12 ORE DEPOSITS OF TONOPAH, NEVADA. [bull. 219. 
The economic interest in this faulting lies largely in the circum- 
stance that the veins in the early andesite have been cut and displaced 
thereby. 
Recent erosion. — To complete the geological history, we have to con- 
ceive of the considerable period of erosion which stripped from the 
surface a great thickness of volcanic material, leaving the resistant 
volcanic necks standing out as hills, and laying bare the present sur- 
face. Certain blocks of limited extent had been raised by the fault- 
ing above the level of the rest, and here the surface of erosion reached 
the early andesite and uncovered the rich veins contained in it; and 
it is the discovery of these veins which has made the subject of the 
geology of the still covered region of such lively interest to miners. 
Sequence of formations and events. — The following, then, is the 
sequence of events as deciphered for the vicinity of Tonopah : 
Sequence of formations and events in the vicinity of Tonopah. 
Early andesite.. 
Fracturing. 
Vein formation. (Primary minerals, quartz, valencianite, stephanite, pyrite, 
chalcopyrite?) Values good; gold and silver, silver predominant. 
Erosion. 
Later andesite. 
Probable erosion. 
Dacite. 
Dacite breccia. 
Rhyolite breccias, flows, and dikes, intermingled with slightly stratified or inter- 
stratified pumiceous or tuffaceous fragmental material. 
Vein formation. (Primary minerals, quartz, pyrite.) Values relatively low; 
gold and silver, gold apt to predominate. 
Erosion. 
Tuffs, with an occasional thin rhyolite flow. 
Elevation of tuffs. 
Tilting. 
Basalt. 
Chief faulting. (Affects everything preceding.) 
Rhyolite (white) intrusion (probably Ararat, Oddie, Rushton hills). 
Vein formation. Primary minerals, quartz, chalcedony, calcite, siderite, 
pyrite.) Values low; gold and silver, gold apt to predominate. 
Erosion. 
Dacite intrusion (Butler, Brougher, Golden, Siebert mountains). 
Mineralization (chalcedony, manganese). Values slight to insignificant. 
Mud veins. 
Erosion. 
Glassy rhyolite flow (slopes of Oddie and Brougher). 
DESCRIPTION OF THE ROCKS OF THE REGION. 
Distribution and characteristics of the early andesite. — This was 
originally a hornblende-andesite, containing probably some biotite 
and pyroxene. In the immediate vicinity of Tonopah it outcrops 
only, so far as observed, on Mizpah Hill and Gold Hill, but it has been 
