HAYDEN. 1 



GEOLOGY ELK MOUNTAINS. 69 



quartzites of the Dakota group, and thus the wearing out of the 

 channel was interrupted by the great hardness of the rocky bed. 

 The lower falls were formed by a dike, which commences on the left 

 branch, runs parallel with the strata for 20 or 30 feet, forming the top 

 of the falls, then drop down diagonally across the layers of shale about 

 10 feet, and on the opposite side rises again and is lost in the debris. 

 At first the trachj^te is above a hard layer of calcareous sandstone 

 about 2 feet thick, but it passes through that into the softer clays 

 below, where its thickness is increased from 2 to 4 feet. Both the 

 upper and lower sides of the dike-layers the Cretaceous shales are 

 changed into slates and are adherent. As we follow up the channel of 

 the stream to a high hill overlooking the gorge, we see that the entire 

 mass on both sides, and extending for miles, has been thrown over to 

 the west and southwest, and that the occasional sagging or bending 

 down of the mass produces a drainage-channel through which the little 

 streams find their way. In the case of Eock Creek, the curve down- 

 ward or sag is 600 to 1,000 feet. At another locality near this creek 

 there is a dike 4 feet in width, nearly vertical, or running across the 

 strata at right angles. We see, therefore, that the igneous matter was 

 pressed up through every fissure of any form through which it could 

 gain access to the surface, or relieve the pressure from beneath. Some- 

 times it is squeezed out between the layers or strata across them diag- 

 onally, or at right angles, in whatever direction the original fissure 

 might take. As we descend Eock Creek the channel reveals the lower 

 set of beds from the Cretaceous shales of No. 4, downw^ard to the gran- 

 ites, with the various changes which they undergo. The shales, mud 

 sandstones, and clays, of No. 4 and No. 2, as well as the gray limestones 

 of No. 3, are frequently much changed. The black quartzites on Eock 

 Creek, and the blue limestones, with all the intermediate steps of change, 

 are most clearly shown. The gorge or channel is inclosed within nar- 

 row walls, which soon increase in height from 1,200 to 1,500 feet, inclining 

 at a moderate angle, 5^ to 10°, about northwest, or nearly opposite to the 

 dip from Snow Mass range. Gothic Mountain, 12,315, and Crested Butte, 

 11,838 feet high, may be regarded as immense dikes, the melted mat- 

 ter pushed up through the superincumbent strata, but in such a way 

 as not to disturb to any great extent the great thickness of yielding 

 Cretaceous shales and clays, so that they surround the base of the 

 mountains like terraces, apparently in a nearly horizontal j)osition. 

 Gothic Mountain presents an illustration not only of the vertical up- 

 tlirust by which the central mass was brought to the surface, but also 

 shows an irregular layer of the trachyte squeezed out between the strata 

 of shale. 



We have not been able in this chapter to present more than a glimpse 

 of the remarkable geological features of the Elk Mountain range. It is 

 the purpose of the survey to make a careful examination of this entire 

 region as a special study, and therefore only a preliminary notice of it 

 can be given in this report. We have frequently spoken of the chaos 

 into which the beds seem to have been thrown by the numerous centers 

 of uplift, and so it seems at the first glance; but our map will show that 

 the massive peaks are located along a regular line which indicates a 

 true axis of uplift, trending northwest and southeast. Studied in de- 

 tail, the geologist will find faults and dikes without number, strata in- 

 clining in every direction and at all angles, and very often entirely sub- 

 verted, yet the aggregate dip and strike may be reduced to a system, as 

 we see it on the map, from Italian Peak to Sopris Peak. 



