irving.]        ORE    DEPOSITS    OF   THE    NORTHERN    BLACK    HILLS.  137 
mond,  Florence,  Hester  A,  Coletta,  Merritt  No.  2,  Cora,  Carpenter, 
Alexander,  Romeo,  and  El  Refugio. 
The  ore  is  of  a  more  basic  character  than  that  found  in  the  more  west- 
erly ore  deposits  heretofore  described.  When  unoxidized  it  consists 
chiefly  of  pyrite,  which  is  either  massive  or  disseminated  more  or  less 
thickly  through  the  body  of  the  country  rock.  With  the  pyrite  is 
associated  argentiferous  galena  and,  not  infrequently,  small  quantities 
of  sphalerite.  In  many  cases  the  galena  occurs  as  minute  crystals  lining 
the  interior  of  druses,  or  in  seams  in  the  pyrite,  and  wherever  they 
have  not  been  oxidized  these  two  minerals  are  found  associated  in  this 
manner.  The  galena  is,  therefore,  of  later  origin  than  the  pyrite; 
occasionally  the  latter  carries  low  values  in  gold,  but  these  are  unim- 
portant. The  values  that  render  the  mines  workable  are  contained  in 
the  argentiferous  galena.  In  most  cases  there  is  but  little  silica  asso- 
ciated with  the  ores,  but  in  the  Florence  and  Richmond  very  consider- 
able amounts  of  secondary  silica  are  found  in  intimate  association  with 
the  deposits. 
Origin  of  the  ores. — The  refractory  siliceous  ores  have  been  formed 
by  a  process  which  involved  the  gradual  removal  of  the  original  rock 
substance  and  the  simultaneous  substitution  of  the  ore  minerals.  This 
is  commonly  known  as  replacement  or  metasomatic  alteration  and  has 
often  proceeded  with  so  little  disturbance  of  the  original  rock  material 
that  both  stratigraphic  character  and  microscopic  structure  are  pre- 
served in  the  ore,  although  the  original  rock  was  carbonate  and  the 
ore  is  chiefly  silica.  The  mineral  which  has  been  altered  to  form  ore 
seems  to  have  been  exclusively  dolomite,  for  where  verticals  pass 
through  rocks  of  varying  composition  it  is  found  that  the  dolomite 
alone  has  been  appreciably  affected.  The  ore  minerals  substituted  are 
chiefly  silica  and  p}^rite,  with  which  there  are  minute  amounts  of  gold 
and  silver.  Smaller  quantities  of  fluorite,  barite,  gypsum,  and  several 
other  accessory  minerals  are  also  of  frequent  occurrence.  To  the 
dolomite,  whether  present  as  comparatively  pure  beds  or  as  cement- 
ing material  of  sandy  and  shaly  rocks,  the  ore  minerals  have  been 
transported  by  circulating  waters.  Such  waters  have  found  in  the 
fractures  trunk  channels,  by  means  of  which  they  have  been  enabled 
to  penetrate  the  encompassing  and  comparatively  insoluble  rocks  and 
reach  the  more  readity  replaced  material.  The  mass  of  evidence 
seems  to  show  that  these  waters  have  ascended. 
ORES   IN    CARBONIFEROUS    ROCKS. 
In  the  heavy,  gra}T  limestones  of  the  Carboniferous  are  found  two 
distinct  varieties  of  ore:  (1)  Gold  and  silver  ores  of  a  refractoiy  sili- 
ceous type;  (2)  lead -silver  ores.  In  general,  the  Carboniferous  rocks 
have  not  figured  largely  in  the  mineral  production  of  the  region. 
The  refractory  siliceous  ores  are  in  bodies  of  comparatively  small 
