shirr.]  ORE    DEPOSITS    OF    TONOPAH,    NEVADA.  103 
where  a  small  branch  vein  leaves  the  main  one  is  generally  impover- 
ished, often  to  a  much  greater  degree  than  the  size  of  the  branch  vein 
would  seem  to  warrant.  The  recognition  of  this  fact  by  the  miners 
has  led  to  the  use  of  the  highly  descriptive  term  "vein  robbers,"  for 
these  small  branches. 
From  the  above  it  will  be  seen  that  the  chief  developed  Tonopah 
veins  have  a  decided  east-west  trend.  From  the  fact  that  there  is  a 
predominant  north- south  trend  in  some  of  the  chief  veins  of  Cali- 
fornia and  the  western  part  of  Nevada,  east-west  veins  have  been 
looked  upon  with  some  prejudice  in  Tonopah,  many  miners  being 
inclined  to  doubt  the  value  of  the  veins  on  account  of  their  strike, 
and  others  forming  the  theory  that  they  were  simply  the  offshoot  of  a 
great  undiscovered  north-south  vein  which  would  prove  to  be  vastly 
richer  than  these;  but  the  Tonopah  field  is  in  an  entirely  different 
region,  geologically,  from  California,  and  this  objection  seems  to  the 
writer  to  have  little  weight.  In  Idaho,  where  the  geological  condi- 
tions are  perhaps  more  like  those  of  Nevada  than  they  are  in  Cali- 
fornia, the  usual  trend  of  the  veins  is  east  and  west. 
Minor  veins  run  northwest  and  northeast  and  even  north  and  south, 
but  so  far  as  yet  observed  the  east- west  veins  are  the  chief  ones,  and 
it  seems  likely  that  the  mineralization  will  be  extensive  in  an  east- 
west  belt  rather  than  in  a  north-south  one. 
OCCURRENCE  OF  ORES  IN  THE  VEINS. 
Physical  character. — The  veins  of  the  Tonopah  district  are  usually 
strong,  straight,  and  well  defined,  yet  they  are  not  fissure  veins.  They 
have  at  first  sight  all  the  appearance  of  fissure  veins,  but  a  little  close 
examination  shows  that  they  have  been  formed  almost  entirely  by 
replacement  of  the  andesite  in  which  they  occur.  They  seem  to  have 
originated  along  zones  of  especially  strong  fracturing  in  the  andesite, 
formed  during  a  period  of  movement  subsequent  to  the  consolidation 
of  this  rock.  These  zones  of  maximum  fracturing,  which  are  usually 
4  to  6  feet  wide,  but  may  be  much  wider  or  narrower,  became  the  chief 
channels  of  circulation  for  the  mineralizing  waters.  It  has  already 
been  stated,  in  describing  the  alteration  of  the  early  andesite,  that  the 
andesite  near  the  veins  has  been  silicified  to  a  very  great  extent,  and 
the  veins  themselves  seem  to  be  the  final  stage  of  alteration,  the  ande- 
site being  mostly  or  entirely  altered  to  quartz.  A  slight  detailed  study 
of  the  veins  gives  abundant  proof  of  this  origin,  for  all  stages  in  the 
development  can  be  seen  in  different  portions.  In  many  cases  the 
vein  consists  simply  of  a  zone  of  more  or  less  altered  andesite,  not 
essentially  different,  except  perhaps  for  a  somewhat  greater  silicifica- 
tion,  from  the  andesite  which  forms  the  walls.  This  zone  is  cut  by 
parallel  fractures  having  the  same  strike  and  dip  as  the  walls,  and  the 
walls  themselves  are  nothing  more  than  stronger  fractures  of  the  same 
kind.     In  the  next  stage,  Avhere  part  of  this  fracture  zone  becomes 
