420 



University of California Publications. 



[Geology 



apatite, zircon, and ehalcopyrite, which by preference replaces 

 the chlorite. The quartz monzonite shows evidence of severe 

 movement and fracturing, and is often highly sulphuretted. 



Another type of igneous rock is found in the hanging wall 

 country just east of the mill. This is an aphanitic white rock, 

 heavily pyritized, showing a few small glassy feldspars, which 

 the microscope shows to be largely orthoelase. It may represent an 

 offshoot from the quartz monzonite, which a prospect shaft shows 

 is intrusive in the greenstones a hundred feet or so to the east. 



Northward from the original location the Copperopolis Lode 

 occupies the center of a broad strike valley ; to the south, it forms 

 the summit of a low, gentle ridge. At present the ores are being 

 entirely hoisted from the northern portion of the lode, consist- 

 ing of microcrystalline chlorite schists (locally known as "black 

 pyritous slates") ; in the southern portion the ore rock is thor- 

 oughly seamed with quartz stringers, which by their resistance 

 to weathering, have elevated the lode above the surrounding 

 topography. 



The ore bodies consist of a series of lenses parallel to the 

 foliation of the schists, about 200 feet long, 150 feet high, and 

 4 to 60 feet in width. They are not necessarily located in align- 

 ment, but frequently occur en enchelon. The shoots are stated 

 to pitch 45° to the north in the plane of the dip.* The footwall 

 is often defined by soft, putty-like talc, which has a tendency to 

 run. There is no definite hanging wall, the ore shoots fading 

 into barren rock, and the extent to which ore is stoped out is 

 governed by the dictates of economic expediency. 



Below the shallow zone of oxidation, 30 feet in depth, lie 

 the unaltered sulphide masses. The ore consists of ehalcopyrite, 

 singularly free from admixture with other minerals, included in 

 a fine-grained chlorite schist as gangue. The absence of the ordi- 

 nary gangue minerals is so marked that ore containing quartz 

 is retained as specimen material. The ehalcopyrite occurs as 

 thin bands, from an eighth to a quarter of an inch in thickness, 

 ribboning the schist parallel to its foliation planes, though fre- 

 quently anastomosing irregularly across the schistosity. 



* According to Mr. G. McM. Eoss, Mgr., whose great courtesy it is the 

 writer 's pleasure to acknowledge. 



