spurr.]  OKE    DEPOSITS    OF    TONOPAH,    NEVADA.  93 
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  has  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, 
thresh,  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. 
