spurh] PEEIODS AND NATURE OF MINERALIZATION. 15 
is regarded as probably the oldest dacite exposed at the surface. It 
is a compact, relatively coarse-grained rock, finely brecciated, and 
carrying included fragments of older rocks, notably the later andesite. 
It is surrounded and probably overlain by the softer dacite-breccia 
formation. The Tonopah City shaft after passing through the dacite 
breccia has continued in solid Heller dacite to a total depth of over 
500 feet. East of Butler Mountain, on the edge of the district, 
another strip of this dacite is exposed, also surrounded by the dacite 
breccia. It is regarded as probably intrusive. 
The dacite-breccia formation, on the other hand, is essentially a sur- 
face formation, and not intrusive. It consists of flows of often very 
pumiceous and friable dacite, dacitic mud flows, and some rudely lay- 
ered or even stratified fragmental material. In places it is very thick. 
It is exposed over a large fraction of the southern portion of the dis- 
trict, around Butler, Brougher, and Siebert mountains. The Ohio 
Tonopah and the New York Tonopah shafts passed through consider- 
able thicknesses of this formation, and the Fraction No. 3 is entirely 
in it. The Fraction Nos. 1 and 2, and Wandering Boy, and the West 
End, passed through this formation at the surface and reached the 
underlying formations. 
The later intrusive dacite, which makes up Butler, Brougher, and 
Golden mountains and the central portion of Siebert Mountain, has 
usually a characteristic appearance. It is darker than the rhyolite of 
Mount Oddie, has a slight purplish tinge, and contains more crystals 
(of feldspar, quartz, and mica) embedded in the glassy groundmass. 
The Big Tono shaft, at the east foot of Mount Brougher, starting at 
the outward-pitching contact of this rock and the intruded dacite- 
breccia formation, goes down several hundred feet in the former. 
The Molly, starting near the dacite contact on Golden Mountain, 
passed through several hundred feet of this rock, then through the 
inward-pitching contact to a slight thickness of loose material belong- 
ing to the dacite-breccia formation, and so into the later andesite. 
PERIODS AND NATURE OF MINERALIZATION. 
Mineralization subsequent to the early andesite intrusion. — The most 
important veins of the Tonopah district, and all those that have been 
proved to be of immediate economic importance, occur in the early 
andesite, and do not extend into the overlying rocks. Hence, when 
the early andesite is not exposed at the surface, the later rocks form 
a capping to the veins, and this capping must be passed through 
before anything can be learned of the presence or the nature of the 
veins beneath. This circumstance shows pretty plainly that the vein 
deposition took place before the eruption of the later andesite, and 
immediately after that of the early andesite (for the period of erosion 
between the two andesites seems to have exposed the veins at the sur- 
face, showing that they were formed before this period, or early in it). 
