164 GEOLOGICAL SURVEY OF THE TERRITORIES. 



It is scarcely to be doubted that all these mountains are eruptive in 

 their origin. The evidence pointing to this fact will be referred to as 

 we proceed. The sandstones surrounding them seem to have been but 

 little disturbed by the thrusting-up of these masses. It is difficult to 

 get at the line of junction between the trachyte and the sandstone, as 

 the slopes on the mountains are very steep and the bases are entirely 

 concealed by the mass of debris washed down. It is probable, how- 

 ever, that the ends of the sandstones would be found slightly tipped up. 

 On Anthracite Creek, on the north edge of Mount Marcellina, the sand- 

 stones are tipped up and penetrated with dikes, as shown in Plate 

 XII. On the northern side of the area, marked A, on the map C, the 

 sandstones are tipped up, dipping to the northward at an augle of 20° 

 to 25°. It is a curious fact that these western terminations of the Elk 

 Mountains should all be trachytic, while in the main mass of the Elk 

 Mountains the granitic character should predominate. In the dikes, 

 however, the rock was always trachyte. Proceeding westward, also, 

 trachyte became more abundant. In the region I have under con- 

 sideration at present the varieties of rock that most resembled granite 

 were found in the eastern portion. The dikes always gave the best 

 typical specimens of trachyte. Some of the specimens are rhyolitic. 

 Touly refer to these facts to show the analogy between the trachytes 

 and the eruptive granite of the Elk Mountains. Until these rocks are 

 all subjected to a close microscopic and chemical analysis and the region 

 iu which they are found examined in detail, all opinions must, to a cer- 

 tain extent, be conjectural. The Elk Mountain region presents one of 

 the most interesting fields of study, and one which will yield more 

 material for the study of eruptive granites than any other on our conti- 

 nent. 



I will take up these areas in the order in which they are lettered on 

 the accompanying map (C), which gives a much better idea of the out- 

 line of each than could be given in words. 



The eastern groups are those which, in Lieutenant Buffner's report,* 

 are called the Philosophers' Monuments. 



A. — In ascending Ohio Creek, the hill on which station 30 is located 

 forms a prominent feature of the landscape. It is sugar-loaf in form, 

 and rises over three thousand feet above the level of the valley of Ohio 

 Creek. Its slopes are steep, and at the base is an accumulation of rocks 

 that have been washed down its sides. The rock is a porphyritic 

 trachyte. There is a rather compact matrix, inclosing white crystals 

 of feldspar, free quartz, and small crystals of black mica. The mass 

 north of station 30 is broad-topped, and composed of the same kind of 

 rock. East of station 30 is another mass, not shown on the map. It 

 was not visited, but is probably similar to that of station 30. The west- 

 ern part of the area, marked A' on the map, is a ridge, with numerous 

 sharp points. The pa ss from Ohio Creek to the head of Anthracite Creek, 

 is at the eastern end of this ridge, at the point b on the map. This pass 

 is 1,800 feet below the summits of the ridge. I am in doubt whether or 

 not this area is separated from the mass of station 30, marked A. I have 

 provisionally connected them on the map. At the western end I am also 

 I doubtful, as I have never been on the saddle that separates it from the 

 ' area E. This saddle seems to be comparatively low, as seen from the 

 surrounding country, and I think it probable that the Cretaceous sand- 

 stones connect across it. The dotted lines indicate the doubtful part. 

 On the north side there are Cretaceous sandstones dipping to the north, 



* Report of a Reconnaissance in the Ute Conntry, page 40. 



