182 University of California Publications [Geology 



ing both field and laboratory work. The microscopical evidence 

 of Hague and Iddings needs corroboration on but one possible 

 point — that of the undoubted occurrence of fresh augite as the 

 chief ferromagnesian mineral in the diorite. The field evidence, 

 however, seems to be insufficient regarding the diabase and dio- 

 rite. The present writer, through late mining work, is in posses- 

 sion of the facts necessary to prove beyond the shadow of a doubt 

 the conclusions of Hague and Iddings. The Hale and Norcross 

 tunnel into Mt. Davidson, just north of Bullion Ravine, has 

 brought to light these facts, and also a number of structural facts 

 of the most vital importance. The structures shown will be dis- 

 cussed later, and a note on the rocks will be placed near the end of 

 this paper. 



Forks of Cornstock Lode. — The main forks of the Oomstock 

 lode, both north and south, are quite similar in their action on the 

 formation of the fissure as seen to-day. Of the Sierra Nevada 

 fork, which strikes nearly east and west, the writer can say 

 nothing, for so much of the old ground is not open. However, 

 the east-west slips are well shown underground in the present 

 workings, and this portion of the lode country appears to be a 

 locus of these movements. The large southeastward bearing 

 "fork" of the lode which runs toward Silver City, has some pecu- 

 liarities which deserve mention. In the first place, this so-called 

 branch is not a mere fork of the lode, but as a fault continues west 

 of the lode into the mass of Mt. Davidson west of the summit. 

 Bullion Ravine exhibits this fault in the best manner. As already 

 noted, the south wall of the ravine is diorite for a few hundred 

 yards west up the gulch, when nndesite appears from the bottom 

 of the south wall clear to the top. The topography also shows the 

 change, the diorite portion being steep and precipitous, while the 

 andesite is smoother and of more gentle slope. The transition 

 from one to the other is abrupt. Some prospecting has been 

 done along this line and some ore found. The gangue in the 

 Silver City portion or "fork," as of a few of the east-west veins 

 east of Bullion Ravine, is largely caleite. Granting the identity 

 of the rocks as set forth in the work of Hague and Iddings, this 

 difference in gangue mineral from the silica of the lode proper 



