spurr.]  ORE    DEPOSITS    OF    TONOPAH,    NEVADA.  99 
of  white  rhyolite  forced  up  into  the  earlier  gray  rhyolite.  This  later 
white  rhyolite  shows  near  its  contacts,  and  to  a  less  extent  away  from 
these,  in  itself  or  in  the  intruded  rock,  veins  which  are  ascribed  to 
causes  similar  to  those  which  probably  brought  about  the  preced- 
ing mineralizations — namely,  ascending  heated  waters  following  the 
lava  intrusion,  genetically  connected  with  it,  and,  like  the  previous 
similar  processes,  dying  out  after  the  lapse  of  some  time.  These 
veins,  which  are  in  places  very  large,  are  of  calcite,  siderite  or  fer- 
riferous calcite,  chalcedony,  etc.  The}^  contain  some  pyrite  and  some- 
times give  low  values  in  the  precious  metals,  gold  preponderating. 
Mineralization  phenomena  may  also  be  observed  in  some  places  near 
the  contact  of  the  Oddie  Mountain  rhyolite,  where  there  is  silicifica- 
tion  and  the  formation  of  pyrite,  with  quartz  veins  of  various  sizes, 
sometimes  containing  calcite.  These  veins  are  generally  small,  but 
may  be  large.  Galena  and  chalcopyrite  have  been  exceptionally  found 
in  them.  They  carry  low  values  in  gold  and  silver,  the  gold  being 
apt  to  predominate. 
Mineralization  subsequent  to  the  dacite  intrusions. — Finally,  the 
contacts  of  the  large  dacite  necks  (Butler,  Golden,  and  other  moun- 
tains) are  accompanied  in  many  places  by  the  deposition  of  chalce- 
dony, manganese,  etc.,  in  cracks  and  crevices.  These  are  not  usually 
in  veins,  or  when  so  are  small  and  irregular.  Considerable  values  in 
the  precious  metals  have  been  claimed  in  some  cases,  but  it  is  the 
writer's  experience  that  the  content  of  these  metals  is  insignificant. 
Resume. — To  sum  up,  no  fewer  than  four  periods  of  hot-spring 
action,  accompanied  by  more  or  less  vein  formation  and  mineraliza- 
tion, have  been  noted  at  Tonopah.  Each  of  these  periods  was  conse- 
quent upon  a  lava  intrusion,  and  the  mineralization  was  the  result  of 
a  process  which  is  known  to  accompany  and  follow  volcanic  eruption 
at  the  present  day.  In  general,  the  values  found  in  the  veins  of  these 
different  periods  increase  with  increasing  age.  Also,  the  general  phe- 
nomena indicate  that  the  younger  veins  were  formed  nearer  the  sur- 
face than  the  older  ones.  Although  ascending  hot  waters  are  not 
known  to  deposit  more  than  traces  of  metals  at  the  surface,  it  is  sup- 
posed that  an  important  precipitation  takes  place  at  some  depth ;  so  in 
the  case  of  Tonopah  it  may  be  that  the  later  periods  of  mineralization 
have  produced  some  rich  ores,  but  that  erosion  has  not  yet  had  time 
!to  expose  the  level  at  which  they  lie,  whereas  in  the  case  of  the  oldest 
period  this  has  been  accomplished.  At  Gold  Mountain,  about  4  miles 
south  of  Tonopah,  veins  belonging  to  the  rhyolitic  period  and  having 
jhe  same  characteristics  as  some  of  these  veins  at  Tonopah  contain 
ponsiderable  values  in  gold  and  silver,  while  at  Tonopah  no  such 
Values  have  been  discovered  in  this  class  of  veins. 
At  Tonopah,  therefore,  the  earliest  and  most  important  class  of 
[reins  can  be  found  only  in  the  earlier  andesite.  The  rhyolite  veins, 
>n  the  other  hand,  may  occur  in  the  rhyolites,  as  in  any  of  the  older 
