ALASKA-TREADWELL MINE 63 



little altered where it was known to be gold-bearing, for 

 all descriptions of the ore had led me to expect diffi- 

 culty in recognizing the original character of the rock, 

 whereas in nearly all my specimens the syenitic nature of 

 the rock is clearly visible, and the microscope only con- 

 firms the judgment of the unaided eye as to its compara- 

 tive freshness. 



The foot- wall of the deposit is uniformly formed by the 

 black slate. The hanging-wall also is largely constituted 

 by the same rock, but in places the slate is separated from 

 the ore by the dark green, highly altered, igneous rock 

 called gabbro by Becker. This rock was seen in the 

 hanging wall of the Treadwell mine at the 220 and the 

 440 levels, at numerous points along the vein between the 

 Treadwell and the Mexican, and in the open pit of the 

 Mexican mine. It has the appearance, now of an amphi- 

 bolite, now of serpentine, now of a hornblendic diorite or 

 gabbro. It was not studied in thin section. It contains 

 fragments of the albite-diorite, and is undoubtedly younger 

 than this, as stated by Becker. 



The mineral content of the Treadwell deposit is not 

 much varied. Quartz is the chief gangue, if the original 

 rock constituents are left out of account; but at times 

 calcite, or an iron-bearing carbonate, largely takes the 

 place of the quartz as a vein filling. In the open pit of 

 the Mexican mine specimens of ore were found in which 

 narrow veins of delicate pink rhodochrosite or manganese 

 carbonate traversed the syenite and accompanied the py- 

 rite. This mineral seems not to have been observed there 

 before. Of metallic minerals pyrite is far the most im- 

 portant. Mr. Corbus gave me a specimen of ore showing 

 considerable free gold along with pyrite and films of 

 graphite, but such occurrences are very rare. He said 

 that graphite was sometimes sufficiently abundant to be a 

 source of annoyance, by fouling and greasing the plates 



