Irving.]        ORE    DEPOSITS    OF    THE    NORTHERN    BLACK    HILLS.  133 
Refractory  siliceous  ores. — Of  all  the  ores  occurring  in  rocks  of 
ages  later  than  the  Algonkian,  the  refractory  siliceous  ores  have  thus 
far  been  the  most  important  factor  in  the  gold  production  of  the 
northern  Black  Hills.  They  are  widely  distributed  over  a  large  area 
extending  from  Yellow  Creek  on  the  southeast  to  Squaw  Creek  on  the 
northwest  in  a  broad,  irregular  belt.  This  belt  includes  five  pro- 
ductive areas,  which  will  later  be  discussed.  The  country  rock  in 
which  the  ore  occurs  is  a  crystalline  dolomitic  limestone  of  fine-grained 
texture  and  varying  liKe  the  ore  in  its  degree  of  oxidation.  It  is 
termed  "sand  rock"  by  the  miners.  In  its  fresh  condition  it  is  a 
dense,  gray,  crystalline  rock,  showing  innumerable  small  cleavage 
faces  of  dolomite  that  are  generally  interrupted  by  bands  of  greenish- 
black  shale  of  varying  width.  When  oxidized  it  has  a  deep-red  color, 
but  presents  the  same  crystalline  texture,  while  with  very  advanced 
alteration  it  passes  into  a  red,  earthy  material  termed  "gouge." 
Chemical  analysis  of  this  rock  shows  it  to  be  a  dolomite  of  nearly  normal 
composition,  while  the  microscope  shows  that  it  is  composed  of  either 
(1)  irregular  grains  of  dolomite  with  some  scattered  grains  of  quartz 
or  (2)  of  clearly  bounded  rhombic  crystals  of  dolomite.  When  the 
latter  t}^pe  of  rock  has  been  mineralized  the  rhombic  crystals  of  dolo- 
mite are  altered  to  silica  and  often  beautifully  preserved. 
The  dolomite  beds  which  have  so  far  been  most  extensively  pros- 
pected occur  at  two  positions  in  the  Cambrian  series.  The  first  is 
immediately  above  the  basal  quartzite,  from  15  to  25  feet  above  the 
schists  of  the  Algonkian,  and  is  known  as  the  "lower  contact;"  the 
second  is  from  18  to  30  feet  below  the  Scolithus  or  so-called  "worm- 
eaten"  sandstone  that  forms  the  top  of  the  Cambrian  series,  and  is 
termed  the  "upper  contact."  Many  other  beds  of  dolomite  occur  at 
intervening  levels,  and  prospecting  shows  them  to  be  very  frequently 
mineralized.  There  has  been  as  yet  but  little  systematic  search  upon 
the  latter  beds,  and  it  is  very  probable  that  they  may  become  impor- 
tant ore  horizons  in  the  future. 
The  ore  is  an  extremely  hard,  brittle  rock,  composed,  largely  of  sec- 
ondary silica,  and  carrying,  when  unoxidized,  pyrite,  fluorite,  and, 
at  times,  barite,  wolframite,  stibnite,  and  jarosite.  It  shows  man}^ 
cavities  which  are  lined  with  druses  of  quartz  crystals  or  contain 
cubes  of  fluorite  in  clusters.  Some  of  the  cavities  show  large  crys- 
tals of  barite.  In  some  localities  the  siliceous  ore  is  heavily  charged 
with  wolframite,  so  that  in  many  instances  it  grades  from  beds  of 
siliceous  ore  into  flat  bodies  of  almost  pure  wolframite.  Occurrences 
of  this  kind  are  found  in  the  Yellow  Creek  and  Lead  areas.  When 
^ariying  large  quantities  of  wolframite  the  ore  usually  contains 
£reat  quantities  of  barite.  The  ore  occurs  in  flat,  banded  masses 
n  which  the  banding  is  continuous  with  the  bedding  planes  of  the 
idjoining  strata.  These  masses  possess  a  somewhat  regular,  channel- 
ike  form,  and  follow  zones  of  fracture  that  vary  for  the  separate  dis- 
