108 GEOLOGY AND MINING INDUSTEY OF LEADVILLE. 



The geological structure of this small area is the expression of the 

 ending of the great synclinal fold which is the predominant feature in the 

 structure of the Ten-Mile district on the north. The beds taking part in 

 this fold basin up to the southward, as they pass within the limits of the 

 Mosquito map. In the central part of the fold occur strata of the Upper 

 Coal Measures, the highest horizon represented west of the Mosquito fault 

 within the limits of this map. Forming the dividing line between the Weber 

 Grits and the Upper Coal Measures is the Robinson Limestone, the tracing 

 out of which gave the key to the structure represented. Above the Rob 

 inson Limestone is an intrusive sheet of quartz-porphyry, which has also 

 been folded, and thus assists in bringing out the basin-like form of this 

 fold. The observed dips upon the western and southern sides of the basin 

 vary from 25 to 60, the strike curving as shown upon the map. Upon 

 the east the proximity of the great fault has somewhat complicated matters 

 and the strata dip very steeply westward. Section A A, Atlas Sheet VIII, 

 represents the relations at this point. The intrusive quartz-porphyry men- 

 tioned does not resemble the Chalk Mountain Nevadite, and yet it is a 

 coarsely porphyritic rock, whose prominent constituents are large sanidine- 

 like feldspars and well-formed quartzes, and whose general habit is that of a 

 comparatively recent rock. The facts that on the one hand this body ap- 

 pears as an intrusive sheet which has been folded and that on the other it 

 is cut by a rhyolite of the type occurring in Chalk Ridge have led to its 

 classification as a quartz-porphyry. In the center of this fold are several 

 minor bodies of rhyolite, of quartz-porphyry, and of a hornblendic porphy- 

 rite. These rocks, as well as the fold in which they occur, are much more 

 important features in the geology of the Ten-Mile district, and the further 

 discussion of their relations will be reserved for the report upon that region. 



Mosquito fault. The line of the Mosquito fault is well defined along the 

 base of the steep slope of Bartlett Mountain by the abrupt transition from 

 sedimentary to crystalline rocks; but farther south, where it crosses the open- 

 ing of the Ten-Mile amphitheater and the valley of the Arkansas, owing to 

 the covered nature of the surface, its exact location is difficult to determine. 



At the foot of the steep western slope of Bartlett Mountain, and on the 

 very northern edge of the map, a small cliff-face of rock juts out from the 



