FAULTING. 79 



ORIGIN OF ZIGZAG FAULTS. 



Zigzag fault courses like those described may originate in two ways: (1) By 

 the intersection of independent fault systems which produce a zigzag line of equal 

 dislocation oblique to both the intersecting systems, as explained in the considera- 

 tion of the Wandering Boy fault (pp. 157-161); and, (2) by a simple fault whose 

 initial movement follows a zigzag course along previously existing fractures. 



INTRUSIONS CONTROLLED BY INTERSECTING FRACTURES. 



Rectilinear boundaries or rectilinear boundary scarps do not always indicate 

 faulting in the sense above described, where one of the rocks is intrusive. A case 

 is furnished by the outline of the Golden Mountain intrusion. As shown on the 

 map, the contact of the Golden Mountain dacite with the earlier andesite, on the east 

 side of Gold Hill, is so straight as to suggest the possibility of faulting. Moreover, 

 east of Gold Hill the long south contact of the same intrusion follows alternating 

 straight northwest-southeast and northeast-southwest courses, strongly suggesting 

 the resultant of two intersecting systems of faults, similar to the scarps already 

 described. But excellent evidence that the contact has not been faulted is present 

 in the band of dacite glass which represents the quickly chilled lava along the 

 margin of the intrusion, and which was found to follow the contact along its differ- 

 ent courses. 



It appears that the straight western limit of the intrusive Brougher dacite along 

 Gold Hill, above referred to, has been determined by a preexisting fault, for the 

 continuation of this fault is evident near the California-Tonopah (California fault), 

 where a dike from the main dacite mass follows the fault zone. In this light, also, 

 it seems probable that the rectilinear courses and the set-offs regularly in the 

 same direction on the south side of the Golden Mountain indicate that the intrusive 

 contact was here also determined by a system of preexisting intersecting faults or 

 fractures. 



CORROBORATION OF CONCLUSIONS. 



A number of faults that were located on the surface by the methods above 

 given were subsequently found in mine workings and observed more closely and 

 satisfactorily. The Mizpah fault was recognized at an early stage in the investiga- 

 tion, both on the surface and underground. The Burro fault, distinguished and 

 followed with great difficulty at the surface, was subsequently developed under- 

 ground. The Wandering Boy and Fraction faults, first distinguished on the surface, 

 were subsequently found to be well exhibited in the mine workings. 



