spubr.] GENERAL GEOLOGICAL HISTORY. 11 
rock, the first of the upwellings of the next period of volcanic activity. 
Soon afterwards great columns of dacite and rhyolite made their way 
to the surface. The eruptions from these vents must have been chiefly 
explosive, and the products light and scanty and easily swept away, 
for no trace of them lias been found in the immediate vicinity of 
Tonopah. The volcanic necks, however, have been laid bare by ero- 
sion, and on account of their superior hardness to the tuffs and brec- 
cias, in which they were intruded, now stand up as low detached 
mountains. Brougher Mountain, Butler Mountain, Siebert Mountain, 
and Golden Mountain are all dacite necks of this period, while a group 
of rhyolite eminences, consisting of Oddie and Ararat mountains and 
Rushton Hill, represent necks of very nearly the same period as the 
dacite. 
Faulting of the region. — At about this period occurred an event of 
great scientific and economic interest — the faulting. A considerable 
number of important and complicated faults have been found in the 
region. Their age, as denoted by their relations to the different for- 
mations, seems nearly uniform. All the rocks up to and including 
the stratified tuffs have been displaced by the faulting, and on Siebert 
Mountain the thin sheet of basaltic rock overlying the tuff has been 
faulted with it. But these faults invariably stop at the contact of 
the dacite necks, which are not affected, and the same is probably, 
though less certainly, true of the rhyolite necks above noted. Indeed, 
the dacite certainly (and the rhyolite less certainly) has been intruded 
as dikes along the faults. 
The geological map (PI. I) reveals some important points. It shows 
that the area of observed complicated faulting is in general coexten- 
sive with the region of late dacite intrusion. This region, which 
occupies the southeastern portion of the district, is downsunken in 
comparison with the unfaulted or little-faulted region on the north- 
west. Near the dacite necks the observed faults are rather more 
numerous than elsewhere, and in many instances it may be estab- 
lished that the blocks adjacent to the dacite have been downsunken 
in reference to blocks farther away. From these intrusive necks the 
faults run in a roughly radiating fashion and seem to follow no regu- 
lar system of trend. Detailed study of the contact phenomena of the 
dacite shows that the minute faults in the tuffs at these points gener- 
ally have their downthrown side next the dacite. From these facts 
the following conclusions have been reached. The faulting was 
chiefly initiated by the intrusion of the massive dacite necks (the 
rhyolite necks were probably not so bulky). After this intrusion 
and subsequent eruption there was a collapse and a sinking at the 
various vents. The still liquid lava sank, dragging downward with 
it the adjacent blocks of the intruded rock, accentuating the faults, 
and causing the described phenomena of downfaulting in the vicinity 
of the dacite. This sinking of volcanic centers after eruption has 
been well established by students of volcanism. 
