130 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



same age. On the east, the strong cross vein near the Stone Cabin shaft prob- 

 ably cuts off the complicated vein system in this direction; beyond this cross vein 

 the other veins will be found, for a space at least, abruptly of a different 

 character. Similarly the strong northwest-striking and northeast-dipping vein, 

 which heads off a number of the Valley View veins on the southwest side of 

 Mizpah Hill, seems to mark the boundary of a relatively poorer continuation of 

 the main vein system on the west (see PI. XVII). Nevertheless, some of the 

 veins escape and persist, and are found across the gulch, in outcrops and in 

 the workings of the Wandering Boy. The veins of the Fraction may be a con- 

 tinuation of this system. 



Apparently the mineralizing solutions flowing along the east-west fracture 

 zones were deflected where the transverse fracture zones were strong enough to 

 control the circulation, and did not follow the old channel farther. 



The same principle is shown by the numerous splitting and reuniting 

 branches, all running in the main direction of the vein system. Any of these 

 branches may divert the main strength of the vein along it and into a parallel 

 vein of the group. 



This heading off of the main course of veins by crosscutting veins is entirely 

 analogous, though on a larger scale, to the crosscutting premineral fractures which 

 have produced the cross walls, as studied out on the Mizpah vein, and so have 

 brought about the localization of the ore deposits. The cross walls produce richer 

 shoots, both as regards quartz and precious metals, within the main fracture zone; 

 the cross veins cause relative differences in mineralization and vein formation 

 along portions of a belt of interlacing fracture zones which is similar to though 

 larger than that occupied by the main Mizpah vein. In the ordinary splitting 

 of the veins, as seen in both systems, the diverging branches have not so radically 

 different a direction from the main vein as have the cross veins, but have often 

 operated to deflect the solutions from the main fracture zone, and hence are called 

 vein robbers. 



VEIN STRUCTURE AND ORIGIN. 



On studying the different veins of the system, as exposed excellently in an 

 almost continuous series of surface openings, the fact that these veins are due to 

 the replacement, in varying degrees, of andesite by quartz along a zone of especial 

 fracturing is well illustrated. This is shown by the ever-changing amount of 

 replacement, at one point the vein zone being little more than fractured porphyry 

 and at another solid quartz, with all conceivable transitional stages represented 

 between, these points are illustrated by the sections forming fig. 28. 



