ALASKA-TREADWELL MINE 
6 3 
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 
