THE BUILDING OF THE COLORADO ROCKIES 237 



the granite areas of the main ranges where it would readily escape 

 notice, or within the areas of folded sedimentaries, portions of 

 which might have been raised without materially increasing the 

 intensity of the folding. But on the other hand renewed horizontal 

 thrusting which merely carried further the folding process would 

 cause no harm provided the thickness of the folded shell were the 

 same as in the previous episode of folding. If the thickness were 

 different, however, such later deformation would add a compli- 

 cating factor. The folded Eocene volcanics at Windy Gap on the 

 Grand River show that the folding cannot all be assigned to a 

 single period, and this later folding may have been fully as intense in 

 the region in question as the earlier Laramide diastrophism. 



Although these are admittedly embarrassing factors which may 

 have operated to some extent and which, if they did, lessen the 

 accuracy of the final results, still it is believed that quantitatively 

 they probably were not of sufficient magnitude to alter the general 

 purport of the results. When it is noted that the average height of 

 the folded tract throughout the entire section, according to 

 measurements, is slightly over 13,000 feet above the datum-plane, 

 it is apparent that a concealed, or unrecognized, uplift of 1,000 

 feet, or even 2,000 feet, will not change the order of magnitude or 

 general significance of the results. The former would amount to 

 but 7 . 7 per cent of the total; the latter to 15 . 4 per cent of the total. 

 It is also frankly to be recognized that the method of projecting 

 folds involves at best some error, and that when granite areas 

 are included in the section the possible error becomes just so much 

 greater. Such errors are inevitable owing to the limitations which 

 the section imposes, and in the Colorado Rocky Mountain case 

 they are considerable. Hence it is believed that refinements of 

 method such as the introduction of corrective factors for the uplift- 

 ing of the folded region during the interval between the folding and 

 the peneplain stage, or for changes in the sea-level, or for the 

 possible spreading and settling of the high standing mountainous 

 masses, or for other changes, cease to be very vital in this case, 

 though thoroughness in analysis requires their recognition. In any 

 case they would be likely to offset one another more or less. The 

 measure of inevitable error inherent in the section itself is so 



