554 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



MOUNT GUYOT AREA. 



i^orthwest of South Park, io the divide between it aud Middle Park, 

 is a group of mountains that, topographically, are connected with both 

 the Park and Front Ranges, the continental divide following its crest 

 from one range to the other. Geologically, however, this group is dis- 

 tinct from them, being of later origin and due to eruptive action. The 

 mountains are composed of gray eruptive granite, porphyritic trachyte, 

 and metamorphosed sedimentaries, with tuterbedded igneous rocks. 

 There are two sub-groups, viz. Mount Guyot and Silverbeels. 



Mount Guyot — The rock of Mount Guyot is compact aud light-colored, 

 not to be distinguished in appearance from fine-grained granite. It lies 

 on the slopes of the peak in slab-like masses, with rusty-colored surfaces, 

 and rings beneath the blows of the hammer. Near the summit of the 

 jjeak, it is intersected by a dike-like mass of very compact green rock, in 

 which crystals of feldspar are porphyriticall.v imbedded. Between the 

 peak and the metamorphic Archaean rocks that prevail to the eastward 

 in the Front Eange, are fragments of metamorphosed sandstone and ar- 

 gillaceous slate. They could not be traced north or south of the saddle 

 in which they are exposed, and appear to have been caught during the 

 upheaval of the mountain aud wedged between the Archaean rocks and 

 the eruptive rock of the mountain (see page 213, Report United States 

 Geological Survey, 1873). South of Mount Guyot, the mountains are 

 composed of porphyritic trachyte. It weathers like the rock of Mount 

 Guyot, and, like it, is phouolitlc. On the east side of Tarryall Creek, 

 above the village of Hamilton, it rests on the Dakota sandstones, which 

 dip under it, changed into quartzites, 



Silverheels. — The Silverheels group is separated from the Mount 

 Guyot group by Tarryall Creek. Silverheels is the principal peak, and 

 is composed of stratified rocks of probable Upper Carboniferous age, 

 with interlamiuated igneous rocks. The strata dip approximately to 

 the eastward at an angle of 30°, and are much metamorphosed. From 

 the summit of the peak, the outcrops can be traced, extending south- 

 ward into South Park, where the sections show them to be unchanged.* 

 At the head of Tarryall Creek, dikes penetrate the sedimentaries, and 

 are doubtless connected with those of Silver Heels Mountain. East of 

 Silver Heels is a comparatively low rounded hill of porphyritic rock, 

 and in the Park are several igneous ridges, all probably of the same age 

 and alike in general character. The Park Range in the region of Mount 

 Lincoln and southward presents abundant evidences of the same igneous 

 action in the intrusion of porphyritic igneous rocks in schists and rocks 

 of Palaeozoic age.t The proximity of the Sa watch Range, of which the 

 Park Range is really a part, tends to complicate matters, and leaves us 



•'' Animal Report Uuited States Geological Survey for 1873, 1874, pp. 214-219. 

 ilhid., pp. 41-47, 225-236. 



