172 A. C. Peak— Age of the Rocky Mountains in Colorado. 



In the " Keport upon Geographical Explorations and Surveys 

 west of the 100th Meridian, in charge of Lieut. Geo. M. 

 Wheeler, vol. iii, Geology," Prof. John J. Stevenson, on the 

 " Structure and age of the Eocky Mountain System," in Chap- 

 ter XVII of his report, makes the following statement (p. 501) : 

 "The Eocky Mountain system, then, is the result of four es- 

 pecially marked upheavals, the first at the close of the Carbon- 

 iferous, the second at the close of the Trias, the third at the 

 close of the Cretaceous, and the fourth during the Tertiary. 

 Of these, the first and third were the most general in their 

 effects." 



These conclusions he deduces from his investigations during 

 the season of 1873, in the "area embraced between the meridians 

 of 105° and 107° west from Greenwich, and between the north 

 latitude 39° 45' and the southern boundary of Colorado, giving 

 a length of one hundred and ninety miles, and a breadth of 

 about one hundred and six." (p. 307.) 



My duties, as one of the Assistants of Dr. Hayden's Geologi- 

 cal Survey of the Territories, have taken me over the same 

 area, and as far west as the meridian of 109° 40'. From my 

 study of this region, and the results of the work of my col- 

 leagues on the Survey, I am led to conclusions differing 

 from those reached by Prof. Stevenson in regard to the age of 

 the Eocky Mountains in Colorado. The data obtained are, 

 however, too few to enable us to extend the generalizations to 

 the entire Eocky Mountain system. 



The first statement of Prof. Stevenson that I shall consider is 

 the following. "The fiist great epoch of accelerated disturb- 

 ance in the Eocky Mountain region, resulting in permanent 

 elevation of the surface, was synchronous with that during 

 which the Appalachian chain was completed." (pp. 499, 500.) 



He bases this statement upon the following facts (?) : 



1st, The occurrence of Paleozoic strata high up on the flanks 

 of the mountains and the absence of the Trias in the interior, 

 and the abutting of the Cretaceous against the Paleozoic, (with 

 the exception of the Elk Mountains.) p. 499. 



2d. The want of conformability between the Paleozoic rocks 

 and the Mesozoic rocks, (p. 499.) 



I will take these up in order: 



1st. Tfie occurrence of Paleozoic strata and the absence of the Trias 

 in the Interior. I use the name Triassic to denote the Eed Beds 

 that have been so called generally in the west. It is so used by 

 Prof. Stevenson. Prof. Stevenson says, (p. 499) : " In the in- 



