spttrr/I  ORE    DEPOSITS    OF    TONOPAH,    NEVADA.  107 
the  fault,  planes,  which  are  later  than  the  original  veins  and  have  dis- 
placed them.  This  action,  however,  is  only  exceptionally  the  case, 
and  as  a  whole  is  insignificant.  The  main  ore  shoots  of  the  oxidized 
portion  of  the  Mizpah  mine  are  regarded  as  primary,  since  evidence 
has  been  found  that  these  coincide  with  the  original  ones  and  have 
been  oxidized  nearly  in  place;  therefore  it  is  improbable  that  the 
transfer  of  material  during  oxidation,  bringing  about  local  concen- 
tration or  impoverishment,  has  been  very  great. 
Ore  shoots. — In  the  oxidized  portion  of  the  Mizpah  vein  the  richer 
ores  lie  in  roughly  defined,  broad  shoots,  which  pitch  east  on  the  vein. 
At  least  three  of  these  shoots,  parallel  to  one  another  and  with  poorer 
ore  between,  have  been  recognized.  In  the  unoxidized  ores  of  the 
Montana  Tonopah  there  is  evidence  of  the  existence  of  similar  shoots 
or  bunches,  but  enough  exploration  work  has  not  been  done  to  enable 
the  writer  to  describe  them  more  closely. 
APPLICATION   OF  GEOLOGICAL  PRINCIPLES. 
From  the  short  description  of  the  occurrence  of  the  veins  which  has 
been  given,  it  is  seen  that  it  is  of  the  first  and  highest  importance  to 
distinguish  and  determine,  so  far  as  possible,  the  position  of  the  early 
andesite.  As  has  been  noted,  this  early  andesite  is  likely  to  be  con- 
founded with  the  later  andesite  and  with  some  forms  of  altered  and 
silicified  rhyolite,  especially  that  gray,  glassy  rhyolite  which  occupies 
a  considerable  area  in  the  northern  corner  of  the  district.  As  is  seen 
on  the  accompanying  map,  the  actual  area  where  the  early  andesite 
outcrops  is  very  limited,  and  in  other  portions  of  the  district  it  is 
covered  by  later  rocks  or  is  cut  through  and  displaced  by  them. 
These  later  cappings  may  consist  of  the  biotite-andesite,  the  various 
rhyolites  and  dacites,  the  volcanic  breccias,  or.  the  stratified  tuff  for- 
mation ;  or  they  may  consist  of  two  or  more  of  these,  superimposed 
one  on  the  other.  Those  later  rocks  which  are  intrusive  rather  than 
in  the  form  of  flows  (including  some  of  the  siliceous  rhyolites,  some 
of  the  dacites,  and  probably  even  some  of  the  later  andesites)  can  not 
be  expected,  however,  to  give  way  to  the  early  andesite  in  depth.  A 
shaft  sunk  in  a  dike  or  volcanic  neck  is  very  apt  to  continue  in  this 
same  rock  indefinitely  downward  (see  fig.  5). 
Of  the  different  rocks  which  cap  the  early  andesite  and  the  included 
veins,  the  oldest,  the  later  andesite,  is  regarded  as  the  most  favor- 
able for  prospecting;  and,  in  the  light  of  present  knowledge,  a  belt  of 
this  rock  lying  east  and  west  of  Mizpah  Hill  is  probably  especially 
favorable.  On  the  other  hand,  although  the  veins  indicate  a  general 
east- west  trend  of  mineralization,  it  is  by  no  means  proved  that  the 
outcrops  on  Mizpah  Hill  occupy  the  center  of  the  mineralized  belt  and 
show  the  greatest  mineralization.  It  may  eventually  be  proved  that 
the  strongest  portion  of  this  belt  lies  north  or  south  of  the  locality 
known  at  present,  which  would  make  the  whole  width  of  the  belt  con- 
