168 ■ GEOLOGICAL SURVEY OF THE TEERITOEIES. 



the summits of the peaks they seem merely irregularities iu the surface. 

 Station 38 is 10,634 feet above sea-level. It is the last peak of the 

 Elk Mountains to the west, and from its summit one can see the dim 

 outline of the Sierra la Sal Mountains, 100 miles farther west. Station 

 39 is higher, having an elevation of 11,337. 



^. — This area consists of a double-topped hill of trachyte, rising 

 about 1,000 feet above the cretaceous rocks that surround it. It is well 

 wooded to its summit, which is broad. The slopes are not very steep. 

 The JSTorth Fork of Smith's Fork curves around its northern end. The 

 trachyte composing it presents no features different from those of the 

 rocks already described. 



H. — Opposite this double-topped mountain, which we called Saddle 

 Mountain while in the field, is a trachyte point rising between 300 and 

 400 feet above the level of the creek, standing like a finger in the midst 

 of the Cretaceous rocks. 



K. — This is the last trachytic area to the westward. The hill in 

 which it is shown is low and rounded, being only 1,800 feet above 

 the level of Smith's Fork. The face toward Smith's Fork shows the 

 trachyte best. It is of a light-gray color and porphyritic. It does 

 not differ materially from that of station 38. East and north there are 

 Cretaceous shales, and on a low hill back of it there is a dike of similar 

 rock lying at the base. This is on the layer marked Ko. 1, in section 

 16, given in chapter VI. The eruptive force seems to have been dying 

 out to the westward, and the last evidence we have of its action is in 

 the caSon of the south branch of Smith's Fork, where the bending of 

 the No. 1 Cretaceous caused a break, which gave origin to the canon. 



In the consideration of the areas just described it will have been no- 

 ticed that there were numerous i)oints at which the intrusive character 

 of the trachyte was not to be doubted, as near station 34 and near 

 Mount Marcellina. These rocks are the same that are seen in the large 

 areas, and if intrusive in one place are probably so in the others. The 

 elevation of these mountains is Post-Cretaceous, and probably of more 

 recent date than the rhyolitic flow from the south, for the northern edge 

 adjoining this area is tipped up, where the trachyte has not been re- 

 moved, and, where it has, the underlying breccia forms the summits of 

 the mountains, its level being much higher there than along the Gunni- 

 son Eiver. 



TRACnOEHEITES. 



Under this head I will describe the rocks of the mesa-like country 

 extending along the Gunnison, on both sides, as far as the Grand 

 Cafion. The accompanying map (map D) will give the extent and dis- 

 tribution of the rocks comprehended. Their character and order of 

 superposition will be given in detail as I proceed. The source of the 

 flow is to the southward in Dr. Endlich's district, where there is a much 

 greater thickness of the rocks, those exposed on the Gunnison being 

 merely the overlying edges of the upper layers, those underneath not hav- 

 ing spread so far to the north. East of Ohio Creek there are two mesa- 

 like summits crowning the broad ridge between Ohio Creek and East 

 Eiver. These mesas are trachytic, and seem to rest immediately upon 

 beds of Cretaceous age. I do not think there is any breccia beneath, as 

 there is farther west. The general level of the mesas would seem to 

 indicate that they are remnants of the layer that forms the mesas north 

 of the Gunnison. The' hills west of Ohio Creek are composed mainly 

 of breccia, the trachytic capping having been removed. The soft 

 beds have been eroded in the most fa^itastic fashion. The breccia is 



