irving]        ORE    DEPOSITS    OF    THE    NORTHERN    BLACK    HILLS.  127 
There  are  in  general  two  areas  where  ores  of  this  character  have 
been  discovered.  The  first  is  the  great  Homestake  belt;  the  second, 
the  Clover  Leaf  or  Uncle  Sain  mine,  some  distance  southeast  of  the 
first. 
As  a  report  will  soon  appear  by  Mr.  S.  F.  Emmons,  discussing  in 
detail  the  geological  structure  of  the  Homestake  mine,  the  writer  will 
give  only  a  brief  summary  of  this  important  ore  zone,  gathered  in 
large  part  from  previously  published  reports  and  in  part  from  per- 
sonal observations. 
Homestake  belt. — The  Homestake  belt  is  a  term  which  has  been 
applied  to  a  series  of  mines  operated  on  a  great  gold-bearing  zone  in 
the  metamorphic  schists  in  the  vicinit}^  of  Lead,  S.  Dak.  It  com- 
prises a  group  of  mines  which  are  known  severally  as  the  Homestake, 
Deadwood-Terra,  Father  de  Smet,  and  Caledonia,  but  as  the  Home- 
stake  Company  has  exercised  an  increasingly  important  influence  in 
the  management,  its  name  is  now  generally  applied  to  the  entire  belt. 
The  surface  workings  or  open  cats  from  which  the  ore  was  first 
extracted  in  the  early  days  of  its  history  indicate  in  a  broad,  general 
way  the  location  and  trend  of  the  lode.  The  Caledonia  ore  body  is 
distinct  from  that  operated  in  the  other  mines,  and  lies  to  the  east  of 
it.  The  Homestake  ore  bod}7  is  not  a  true  fissure  vein,  but  is  a  broad, 
impregnated  zone  in  the  schists,  which  strikes  approximately  N.  34° 
W.  and  is  slightly  at  variance  with  the  general  direction  of  the  lami- 
nation of  the  schists.  There  seems  to  be  a  rough  dip  to  the  east,  but 
the  ore  is  so  irregularly  related  to  the  rocks  in  which  it  occurs  that 
the  general  inclination  can  not  be  given  with  any  degree  of  accuracy. 
The  ore  body  pitches  noticeably  toward  the  south,  so  that  at  the 
southernmost  portion  yet  opened  up  it  is  much  more  deeply  buried 
than  at  the  northern  end.  Alternating  with  the  lenses  of  ore,  and  also 
to  the  east  of  them,  are  many  dikes  of  white,  fine-grained  rhyolite 
which  have  passed  upward  between  the  lamination  planes  of  the  schists 
and  spread  out  in  broad,  flat  masses  in  the  nearly  horizontal  Cambrian 
strata  that  cap  the  hills  to  the  west,  north,  and  east  of  the  ore 
zone.  In  places  most  of  the  stratified  rock  into  which  these  porphyry 
masses  have  been  intruded  is  now  eroded,  and  on  the  summits  of  the 
divides  which  separate  the  open  cuts  little  is  left  but  the  thick  sheets 
of  rhyolite.  As  the  porphyry  bodies  which  form  these  sheets  were 
followed  downward  in  the  mine  workings  the}7  became  gradually  thinner 
md  fewer  in  number,  the  eruptive  rock  having  apparently  spread  out 
is  it  came  nearer  to  the  surface  and  formed  branching  masses  of  a  len- 
ticular form.  The  first  ore  which  was  mined  in  the  early  days  formed 
j^reat  irregular  lenses  included  almost  wholly  within  these  dikes  of 
porphyry,  but  as  it  was  followed  downward  it  seemed  to  diverge 
from  the  porphyry  bodies,  and  in  the  deeper  levels  of  the  mine  is 
ipparently  entirely  independent  of  them.     It  is  an   interesting  fact 
