510 ^ DESCRIPTIVE GEOLOGY. 



tion. Like the lower end of the main range, it consists chiefly of heavy 

 limestone strata, which have been referred to the Lower Coal-Measure 

 group. In its geological features. Spruce Mountain shows a very com- 

 plicated structure, but in general would appear to be an anticlinal fold, the 

 eastern side forming a synclinal with the beds of the main range. The 

 occurrence of mica slates and schists would indicate that the beds rested 

 directly upon an Archaean foundation, while the crumplings and phcations 

 of strata, with their local displacements, are rendered still more complex by 

 the intrusion of both small and large bodies of diorite and feldspar-porphyry, 

 the more prominent ones only being represented on the map. From these 

 crystalline rocks, the Palaeozoic strata incline in all directions, with abrupt 

 changes in both dip and strike, showing that their present position is largely 

 dependent upon the intrusive bodies. On the southern point of the high 

 ridge, the strata strike northwest and southeast, and dip easterly. On the 

 saddle between the peaks, a diorite body disturbs the original position of the 

 beds ; but, on the northern and higher peak, the beds strike north 8° to 12° 

 west, apparently at a slight angle obliquely to the trend of the uplift. The 

 dip is about 15° to the eastward. Northwest from here, and about 2,000 

 feet below the summit, occurs a large feldspar-porphyry body, to the west 

 of which the strata mainly dip westerly, and from there northward to Blue 

 Point Spring occur a number of minor folds and flexures. At Crawford 

 Pass, a well-marked anticlinal fold in the hard siliceous limestone occupies 

 the ridge, which is here not much more than 600 feet above the valleys. 

 The axis of the fold strikes obliquely across the trend of the ridge, the beds 

 upon each side dipping away at about an angle of 15°. 



No measurement of the thickness of the Spruce Mountain beds was 

 made, but it would seem highly probable that there are at least between 

 2,000 and 3,000 feet of Wahsatch limestone represented. Lithologically, 

 the formation shows the same habit as characterizes the beds in other places, 

 that is, heavily-bedded limestones, more or less siliceous, with occasional 

 layers of calcareous shales and fine bands of quartzite. On the west face 

 of the mountain, cropping out in several localities, occurs a well-marked 

 bed of hard, black slate, carrying cubes of pyrites. A recorded strike gave 

 north 25° west. 



