96  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1903.  [bull.  225. 
altered  and  bleached  it  lias  a  dull  elayey  luster,  which  is  rarely  seen 
in  the  early  andesite.  Rocks  which  are  highly  siliceous  from  altera- 
tion are  apt  to  belong  to  the  early  rather  than  the  late  andesite. 
Distribution  and  characteristics  of  the  rhyolites. — There  have  been 
several  different  rhyolite  flows  and  intrusions,  some  before,  some 
mainly  subsequent  to  the  f aultings ;  and  the  rocks  have  a  variety  of 
appearances  which  defies  any  description  in  common.  The  lava  of 
Oddie,  Rushton,  and  Ararat  hills  is  a  characteristic  siliceous  white 
rhyolite.  The  Rescue  shaft  is  mainly  in  this  rock,  and  two  long 
tunnnels,  the  North  Star  and  the  G.  &  H. ,  have  been  driven  in  it  into 
Mount  Oddie. 
Besides  this,  however,  there  is  a  formation  of  fine-grained  gray  or 
red,  largely  glassy  rhyolite,  which  covers  a  still  larger  area.  A  large 
body  of  it  occupies  the  northern  portions  of  the  district,  where  it  is 
intrusive,  with  a  sinuous  contact,  into  the  later  andesite.  It  continues 
from  here,  along  the  western  edge  of  the  district,  to  the  vicinity  of 
the  Ohio  Tonopah,  and  occupies  a  considerable  part  of  the  basin 
inclosed  between  Butler,  Brougher,  and  Siebert  mountains.  The  for- 
mation here  occurs  as  complicated  dikes  cutting  the  dacite-breccia 
formation,  and  as  sheets  underlying  the  tuff,  or,  to  a  less  extent,  inter- 
calated in  the  tuff.  In  the  rest  of  the  district  it  is  practical!}7  absent. 
It  is  in  general  between  the  dacite  breccia  and  the  stratified  tuff  in 
point  of  age,  is  older  than  the  faults,  and  is  therefore  displaced  by 
the  fault  movements.  In  the  northern  portion  of  the  district  a  num- 
ber of  shafts  are  located  on  or  near  the  contact  of  this  rhyolite  and 
the  later  andesite.  Among  these  may  be  mentioned  the  King  To- 
nopah, Belle  of  Tonopah,  Miriam,  Silvertop,  Little  Tonopah,  etc.  I 
the  western  portion  the  Ohio  Tonopah  and  the  New  York  Tonopa 
each  encountered  a  considerable  quantity  of  this  rhyolite  mixed  with 
dacite  breccia,  while  the  Fraction  Extension  is  entirely  in  this  rock, 
after  having  passed  through  the  overlying  tuff. 
This  finer  grained  gray  or  red  rhyolite  has  many  different  aspects 
Fine  brecciation,  with  bright  red,  gray,  and  white  colors,  is  frequent 
in  other  cases  it  is  dense  and  characterless.     The  best  test  of  it  is  the 
detection  of  the  small  quartz  crystals  that  dot  the  glassy  groundmass, 
which  distinguishes  the  rock  satisfactorily  from  the  andesites.     Some- 
times, however,  these  quartz  crystals  may  be  rare  and  small.     Near, 
the  contacts  of  the  rhyolite  (intrusive  into  the  earlier  rocks)  there 
has  also  frequently  been  alteration — silicification,  the  formation  ofj 
pyrite,   etc. — and   then   the   rock   may   become   almost   identical   in 
appearance  with  the  early  andesite  which  has  been  similarly  altered. 
The  best  method  for  distinguishing  the  rhyolite  from  andesite  in  this 
case  is  the  tracing  of  the  connection  with  the  less-altered  rhyolit 
farther  away  from  the  contact. 
Distribution   and   characteristics   of  the  dacites. — The  bold   kno 
north  of  Butler  Mountain,  known  as  Heller  Butte,  is  made  up  of  what 
