664 . BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



region, they are still igneous in all their history. Another interesting 

 question is as to the age of the eruptions. In most of the areas, all that 

 can be positively said is that they are Post- Cretaceous. 



In one of the West Elk areas,* the rocks on the north are mainly of 

 Upper Cretaceous age. On the south, a volcanic breccia is in contact 

 with the igneous area, and on this breccia, a short distance farther south, 

 is a layer of rhyolitic rocks with obsidian at the base. This layer dips 

 somewhat steeply to the south, and the only explanation of the dip is to 

 be found in the eruption of the igneous mass of the area under considera- 

 tion. The layer becomes horizontal as we go south. The breccia, 

 which presents evidence of deposition in water, rests on the upturned 

 edges of the Cretaceous shales and the sandstones of the Dakota group, 

 between the eruptive area and the Gunnison River, and south of the 

 river is on the granite, gradually thinning out until the rhyolitic layers 

 are superimposed to the granite.t These facts show that after the Cre- 

 taceous strata were deposited, there was a period in which they were 

 above water, and subjected to considerable erosion. Then followed a 

 subsidence, during which the breccia was deposited. Then came the 

 flow of the rhyolitic layers from some point south, spreading out on the 

 breccia, and subsequently the eruption of the trachytic mass of the 

 West Elk groups followed, tipping upthe rhyolitic layers. Here we have 

 a clew to the age of at least one area, whether or not it can be applied 

 to the others. Not only was the eruption post-Cretaceous, but it was 

 subsequent to considerable erosion that was also post-Cretaceous. In 

 Park View Mountain and at Spanish Peaks, the trachytic dikes intersect 

 horizontal liguitic strata; and, if we accept a Tertiary age for the latter, 

 we have a Tertiary or post-Tertiary age for the eruption. I think it 

 probable that the eruptions are all of the same age, and that they oc- 

 curred after the elevation of the mountain ranges. If that be so, we 

 may sketch the following as the probable plan: — 



1. Progressive subsidence of the region, accompanied by the deposi- 

 tion of sediments, the successive formations overlapping each other ;| 



2. Following this subsidence, the contraction accompanied by lateral 

 pressure, resulting in the formation of the mountain ranges ; 



3. Attending the plication of the strata, heat resulting in the fusion 

 of the metamorphic and other rocks; and, also, 



4. Causing fractures and fissures in the sedimentary strata, through 

 which the igneous rock resulting from the fusion of the metamorphics 

 was forced, forming, in connection with subsequent erosion, the mount- 

 ain masses under consideration in this paper.§ 



*Area F, on map opposite p. 166, Annual Report United States Geological Survey for 

 1874. 



t Annual Report United States Geological Survey for 1874, 1876, pp. 168-171. 



t See article on Age of Rocky Mountains in Colorado, in Silliman's Journal, March;. 

 1877, pp. 17-2-181. 



§ It is due to myself to state that this article was prepared in February, 1867. 



