26 
ORE DEPOSITS OF TONOPAH, NEVADA. 
[BULL. 219. 
siderable. The whole later andesite region is worthy of careful 
yet cautious prospecting. The localities where the later andesite is 
intruded by the rhyolite ought, as a rule, to be avoided in this pros- 
pecting, for such places are not so favorable as those where the ande- 
site is undisturbed. Such places, however, are likely to be pre- 
ferred by many mining men, since they are apt to show some of the 
smaller and poorer veins of the later rhyolitic mineralization. Again, 
the region of faults should be avoided, for these faults have affected 
the ore-bearing veins, and if veins are found near them they are diffi- 
cult, and sometimes almost impossible, to follow. The rocks along 
fault zones are often decomposed and the decomposed material may 
-'\y 
i ?fl 
. ; 
□ 
n?n mm via 
Hypothetical 
deep-seated 
granite 
Hypothetical Early andesite Later andesite Dacite breccia Brecciated 
deep-seated earlier rhyolites 
later dacite Early andesite veins 
and rhyolite Cesser veins belong- 
intrusions i D g to other periods 
not represented ) 
have the appearance of what is known as " vein matter." From this 
material real or fictitious assays may be obtained, and such a locality 
may appear to the miner as more hopeful than less altered rock which 
does not look like a vein and does not show any assays. Some of the 
principal faults are shown on the accompanying map, but certainly 
not all of them have been detected, and underground developments 
will probably reveal many more. 
Concerning the dacites and rhyolites of the later intrusions, the 
volcanic necks and the dikes should, as a rule, be avoided b}^ pros- 
pecting operations. The greater parts of the principal low mountains 
and hills in the vicinity, such as Butler, Brcugher, Siebert, most of 
Golden, etc., are regarded as offering very little hope for prospecting, 
and even near the rhyolite hills, like Rushton, Oddie, and Ararat, 
there has been a displacement of the veins, for the hills are essen- 
tially intrusive. The writer is aware that the proximity of the rich 
veins of Mizpah Hill to Oddie Mountain has caused this mountain to be 
