lOfi  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1903.  [bull. 225. 
in  the  country  rock.  At  400  feet  from  the  surface  the  Valley  View 
veins  show  oxidized  ore,  while  100  feet  higher  the  country  rock  away 
from  the  veins  is  nnoxidized.  In  the  Stone  Cabin  shaft  the  change 
from  the  oxidized  to  the  nnoxidized  andesite  came  just  below  the  200- 
foot  level,  but  at  400  feet  the  ore  is  partly  oxidized.  The  reason  for 
this  appears  to  be  that  the  quartz  veins,  on  account  of  their  brittle- 
ness,  have  been  more  readily  fractured  by  strains  than  has  the  softer 
country  rock,  and  so  the  veins  afford  channels  for  oxidizing  waters 
coming  from  the  surface.  Strong  fractures,  such  as  those  in  the 
vicinity  of  faults,  have  the  same  effect  as  the  open  fractures  in  veins 
and  carry  down  the  oxidizing  influences  to  a  depth  of  several  hundred 
feet.  Where  the  rocks  are  protected  above  by  relatively  impervious 
formations  the  oxidation  may  be  comparatively  slight.  Thus,  in  the 
Silvertop  workings  (near  the  Stone  Cabin)  the  later  andesite  is  unoxi- 
dized  at  a  depth  of  120  feet.  Here  it  is  protected  at  the  surface  by  a 
deposit  of  fine-grained,  stratified  tuff. 
In  the  oxidized  zone  of  the  veins  the  sulphides  are  mostly  entirely 
altered  or  can  be  made  out  only  with  a  microscope,  yet  in  many  cases 
the  alteration  is  not  complete  and  the  purple  color  of  the  richer  quartz 
is  as  a  rule  due  to  these  finely  disseminated  sulphides.  Most  of  the 
sulphides,  however,  are  altered  to  silver  chloride,  with  some  bromides 
and  iodides,  and  to  iron  oxide.  Nearly  all  of  the  ore  in  the  Mizpah 
vein,  down  to  the  lowest  depths  explored,  is  oxidized,  while  the  ores 
of  the  Montana  Tonopah  are  only  slightly  and  exceptionally  oxidized. 
In  the  latter  case  the  veins  and  the  inclosing  early  andesite  have  been 
Xn-otected  from  the  atmosphere  by  several  hundred  feet  of  decomposed, 
soft,  later  andesite,  whereas  in  the  former  case  the  veins  outcrop  at 
the  surface  and  oxidizing  waters  sink  readily  into  them. 
In  the  process  of  oxidation  and  alteration  of  the  metallic  minerals 
there  has  probably  been  some  transfer  of  material.  Little  is  known 
of  the  primary  or  nnoxidized  veins,  but  some  portions  seem  almost 
entirely  barren  and  some  very  rich.  It  will  probably  be  found  that 
these  rich  portions  form  definite  shoots  or  masses  of  ore  in  the  rela- 
tively barren  veins.  In  the  oxidized  ores  the  values  are  by  no  means 
uniform,  but  seem  rather  better  distributed  than  in  the  deeper  regions.  | 
The  indications  are  that  during  oxidation  some  of  the  values  have 
been  distributed  from  the  richer  portions  through  the  more  barren 
portions,  producing  a  larger  supply  of  fair  quality  ore.  It  is  also 
likely  that  during  this  process  there  has  been  some  concentration  of 
values,  so  that  the  oxidized  ores  as  a  whole  may  be  found  to  be  some- 
what richer  than  the  sulphide  ores.  It  is  possible,  for  example,  that 
those  portions  of  the  veins  which  have  been  eroded  furnished  during 
the  process  of  erosion  a  small  share  of  their  precious  metals  to  the 
underlying  portions  of  the  vein,  these  underlying  portions  now  hav- 
ing become  the  present  oxidized  zone. 
It  also  seems  probable  that  in  places  some  relatively  slight  trans- 
fer and  redeposition  of  the  precious  metals  has  taken  place  along  j 
