spurb.]  ORE    DEPOSITS    OF    TONOPAH,   NEVADA.  97 
is  regarded  as  probably  the  oldest  dacite  exposed  at  the  surface.  It 
is  a  compact,  relatively  coarse-grained  rock,  iinely  brecciated,  and 
carrying  included  fragments  of  older  rocks,  notably  the  later  andesite. 
It  is  surrounded  and  probably  overlain  by  the  softer  dacite-breccia 
formation.  The  Tonopah  City  shaft  after  passing  through  the  dacite 
breccia  has  continued  in  solid  Heller  dacite  to  a  total  depth  of  over 
500  feet.  East  of  Butler  Mountain,  on  the  edge  of  the  district, 
another  strip  of  this  dacite  is  exposed,  also  surrounded  by  the  dacite 
breccia.     It  is  regarded  as  probably  intrusive. 
The  dacite-breccia  formation,  on  the  other  hand,  is  essentially  a  sur- 
face formation,  and  not  intrusive.  It  consists  of  flows  of  often  very 
pumiceous  and  friable  dacite,  dacitic  mud  flows,  and  some  rudely  lay- 
ered or  even  stratified  fragmental  material.  In  places  it  is  very  thick. 
It  is  exposed  over  a  large  fraction  of  the  southern  portion  of  the  dis- 
trict, around  Butler,  Brougher,  and  Siebert  mountains.  The  Ohio 
Tonopah  and  the  New  York  Tonopah  shafts  passed  through  consider- 
able thicknesses  of  this  formation,  and  the  Fraction  No.  3  is  entirely 
in  it.  The  Fraction  Nos.  1  and  2,  and  Wandering  Boy,  and  the  West 
End,  passed  through  this  formation  at  the  surface  and  reached  the 
underlying  formations. 
The  later  intrusive  dacite,  which  makes  up  Butler,  Brougher,  and 
Golden  mountains  and  the  central  portion  of  Siebert  Mountain,  has 
usually  a  characteristic  appearance.  It  is  darker  than  the  rhyolite  of 
Mount  Oddie,  has  a  slight  purplish  tinge,  and  contains  more  crystals 
(of  feldspar,  quartz,  and  mica)  embedded  in  the  glassy  groundmass. 
The  Big  Tono  shaft,  at  the  east  foot  of  Mount  Brougher,  starting  at 
the  outward-pitching  contact  of  this  rock  and  the  intruded  dacite- 
breccia  formation,  goes  down  several  hundred  feet  in  the  former. 
The  Molly,  starting  near  the  dacite  contact  on  Golden  Mountain, 
passed  through  several  hundred  feet  of  this  rock,  then  through  the 
inward-pitching  contact  to  a  slight  thickness  of  loose  material  belong- 
ing to  the  dacite-breccia  formation,  and  so  into  the  later  andesite. 
PERIODS  AND  NATURE  OF  MINERALIZATION. 
Mineralization  subsequent  to  the  early  andesite  intrusion. — The  most 
important  veins  of  the  Tonopah  district,  and  all  those  that  have  been 
proved  to  be  of  immediate  economic  importance,  occur  in  the  early 
andesite,  and  do  not  extend  into  the  overlying  rocks.  Hence,  when 
the  early  andesite  is  not  exposed  at  the  surface,  the  later  rocks  form 
a  capping  to  the  veins,  and  this  capping  must  be  passed  through 
before  anything  can  be  learned  of  the  presence  or  the  nature  of  the 
veins  beneath.  This  circumstance  shows  pretty  plainly  that  the  vein 
deposition  took  place  before  the  eruption  of  the  later  andesite,  and 
immediately  after  that  of  the  early  andesite  (for  the  period  of  erosion 
between  the  two  andesites  seems  to  have  exposed  the  veins  at  the  sur- 
face, showing  that  they  were  formed  before  this  period,  or  early  in  it). 
Bull.  225—04 7 
