spubr.]  ORE    DEPOSITS    OF   TONOPAH,   NEVADA.  95 
discovered  under  younger  rocks  in  a  number  of  different  mines,  such 
as  the  Montana  Tonopah,  the  North  Star,  the  Ohio  Tonopah,  the 
California  Tonapah,  the  Wandering  Boy,  the  Fraction,  and  probably 
the  West  End  and  MacNamara,  the  Midway,  etc.  Nowhere  is  it  in 
even  an  approximately  fresh  state,  but,  as  before  noted,  it  has  altered 
largely  to  calcite  and  chlorite,  and  (more  commonly  in  the  vicinity  of 
the  veins)  to  quartz  and  sericite.  In  the  former  condition  it  has  a 
fairly  deep-green  or  blue  color,  while  in  the  latter  phases  it  is  lighter 
green  or  blue,  and  very  often  nearly  white,  or  assumes,  near  the  sur- 
face and  along  faults  or  fractures,  a  pale,  yellowish,  brownish,  or  red- 
dish color.  This  common  quartz-sericite  type  has  the  appearance  of 
rhyolite  rather  than  of  anything  else,  and  does  not  at  all  resemble 
andesite,  while  the  caleite-chlorite  phase  has  more  nearly  the  rightful 
aspect  of  a  highly  altered  andesite. 
It  is  difficult  always  to  distinguish  this  early  andesite  with  certainty 
on  account  of  its  many  different  phases.  It  is  most  likely  to  be  con- 
founded with  the  later  andesite  on  one  hand  and  with  certain  types 
of  rhyolite  on  the  other.  The  main  points  of  difference  will  be  out- 
lined shortly. 
Distribution  and  characteristics  of  the  later  andesite. — The  later 
andesite  outcrops  over  a  considerable  region  in  the  northern  part 
of  the  district.  The  upper  portions,  and  in  some  cases  the  whole,  of 
the  following  shafts  are  in  it:  Montana  Tonopah,  North  Star,  Mid- 
way, Golden  Anchor,  Little  Tonopah,  Boston  Tonopah,  Halifax,  etc. 
It  varies  greatly  in  state  of  preservation,  being  frequently  found 
nearly  fresh,  and  again  highly  decomposed.  In  its  fresher  phases 
it  is  a  dark-colored,  hard  rock,  with  large  crystals  of  pyroxene,  feld- 
spar, and  biotite.  It  has  usually  been  more  or  less  altered,  and 
as  secondary  minerals  calcite,  chlorite,  serpentine,  etc.,  have  been 
formed,  giving  the  rock  a  dark-green  color.  Frequently,  also,  the 
altered  rock  has  a  deep-blue  color.  Near  the  surface  the  red  of  the 
oxidized  iron  combines  with  these  colors  to  produce  a  characteristic 
rich  purple,  hi  some  places  the  rock  has  been  thoroughly  altered  to 
calcite,  chlorite,  and  pyrite,  with  other  secondary  minerals,  and  again 
has  been  so  completely  leached  as  to  be  soft  and  white. 
In  these  highly  altered  phases  the  later  andesite  becomes  with  great 
difficult}^  distinguishable  from  the  early  andesite,  and  where  it  is  fine 
grained  the  resemblance  may  become  almost  exact. 
For  the  distinction  of  the  two  rocks  in  general  it  may  be  noted  that 
the  early  andesite  is  characteristically  finer  grained  than  the  later 
andesite,  and  that  in  the  former  the  feldspar  crystals  are  slimmer  and 
rectangular,  while  in  the  latter  they  are  stouter  and  often  of  complex 
shape.  In  some  of  the  hornblende-andesite  (though  not  generally)  the 
long  dark  crystals  of  altered  hornblende  may  be  distinguished,  while 
in  the  later  andesite  the  crystals  of  biotite  are  usually  plainly  visible, 
even  in  the  considerably  altered  phases.  The  characteristic  later 
andesite  has  a  coarse,  mottled  appearance,  and   even  when   highly 
