382 DESCRIPTIVE GEOLOGY. 



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while the eastern half cuts into the beds of the Fox Hill and Colorado 

 Cretaceous. 



Directly south of the westward opening of this valley, on the east side 

 of the canon, there is a second development of trachyte, which occupies the 

 hill-slope for a distance of about 3 miles, but apparently does not cross to the 

 western side. It occurs in bluifs with horizontal bedding, and it seems as if it 

 might have broken out through the Tertiary, and outflowed after the forma- 

 tion of the canon, or at least after the cafion was cut down almost to its present 

 level. Petrographically, it is very similar to that in City Creek, is of a 

 prevailing purplish-gray color, and, in rather a coarse groundmass, contains 

 crystals of sanidin, hornblende, biotite, and augite. Like the City Creek 

 trachyte, it is also rich in tridymite. The tridymite, it should be said, is not 

 visible to the naked eye, and does not, according to Professor Zirkel, par- 

 take in the structure of the groundmass, but only occurs as an incrustation 

 of the small cavities. At City Creek, the interior^f the hornblende is color- 

 less, while the exterior is black ; here the iiiterior is black, and the exterior 

 yellowish-brown ; the biotite is unchanged here, as is also the feldspar. The 

 habit of the rock is decidedly like an andesite, but, although triclinic feldspars 

 are not wanting, orthoclase certainly predominates, and the rock can there- 

 fore only be considered a trachyte. 



The summits of the Wahsatch on either side of the trachyte body 

 are of the Vermillion Creek Eocene, lying nearly horizontal. Where 

 the Mountain Dell trail crosses the main ridge, however, it is formed 

 of coarse gritty sandstones, underlaid by heavy conglomerate beds, formed 

 of pebbles of limestone and quartzite, having a strike of north 60° east 

 and a dip of 25° to the southeast. This is evidently the same conglom- 

 erate which has been already mentioned as overljang unconformably the 

 Jurassic and Triassic beds in Emigration and Parley's Canons, and is con- 

 sidered to represent the lowest member of the Dakota Cretaceous. It here 

 rests on the upturned edges of the Upper Coal-Measure limestones ; these 

 are very much obscured by drift and the disintegrated material of the 

 Cretaceous, so that its position is somewhat uncertain. It is clear, however, 

 that these supposed Cretaceous beds are unconformable with the Jurassic, 

 Triassic, and underlying formation, and it seems also evident that they are 



