118 



GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



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Flo. 19. Detail sections from Mizpah vein showing the effect of premineral cross fractures. A, Sketched vertical cross sect ii m 

 of Mizpah vein at a point In the west workings (the whole vein sloped out). B, Sketched vertical cross section of 

 Mizpah vein, Oddie and McMann lease. C, Detail of Mizpah vein in the Golden big stope, about 160 feet from the 

 surface. Hori/xmtal plan along a crosscuttlng fracture vein; the vein abruptly increases in width from 2J to 10 feet. 

 Fracture dips 43 southwest, and the vein is here perpendicular. There is no evidence of movement along the cross- 

 cutting fracture, but it appears to be premineral. Its function as a fracture plane limiting the circulation is like that 

 of a wall. It may therefore be called a cross wall. />, Cross section of Mizpah vein, west face of big stope, Lynch 

 and Omcara workings, about KX) feet from the surface, illustrating the influence of crosscutting fractures on the 

 original ore. The ore, which was a solid mass east of here, is cut off ulong u premineral fracture plane (strike N. 56 E., 

 dip 60 northwest), above which the vein is divided Into a foot-wall and a hanging-wall streak, with altered andesitc 

 between. 



