176 A, G. Peak — Age of the Rocky Mountains in Colorado. 



Golden, near Colorado Springs, and near Canon City (pp. 490, 

 500), and also in Huerfano Park. Near Colorado Springs we 

 have the following section.* Eesting on the Archasan rocks 

 ping eastward about 

 10°. Going eastward we next meet with Triassic red sand- 

 stones, standing on edge, or inclined past the vertical. From 

 the Trias to the Dakota group of the Cretaceous the dip de- 

 creases to 40°, and this fishes eastward until at 

 Monument Creek it is only 4°. At first sight there wouh 1 a pi.t-ar 

 to be unconformability, especially close to the Mountains, but 

 the beds are conformable, and the varied dips show the presence 

 of an abrupt fold, which is somewhat obscured by the subse- 

 quent erosion.f The same thine' occurs near Golden, (seesection 

 13 on plate opp. p. 136, Eeport U. S. Geol. Survey, 1873.) Mr. 

 Marvine says,$ "Within exceedingly short distances, then, _ 

 great changes of dip may occur, and from them, with but 

 slight changes of exposure, unconformability might be inferred. 

 Yet all are perfectly conformable ; the sudden change really 

 I only a very abrupt flexure in the main fold, as 

 indicated by the dotted lines." Near Canon City, neither Dr. 

 Hayden, Dr. Endlich, Mr. Holmes, nor myself have noted any 

 unconformability. The other cases are probably similar to 

 I rolden and Colorado Springs. 



What I now wish to show is that in Colorado the evidence 

 exists that there was a subsidence commencing in, or prior to 

 the Carboui ; ring through the Triassic, Juras- 



sic, Cretaceous and into the Tertiary. This may have been, and 

 doubtless was interrupted by oscillations, but the general move- 

 ment was depression until the close of the Cretaceous, when 

 there were probably local elevations. A general gradual eleva- 

 tion of the whole West is not incompatible with local depression 

 in Colorado. There are evidences that the upward movement 

 began in Cretaceous time, but as pointed out by Dr. Hayden 

 and Dr. Newberry the elevation of the Eocky Mountains t>>ok 

 place between the close of the Cretaceous and that of the 

 Miocene Tertiary. 



From a study of the Carboniferous rocks in Colorado, we 

 learn that during the earlier part of the Carboniferous age deep 

 seas prevailed with large continental areas or islands. As the 

 time progressed the seas became shallower and encroached more 

 and more upon the laud, which was composed mainly of 

 Archaean rocks. 



The Coal-measure rocks at the northern end of the Sa watch 

 Eange denote in their structure, the proximity of land during 



t Report of VJ. S. Geol. & 



