spubb.] OCCURRENCE OF ORES IN THE VEINS. 21 
where a small branch vein leaves the main one is generally impover- 
ished, often to a much greater degree than the size of the branch vein 
would seem to warrant. The recognition of this fact by the miners 
has led to the use of the highly descriptive term "vein robbers," for 
these small branches. 
From the above it will be seen that the chief developed Tonopah 
veins have a decided east- west trend. From the fact that there is a 
predominant north-south trend in some of the chief veins of Cali- 
fornia and the western part of Nevada, east- west veins have been 
looked upon with some prejudice in Tonopah, many miners being 
inclined to doubt the value of the veins on account of their strike, 
and others forming the theory that they were simply the offshoot of a 
great undiscovered north-south vein which would prove to be vastly 
richer than these; but the Tonopah field is in an entirely different 
region, geologically, from California, and this objection seems to the 
writer to have little weight. In Idaho, where the geological condi- 
tions are perhaps more like those of Nevada than they are in Cali- 
fornia, the usual trend of the veins is east and west. 
Minor veins run northwest and northeast and even north and south, 
but so far as yet observed the east- west veins are the chief ones, and 
it seems likely that the mineralization will be extensive in an east- 
west belt rather than in a north-south one. 
OCCURRENCE OF ORES IN THE VEINS. 
Physical character. — The veins of the Tonopah district are usually 
strong, straight, and well defined, yet they are not fissure veins. They 
have at first sight all the appearance of fissure veins, but a little close 
examination shows that they have been formed almost entirely by 
replacement of the andesite in which they occur. They seem to have 
originated along zones of especially strong fracturing in the andesite, 
formed during a period of movement subsequent to the consolidation 
of this rock. These zones of maximum fracturing, which are usually 
4 to 6 feet wide, but maybe much wider or narrower, became the chief 
channels of circulation for the mineralizing waters. It has already 
been stated, in describing the alteration of the early andesite, that the 
andesite near the veins has been silicified to a very great extent, and 
the veins themselves seem to be the final stage of alteration, the ande- 
site being mostly or entirely altered to quartz. A slight detailed study 
of the veins gives abundant proof of this origin, for all stages in the 
development can be seen in different portions. In many cases the 
vein consists simply of a zone of more or less altered andesite, not 
essentially different, except perhaps for a somewhat greater silicifica- 
tion, from the andesite which forms the walls. This zone is cut by 
parallel fractures having the same strike and dip as the walls, and the 
walls themselves are nothing more than stronger fractures of the same 
kind. In the next stage, where part of this fracture zone becomes 
