hiving.]  OURAY    DISTRICT,   COLORADO.  73 
siliceous  replacement  is  slight,  and  the  gangue  of  the  ore  often  con- 
sists of  immense  masses  of  barite  in  otherwise  unaltered  limestone. 
In  the  massive  sulphide  bodies  the  gray  copper  can  be  readily  seen, 
but  in  the  usual  run  of  ore  it  is  often  disseminated  through  the  gangue 
in  grains  so  small  as  to  be  hardly  perceptible  to  the  eye. 
The  larger  structures  of  the  country  rock,  such  as  brecciation,  bed- 
ding planes,  layers  of  shale,  etc.,  always  extend  uninterruptedly  into 
the  ore,  except  where  the  mineralization — as  in  the  ease  of  the  bodies 
of  sulphides — has  obliterated  them. 
The  values  are  mainly  in  silver.  The  gold  occasionally  rises  to 
$1  or  $5  per  ton,  but  is  generally  so  low  as  to  be  negligible.  In  the 
largest  masses,  such  as  the  Pony  Express  and  Mineral  Farm  mines, 
the  values  in  the  ore  are  extremely  irregular  and  are  uniformly  low. 
The  ore  from  the  Pony  Express  is  reported  to  average  as  a  whole 
about  $30  per  ton,  although  much  higher  values  are  encountered. 
In  the  Mineral  Farm  mine  the  average  is  so  low  and  the  richer  por- 
tions of  the  ore  are  so  irregularly  distributed  that  the  deposit  has 
never  yet  paid  for  exploitation. 
In  the  Newsboy  the  values  are  uniformly  higher,  and  frequently 
carload  lots  will  carry  as  much  as  100  ounces  of  silver. 
It  is  difficult  to  form  a  correct  estimate  of  the  production  of  this 
class  of  ores,  as  they  have  generally  been  worked  intermittently  under 
lease. 
MAGNETITE-PYKITE  ORES. 
The  magnetite-pyrite  ores  are  of  greater  scientific  than  commercial 
interest.  They  have  been  found  at  only  one  locality — in  the  Bright 
Diamond  and  Iron  Clad  mines,  on  the  east  wall  of  the  Uncompahgre 
Canyon,  about  600  feet  below  the  American  Nettie  mine.  The  or? 
occurs  in  a  fine-grained,  dark-blue  limestone,  apparently  quite  pure 
and,  where  unmineralized,  without  any  evidence  of  alteration.  It  is 
about  10  or  15  feet  thick  and  is  overlain  by  green  shales  that  have 
been  altered  by  the  monzonite-porphyry  intrusions  into  a  greer  por- 
.  I  celain-like  argillite  heavily  charged  with  epidote  and  other  metamor- 
phic  minerals,  and  of  an  exceedingly  dense,  impervious  character. 
The  ore  occurs  in  broad,  flat  shoots,  conformable  to  the  stratification 
and  of  a  uniform  thickness  of  about  G  feet.  It  lies  in  close  contact 
with  the  shale  roof.  The  strata  are  nearly  horizontal  but  show  local 
dips,  and  are  intersected  by  a  series  of  widely  spaced  fractures  which 
run  in  a  nearly  east-west  direction  and  frequently  fault  the  strata 
as  much  as  10  feet,  with  doAvnthrows  now  to  the  north  and  now 
to  the  south.  The  ore  outcrops  for  a  long  distance  in  the  cliff  near 
the  mines.  In  the  Bright  Diamond  mine  a  porphyry  dike  10  to 
30  feet  ih  width  occurs  north  of  the  ore,  and  the  heavy  monzonite- 
porphyry  sheet  above  the  American  Nettie  mine  lies  some  700  feet 
