spdrr.]  DEVELOPMENTS    AT    TONOPAH,   NEV.  149 
in  the  disturbances  to  which  the  veins  have  been  subjected  since  their 
formation.  These  disturbances  consist  of  both  repeated  and  copious 
intrusions  of  later  volcanic  rocks  and  of  faulting  which  accompanied 
the  intrusions,  forming  a  condition  illustrated  in  the  ideal  cross  sec- 
tion published  last  year.a 
By  these  disturbances  the  veins  have  been  broken  in  detail,  and 
sometimes  even  displaced  to  such  an  extent  that  search  for  their 
continuation  is  practically  hopeless.  In  mining,  the  expense  of  fol- 
lowing the  vein  is  increased  by  slight  disturbances,  and  where  the 
vein  is  greatly  displaced  a  large  amount  of  money  must  be  spent  in 
exploration,  however  intelligently  this  may  be  directed.  When  veins 
belonging  to  one  of  the  later  periods,  especially  that  following  the 
Tonopah  rhyolite-dacite  intrusion,  are  encountered,  it  ma}7  be  difficult 
for  the  mining  man  to  identify  their  character  or  class,  and  much 
expense  is  incurred  in  drifting  along  these  in  search  of  ore  bodies, 
which,  if  encountered,  are  usually  not  sufficient  to  pay  for  the  whole 
work  done. 
Moreover,  the  exploratory  shafts  which  have  been  sunk,  particu- 
larly during  the  last  year,  in  the  region  around  the  productive  area, 
indicate  that  the  earlier  andesite,  with  its  productive  veins,  if,  indeed, 
it  can  be  found  in  any  large  amount  in  these  regions,  lies  under  an 
average  great  thickness  of  the  subsequent  barren  formations.  Where 
these  veins  are  very  close  to  the  surface,  as  in  the  present  chief  pro- 
ductive area,  the  difficulties  of  mining,  as  pointed  out,  are  still  so 
great  as  to  make  large  profits  often  a  matter  of  considerable  doubt; 
and  where  similar  mining  and  exploration  must  be  carried  on  at  an 
increasingly  greater  depth  it  becomes  more  and  more  doubtful 
whether  the  results  will  warrant  the  necessary  outlay. 
For  these  reasons,  in  considering  the  balance  struck  between  the 
highly  advantageous  character  of  the  ore  and  the  origin  and  nature 
of  the  veins,  and  the  disadvantageous  circumstances  introduced  by  late 
disturbances,  it  impresses  the  writer  that  the  balance  is  at  present 
rather  on  the  disadvantageous  side. 
"Bull.  U.  S.  Geol.  Survey  No.  225,  p.  108. 
