74  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.        I  bull.  260. 
above.     The  ore  is  not  in  contact  with  the  dike,  but  is  some  distance 
north  of  it. 
All  the  rocks  except  the  unmineralized  portions  of  the  blue  lime- 
stones have  been  profoundly  altered  by  the  intrusions,  so  that  they 
are  now  very  porcelainlike  in  their  character.  The  shoots  where 
explored  are  about  300  feet  wide,  and  have  been  followed  into  the 
hill  in  one  place  for  400  feet.  Wherever  the  workings  emerge  from 
the  ore  they  pass  into  the  unaltered  blue  limestone,  which  seems  to 
be  singularly  free  from  the  contact  metamorphism  which  has  altered 
the  more  argillaceous  rocks  of  the  series. 
The  ore  lies  in  close  contact  with  the  shale  roof.  It  is  an  intimate 
mixture  of  a  dense,  granular  magnetite  and  pyrite,  with  a  little 
chalcopyrite,  interwoven  in  the  most  complicated  manner  with  epi- 
dote,  actinolite,  garnet,  quartz,  and  calcite.  In  the  Bright  Diamond 
mine  a  large  fault-fissure  vein  lies  on  the  north  side  of  the  ore,  be- 
tween it  and  the  monzonite-porphyry  dike,  and  it  was  in  the  prospect- 
ing work  on  this  fissure  that  the  ores  were  first  discovered. 
The  ore  carries  $10  to  $14  in  gold  and  is  of  nearly  uniform  value, 
but  is  concentrated  with  difficulty  and  has  not  yet  proved  profitable. 
It  is  not  improbable  that  magnetic  separation  may  be  of  material 
assistance  in  its  exploitation. 
So  much  interest  has  centered  of  late  on  deposits  of  magnetite  and 
pyrite  intergrown  with  metamorphic  minerals  that  a  brief  state- 
ment of  the  conclusions  which  may  be  drawn  from  these  peculiai 
deposits  seems  warranted.  Magnetite-pyrite  ores  of  this  kind  are  usu  | 
ally  believed  to  have  been  produced  by  emanations  from  igneous 
intrusives,  which  have  not  only  altered  the  sedimentary  rocks  in  then 
vicinity  but  have  deposited  ore  minerals  among  the  metamorphi 
minerals  that  were  produced  from  the  substance  of  the  stratifiec 
rocks. 
In  this  instance  there  are  facts  which  suggest  some  modification! 
of  this  view : 
1.  The    metamorphic    minerals    in    the    blue    limestone    terminat  I 
abruptly  at  the  sides  of  the  ore  shoots;    that  is,  they  are  coextensiw 
with  the  ore.     When  the  ore  minerals  are  no  longer  found  the  meta 
morphic  minerals  also  cease  and  only  the  inclosing  blue  limestone  ii 
found. 
2.  Unaltered  blue  limestone  intervenes  between  the  ore  and  th ; 
porphyry  dike  that  forms  the  nearest  intrusion  observed. 
3.  The  blue  limestone,  at  a  distance  from  the  fissures,  does  not  cor 
tain  metamorphic  minerals,  although  the  shale  beds  do  contain  then 
To  the  writer  it  would  seem  that  these  facts  lead  to  the  followin 
conclusions: 
1.  That  the  blue  limestone,  owing  to  its  purity,  was  not  altered  r. 
the  porphyry  intrusion  which  affected  the  more  impure  shales. 
