No. 9. State Bridge sheet 2 . 24 



No. 10. Castle Peak Sheet 1.62 



No. II. Gypsum sheet 1.98 



No. 12. Dotsero sheet 2.54 



No. 13. Glenwood Springs sheet 2.83 



No. 14. Grand Hogback sheet 0.43 



