BURRO VEINS. 127 



trend, however, appears to be east and west, or perhaps more correctly N. 70 E., 

 and it appears to have a steep northerly dip and a width of several feet. It is 

 somewhat dragged in the neighborhood of the fault. Some very rich ore was 

 found here, which was chiefly oxidized and contained a large amount of silver 

 chloride. The Mizpah fault here has its usual northwest strike and northeast dip, 

 but the dip is steeper than it is farther northwest, being from 35 to 45. 



FORMATIONS ENCOUNTERED IN THE LOWER WORKINGS. 



At a depth of 814 feet from the top of the shaft the rocks to the north and 

 west are extensively explored by drifting. These workings are almost entirely 

 in a white, dense rock which study shows to be Tonopah rhyolite-dacite. The 

 quartz masses characteristic of this formation were encountered, showing the 

 usual irregularities, nonpersistence, and barrenness; on the other hand, some 

 portions are exceptional in containing high-grade ore. 



At a depth of 920 feet the shaft cut a sheet of white Oddie rhyolite, which 

 contained a. flat vein of some size, but showed no values of importance (see 

 p. 193). The bottom of the shaft, at a depth of 1,114 feet, is in the Tonopah 

 rhyolite-dacite. This rock is much like that at the bottom of the Siebert shaft. 



THE BURRO VEINS. 



On the south side of the Mizpah vein there are several weaker auxiliary veins. 

 Most of these converge and unite with the Mizpah on the surface at a point not far 

 west of the outcrop of the Mizpah fault. The principal ones have been called the 

 Burro veins, and they have been numbered 1, 2, and 3, No. 1 being nearest the 

 Mizpah. 



These three veins are all branches of the system of which the Mizpah is the 

 trunk vein; on the surface the No. 2 and the No. 3 probably come together about 

 200 feet south of the Mizpah; this united vein joins the No. 1 just south of the 

 Mizpah, and unites with the trunk a very short distance farther northeast. 



VEIN STRUCTURE. 



These veins are all essentially silicifications of definite fracture zones in the 

 altered andesite. The zones average perhaps 4 feet in thickness, and along 

 them quartz has formed (almost entirely by replacement of the altered and 

 silicified andesite) to a varying degree, so that in places the vein zone may be 

 entirely of andesite, distinguishable from the wall rock only by its peculiar 

 and greater fracturing, and in other places may be entirely filled with quartz, 

 carrying good values in silver and gold. All intermediate stages are also seen 

 (fig. 27). 



