22 
ORE DEPOSITS OF TONOPAH, NEVADA. [bull. 219. 
altered to quartz, the main wall fractures have been the most favor- 
able, so that sometimes a hanging- wall streak of quartz and a foot- 
wall streak are found with only altered andesite between (fig. 3). 
Sometimes, also, either the hanging-wall or the foot- wall streak may 
be wanting. Next, streaks of quartz parallel with the walls may be 
found, or the quartz may form a network in the andesite. Thus the 
process may be traced to the stage where the whole of the andesite is 
replaced by quartz, forming a solid vein several feet in width. As a 
""- 7 lA'ftezed'andesite. \ y\ 
Vein zone 5 feet 
f-6 IN-*] 
\^>>l 
_r-\', -/ 
\l 
' !■- s'<- : < \~. A Iter^d \arid e.s-itey ■. -v.-^ 
IN 
rftttfi 
/ \ ;- / \ s i 
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Fjg. 8. — Detailed vertical section of one of minor Valley View veins at surface. 
rule, however, there is more or less decomposed andesite forming part 
of the vein. 
As exceptions there are found streaks of quartz, usually small, 
within the vein, which show crustification and comb structure, and 
thus bear evidence of having been formed in cavities. These cavi- 
ties, however, were of irregular shape and were not fissures, properly 
speaking, but spaces of dissolution, and were the effect of the miner- 
alizing waters themselves. 
The largest example of a crustified vein is found in certain parts of 
the Montana Tonopah workings, where the cavities were sometimes 2 
or 3 feet in diameter and save rise to well-banded ores. 
