24 ORE DEPOSITS OF TONOPAH, NEVADA. [bull. 219. 
in the country rock. At -400 feet from the surface the Valley View 
veins show oxidized ore, while 100 feet higher the country rock away 
from the veins is imoxidized. In the Stgne Cabin shaft the change 
from the oxidized to the unoxidized andesite came just below the 200- 
foot level, but at 400 feet the ore is partly oxidized. The reason for 
this appears to be that the quartz veins, on account of their brittle- 
ness, have been more readily fractured by strains than has the softer 
country rock, and so the veins afford channels for oxidizing waters 
coming from the surface. Strong fractures, such as those in the 
vicinity of faults, have the same effect as the open fractures in veins 
and carry down the oxidizing influences to a depth of several hundred 
feet. Where the rocks are protected above by relatively impervious 
formations the oxidation may be comparatively slight. Thus, in the 
Silvertop workings (near the Stone Cabin) the later andesite is unoxi- 
dized at a depth of 120 feet. Here it is protected at the surface by a 
deposit of fine-grained, stratified tuff. 
In the oxidized zone of the veins the sulphides are mostly entirely 
altered or can be made out only with a microscope, yet in many cases 
the alteration is not complete and the purple color of the richer quartz 
is as a rule duo to these finely disseminated sulphides. Most of the 
sulphides, however, are altered to silver chloride, with some bromides 
and iodides, and to iron oxide. Nearly all of the ore in the Mizpali 
vein, down to the lowest depths explored, is oxidized, while the ores 
of the Montana Tonopah are only slightly and exceptionally oxidized. 
In the latter case the veins and the inclosing early andesite have been 
protected from the atmosphere by several hundred feet of decomposed, 
soft, later andesite, whereas in the former case the veins outcrop at 
the surface and oxidizing waters sink readily into them. 
In the process of oxidation and alteration of the metallic minerals 
there has probably been some transfer of material. Little is known 
of the jn-imary or unoxidized veins, but some portions seem almost 
entirely barren and some very rich. It will probably be found that 
these rich portions form definite shoots or masses of ore in the rela- 
tively barren veins. In the oxidized ores the values are by no means 
uniform, but seem rather better distributed than in the deeper regions. 
The indications are that during oxidation some of the values have 
been distributed from the richer portions through the more barren 
portions, producing a larger supply of fair quality ore. It is also 
likely that during this process there has been some concentration of 
values, so that the oxidized ores as a whole may be found to be some- 
what richer than the sulphide ores. It is possible, for example, that 
those portions of the veins which have been eroded furnished during 
the process of erosion a small share of their precious metals to the 
underlying portions of the vein, these underlying portions now hav- 
ing become the present oxidized zone. 
It also seems probable that in places some relatively slight trans- 
fer and redeposition of the precious metals has taken place along 
