PPURK.]  DEVELOPMENTS    AT    TONOPAH.    NEV.  145 
Extension,  and  Ohio  Tonopah.  On  account  of  their  resemblance  to 
he  earlier  andesite  veins  they  have  been  the  object  of  a  good  deal 
)f  exploration  and  development  work,  which,  on  the  average,  has 
>een  decidedly  unprofitable. 
CHARACTERISTICS  OF  RHYOLITE-DACITE  VEINS. 
The  veins  of  this  period  are  characterized  everywhere  by  irregu- 
arity  and  by  a  lack  of  definition  and  persistence,  though  their  size 
nay  locally  be  great.     As  a  rule  they  are  elongated  and  have  the 
ippearance  of  regular  veins,  but  can  not  be  followed  as  far  on  either 
:he  strike  or  dip  as  can  the  regular  veins.     They  may  disappear  by 
scattering  and  passing  into  a  silicined  wall  rock,  or  they  may  be  cut 
)ff  along  a  cross  fracture.     The  quartz  is,  as  a  rule,  dense  and  jaspery 
md  either  white,  gray,  or  black  in  color,  and  is  therefore  usually 
lifferent  in  appearance  from  the  white  quartz  of  the  earlier  andesite 
t^ems.     The  veins  are  commonly  barren  or  contain  only  very  small 
uantities  of  gold  and  silver,  except  locally,  where  rich  bunches  of 
re  may  occur,  though  of  limited  and  irregular  extent.     In  the  Ohio 
onopah  barite  occurs  as  a  gangue  mineral  with  the  rhyolite-dacite 
eins.     This  mineral  has  not  yet  been  found  in  connection  with  the 
rlier  andesite  mineralization.     In  the  Desert  Queen  and  the  North 
tar,  where  quartz  of  the  rhyolite-dacite  period  has  been  opened  by 
rifting,  a  green  stain  forms  on  the  walls,  which  is  a  basic  copper 
jilphate.     This  phenomenon  has  not  yet  been  noted  in  connection 
jith  the  earlier  andesite  mineralization.     Characteristic  of  the  rhyo- 
|te-dacite  veins,  to  which,  however,  there  are  numerous  exceptions,  is 
^e  greater  ratio  of  gold  to  silver  in  them  as  compared  to  that  in  the 
irlier  andesite  veins.     In  the  latter  class  of  veins  the  gold  averages 
)out  two-fifths  of  the  value,  the  silver  three-fifths,  while  in  the 
»rmer  class  the  gold  is  apt  to  exceed  this  amount  and  sometimes  to 
cur  with  practically  no  silver,   although   the  proportion    is   very 
angeable.     Often,  again,  the  proportion  of  gold  and  silver  is  the 
me  as  in  the  earlier  andesite  veins. 
[A  favorite  place  for  these  veins  is  on  the  upper  contact  of  a  rhyolite- 
(tcite  sheet  with  overlying  decomposed  andesite.     Such,  for  exam- 
.3,   is   the   situation   in   the   Mizpah    Extension,    the   MacNamara, 
nopah   Extension,   and  West  End.   and   to   a   less   degree   in  the 
lio    Tonopah.     This    fact    is    explained    by    the  conception    that 
■sending  hot  waters   circulating   through   the   fractured    rhyolite- 
Jeite  rose  until  at  the  contact  with  the  overlying  soft,  decomposed 
Xdesite   they   found   a    partially   impervious   barrier,    along   whose 
l<ver   contact   they   circulated   and   deposited   the   materials   which 
t%  held  in  solution. 
Bull.  260—05  m 10 
r 
