184 GEOLOGY AND MINING INDUSTEY OP LEADVILLE. 



them; this, taken in connection with the observed angle of 15, leads to 

 the inference that the mountain mass has been elevated to some degree 

 above the Arkansas Valley since the beds were deposited. 



NORTHWESTERN DIVISION. 



The northwestern division comprises the area west of the Mosquito 

 fault and north of the area of the Leadville map. It is a region which is 

 comparatively unbroken by faults, and from its lower elevation one in which 

 the structure lines are more difficult to read, owing to want of continuity in 

 the outcrops. Between this and the area last described lies the complicated 

 region represented on the Leadville map, which will be treated in the fol- 

 lowing chapter. Its broad general features, so far as are necessary for the 

 comprehension of what follows, may be given in a few words. The sedi- 

 mentary beds, within which was an enormous development of eruptive 

 rocks, largely in the form of intrusive sheets, have by the force of contrac- 

 tion been com ressed into a series of anticlinal and synclinal folds, and 

 broken by tiansverse fractures or faults, only two of which extend out to 

 any considerable distance beyond this area, viz, the Weston fault on the 

 south and the Mosquito fault on the north, which are practically part of one 

 great displacement. As shown on the western ends of Sections D and E, 

 by these faults the area is broken into blocks, which have been successively 

 lifted one above the other toward the crest of the range. These faults have 

 in general some definite relation to the axis of the folds, and as they pas& 

 northward merge into them. Thus out of the six faults represented on Sec- 

 tion E two have already disappeared before reaching the line of Section D, 

 and on the line of Section C, which passes through Prospect Mountain 

 along the northern edge of the Leadville map, the structure has simplified 

 itself into two broad anticlines, with an included syncline, and there is no- 

 visible fault west of the Mosquito fault. 



Prospect Mountain. The surface of the massive of Prospect Mountain, 

 which lies between Big Evans and Bird's Eye gulches and extends from the 

 Mosquito fault to the east fork of the Arkansas, is covered to a considera- 

 ble depth by broken masses of porphyry and of sandstones and schists of 

 the Weber formation, so that but few outcrops are found. Fortunately 



