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University of California Publications. 



[Geoloc.y 



tion is directly in line with Ophir Ravine, and no doubt was the 

 main cause of its formation, as noted in general by Becker, 

 quoted above. 



Bullion Ravine Fault. — Bullion Ravine is likewise determined 

 by a fault plane, in this case sbown largely by stratigraphic and 

 physiographic grounds. A new geologic map is necessary to show 

 this fully, as that of Becker * is in error at this point. In the 

 first place, this rocky ravine shows plainly that erosion has had 

 nothing to do with its formation, for its bottom has never been 

 occupied by a stream. Becker notes many times that the erosion 

 of the region has been little or nothing since the vein formation, 

 and this is merely a case in point. Moreover, in the second place, 

 here is a structural discordance between the walls of the ravine. 

 The north wall is solid diorite to the summit of Mt. Davidson. The 

 south wall is diorite farthest east, but andesite a short distance 

 west up the ravine. The contact between the two rocks appears to 

 be in the exact center of the ravine, but is obscured by surface 

 wash. 



Other Ravine Faults. — The ravine leading up to Mt. Butler, 

 and Crown Point Ravine farther south, have not been examined 

 in detail, but they partake of the characteristics of the others : 

 typical fault ravines. Becker, though noting this, fails to give 

 supporting evidence because he attached no importance to their 

 structure. Such evidence is plentiful for all these east-west 

 gulches, the most important of which is as follows : 



1. The erosion of the country has been very little since the 

 faulting, so that we are driven to look for other causes for the 

 formation of the topographically striking ravines. In the case of 

 Bullion and Ophir Ravines, at least, this cause has been shown to 

 be faulting. Becker's reason for a lack of erosion seems to lie 

 in the close similarity of the contours of the actual faulted sur- 

 face with the theoretically deduced one. Further reasons, and 

 conclusive proofs, are : ( 1 ) there is little or no removal of the 

 thoroughly decomposed country rock near the lode; and (2) the 

 present surface of the lode, or its outcrop, is that of the original 

 lode apex, formed under practically no pressure from above and 



* G. F. Becker, Mon. Ill, Atlas, U.S.G.S. 



