148  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.        [bull.  260. 
Outside  of  the  known  productive  area  exploration  has  been  con- 
tinued in  those  formations  and  in  those  portions  of  the  district  which 
were  found  by  the  writer  to  be  more  favorable  than  the  rest.  The 
chief  work  has  been  done  east  and  west  of  the  productive  region,  but 
it  can  not  be  said  that  so  far  the  results  are  highly  gratifying.  To 
the  east  the  Halifax  shaft  has  been  pushed  down  to  a  depth  of  800 
feet  and  is  in  the  later  andesite  from  top  to  bottom.  Work  has 
been  continued  on  the  Rescue  shaft,  which  is  in  a  white  rhyolite  vol-, 
canic  neck.  Work  in  which  a  larger  amount  of  hope  could  perhaps 
have  been  reasonably  placed  has  been  carried  on  in  the  area  west  of  die 
productive  region.  In  this  area,  as  already  noted,  the  Tonopah  Exten- 
sion has  been  successful,  while  the  other  workings  have  developed 
nothing  of  importance.  The  Ohio  Tonopah  shaft,  770  feet  deep,  en- 
countered a  large  body  of  the  Tonopah  rhyolite-dacite,  and  in  this  most 
of  the  drifting  was  done.  The  Golden  Anchor  shaft,  situated  in  as 
favorable  a  position  as  could  be  chosen  in  this  neighborhood,  has 
reached  a  depth  of  640  feet  without  definite  developments.  This  shaft 
is  mostly  in  the  later  andesite,  with  some  rhyolite-dacite  near  the  lower  | 
portion.  .  Farther  west,  near  the  western  limit  of  the  area  shown  on  the j 
published  geologic  map,  the  new  Pittsburg  shaft  was,  at  the  time  of  tffl 
writer's  visit,  in  November,  1904,  570  feet  deep  in  soft  volcanic 
breccia  belonging  to  a  period  later  than  the  ore-bearing  formations  j 
North  of  this  point,  but  also  on  the  edge  of  the  area  mapped,  th( 
Little  Tonopah  shaft  was  at  that  time  585  feet  deep,  and  its  bottorr 
was  still  in  the  later  andesite. 
SUMMARY. 
The  Tonopah  veins  are  similar  in  nature,  composition,  and  origii  < 
to  those  found  in  some  of  the  richest  mining  districts,  such  as  th< 
Comstock  of  Nevada  and  the  Pachuca  district  of  Mexico.  Indeed 
it  is  probable  that  all  of  these  districts  and  many  others  lie  withii 
a  single  petrographic  and  metallographic  province  characterized  bj  ]  ' 
similar  rocks  and  similar  veins  formed  at  similar  periods.  In  th< 
Tonopah  district  the  rich  ores  occur  in  the  veins  in  the  shape  o: 
irregular  shoots  or  masses  (bonanzas),  comparable  in  size  and  rich 
ness  to  the  famous  bonanzas  of  the  districts  mentioned.  The  geo 
logical  conditions  also  indicate  that  such  bonanzas  were  formed  dowi 
to  considerable  depths.  These  are  the  striking  advantages  of  th 
district,  which  have  justified  considerable  expenditure  of  money  ii 
exploration. 
The  disadvantage  of  cost  of  production  is  common  to  all  mines  ii 
the  desert  region,  and  if  this  were  the  only  drawback  the  productioi  j 
of  the  district  might  reasonably  be  expected  to  become  enormous 
A  greater  disadvantage,  however,  as  previously  pointed  out,  consist 
