146 GEOLOGICAL SURVEY OF THE TEEEITOEIES. 



quite prominent points of eruption in tlie range of hills on the east side 

 of the valley. 



The mountains on either side are principally Carboniferous and 

 Jurassic, and the valley itself is surrounded with rolling foot-hills, com- 

 posed of the lake deposits passing vq) into a great thickness of local 

 drift. On either side the rounded, dome-like peaks rise up 1,000 

 to 2,000 feet above the valley, which itself is 6,252 feet above the sea. 

 It would be impossible to describe in detail the geological structure of 

 so extended an area of country. Precipitous walls of Carboniferous 

 limestone can be seen on either side ; but so chaotic are the positions of 

 the beds in different localities, so obscured by more modern deposits, or 

 the outpouring of basalt, that it can only be by pictorial illustrations 

 that can be presented to the eye that the mind can form a conception 

 of this remarkable region. I shall therefore hasten on, making a few 

 observations from point to point, referring my readers to a more com- 

 plete and illustrated report hereafter to be prepared for a clearer under- 

 standing of my descriptions. 



On both sides of Sage Creek, about six miles above its junction with 

 Horsfe Plain Creek, we find a series of more modern strata. They form 

 the foot-hills of the mountains on each side, extending in some instances 

 nearly to the summits. On the west side they incline from the range 

 about northwest, and on the east side, southeast. Group one, is a series 

 of strata of sandstones and arenaceous clays of various textures, which 

 I supposed to represent No. 1, or Lower Cretaceous ; group two, com- 

 posed of a bed of earthy lignite, passing up into a dark chalky slate, 

 with many fish-scales and som.e beautiful impressions of ferns and other 

 plants. These shales are nearly vertical, and in some instances dip 

 past a vertical. I regarded these beds as No. 2 Cretaceous, then passing 

 up into yellow chalky beds which might represent No. 3, then upyvard 

 through clays, sandstones, arenaceous limestones, &c., a thickness of 

 several hundred feet. No shells could be found after a patient search 

 of several hours. The more modern beds, Cretaceous or Tertiary, and 

 possibly both, by more readily yielding to atmospheric agencies, have 

 given a smoother and more rounded form to the mountain hills, and 

 permitted them to be covered with a thick growth of vegetation. Near 

 the head of Sage Creek there is a fine group of mountain peaks, 7,500 

 to 9,000 feet high. They extend along the divide from Eed Eock 

 Creek to Horse Plain Creek, thirty to fifty miles, and may be re- 

 garded as remarkable for their symmetrical beauty. At one locality 

 there is an exposure of purplish granulites of the metamorphic group, 

 revealed by the local wearing away of the Carboniferous limestones. 

 As we ascend Sage Creek toward the high divide, we have an occa- 

 sional exposure of gneiss, enough to show that the nucleus of the 

 mountain ranges is composed of the metamorphic series, with its rocks 

 of varied textures. Here are some purplish granulites, micaceous gneiss, 

 with so large a per cent, of mica that the mass presents a brilliant black 

 color in the distance. Over them are the limestones, sometimes lifted 

 high upon the summits of the mountains, almost horizontal or forming 

 nearly vertical walls on the sides inclosing the narrow valleys. Then 

 come the trachytic basalts of various colors and textures, affecting the 

 adjacent rocks more or less. The quartzites, which are the i)rincipal 

 rocks exposed on the immediate divide, have been subjected to the heat 

 of the igneous rocks so that they appear in the distance, dark-brown like 

 compact trachytes. 



I may now delay for a moment and make a few general remarks 

 on the geology of the Eocky Mountain divide. We have already 



