144  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.         [bull.  260 
tain,  but  the  presence  of  a  selenide  of  gold  is  not  impossible,  although 
this  mineral  has  not  yet  been  described  as  occurring  in  nature. 
The  analysis  of  the  so-called  oxidized  ore  shows  considerable 
changes  from  the  primary  condition,  but  the  argentite  seems  to  hav< 
largely  remained  unaltered,  while  the  polybasite  and  stephanite,  an( 
also  much  of  the  silver  selenide,  have  probably  been  attacked  b; 
oxidizing  agents.  It  is  thus  seen  that  the  so-called  oxidized  ore  o 
the  Tonopah  district  is  really  a  modified  ore,  consisting  of  an  intii 
mate  mixture  of  sulphides  and  selenides,  together  with  secondar 
sulphides,  chlorides,  and  oxides. 
VEINS    OF    A    LATER    PERIOD    THAN    THE    EARLIER    ANDESITI  (| 
VEINS. 
In  last  year's  report  a  it  was  noted  that  while  the  most  productil 
veins  were  connected  with  the  earlier  andesite  eruption,  yet  the  sut 
sequent    volcanic    eruptions    also    were    frequently    accompanied    b 
silicification  and  vein  formation,  although  these  veins  are  of  mini 
economic  importance.     It  was  clear  even  at  that  time  that  the  mo:  I 
abundant   mineralization   and   silicification,   besides   that   connects 
with  the  earlier  andesite,  had  occurred  subsequent  to  the  intrusion  an 
extrusion   of   a   more   siliceous   rock   of   later   age.     This   rock   wj  i1 
referred  to  at  that  time  as  the  "  early  "  rhyolite,  and  was  indicated  c  ! 
the  accompanying  map  as  rhyolite  breccia.     In  the  final  report  th 
formation  is  somewhat  more  accurately  classified  as  rhyolite-dacitt 
and  is  specifically  referred  to  as  the  Tonopah  rhyolite-dacite.     ]| 
intrusive  bodies  of  this  rhyolite-dacite,  especially  near  the  contadj 
and   lying  within   either   the   intrusive   or   the   intruded   rock,   b  i 
preferably  in  the  former,  bodies  of  quartz,  more  or  less  mineralize? 
are  very  common.     At  the  time  of  writing  last  year's  report  tli| 
chief  examples  known  were  the  veins  of  the  Mizpah  Extension,  o 
vein  in  the  Desert  Queen  shaft,  and  the  mineralization  found  in  t 
workings  in  the  King  Tonopah  and  the  Belle  of  Tonopah  sha' 
During  the  past  year,  however,  most  of  the  working  mines  in  t 
central   part   of   the   district   have   developed   large   bodies   of   tl 
rhyolite-dacite   and   have   encountered    in   this   connection   many 
the  characteristic  quartz  veins  of  this  period. 
These  veins  may  be  large  and  may  carry  values.'  They  are  eas> 
confounded  with  the  veins  of  the  earlier  andesite,  just  as  the  silicifili 
Tonopah  rhyolite-dacite,  in  which  they  usually  occur,  may  be  c( 
founded  with  certain  highly  silicified  phases  of  the  earlier  andesii 
Such  A^eins  have  been  encountered  in  the  Belle  of  Tonopah,  Ki 
Tonopah,  Mizpah  Extension,  Desert  Queen,  North  Star,  Monta! 
Tonopah.    Mizpah,    Midway,    MacNamara,    West    End,    Tonop 
a  Bull.  U.  S.  Geol.  Survey  No.  225,  p.  98. 
