GRANITE-BIMETALLIC    AND    CABLE    MINES,  MONTANA.  39 
the  vein  splits  at  many  places,  inclosing  horses  of  various  sizes,  and 
here  and  there  smaller  veins  make  off  from  the  principal  fissure.  Of 
these  the  "south  vein,"  which  branches  from  the  main  lode  near  the 
west  end  of  the  Bimetallic  mine,  is  the  most  important,  and  a  consid- 
erable portion  of  it  has  been  stoped  with  profit.  These  minor-  veins 
do  not  appear  to  cross  the  main  fissure,  but  join  it,  making  angles 
nearly  everywhere  of  less  than  25°,  which  point  toward  the  west.  So 
far  as  known,  all  the  minor  veins  lie  to  the  south  of  the  principal 
vein.     Fig.  2  is  a  sketch  plan  of  the  lode. 
In  the  eastern  portion  of  the  mine,  on  level  8,  the  vein  splits  into 
several  branches,  the  line  of  separation  being  nearly  horizontal. 
Below  this  level  two  of  these  branches  reunite;  others,  however, 
diverge  in  depth. 
The  vein  probably  filled  a  fault  fissure.  There  was  movement 
along  the  plane  of  the  vein  before  the  fissure  was  completely  filled 
and  also  after  it  was  filled,  but  the  amount  of  displacement  can  not 
be  estimated.  The  fissure  can  not  be  identified  with  faults  in  the  sedi- 
mentary rocks  beyond  the  contact  of  the  granite,  and  where  it  should 
Fig.  2.— Sketch  plan  cf  the  Granite-Bimetallic  lode. 
appear,  if  present,  these  rocks  are  not  noticeably  displaced.  Post- 
mineral  movement  is  shown  by  the  local  brecciation  of  the  vein  and 
by  the  slickenside  streaks  of  gouge  along  the  walls.  Much  of  this  gouge 
contains  angular  fragments  of  broken  quartz,  and  it  is  therefore,  in 
part,  at  least,  of  postmineral  age.  That  movement  occurred  while 
the  vein  was  being  filled  is  recorded  by  the  numerous  brecciated 
fragments  of  the  country  rock  included  in  the  veins. 
So  far  as  is  shown  by  present  exposures  in  the  mine  none  of  the  post- 
mineral  movement  has  taken  place  along  planes  forming  any  consid- 
erable angle  with  the  vein,  and  although  the  vein  is  exposed  on  many 
levels  for  nearly  a  mile  along  its  strike  at  no  place  could  cross  faulting 
be  detected. 
Primary  ore. — The  primary  ore,  which  is  found  in  the  lower  portions 
of  the  mine,  has  a  gangue  of  quartz  and  rhodochrosite  inclosing  a  large 
quantity  of  pyrite,  arsenopyrite,  tetrahedrite,  and  tennantite,  with 
some  galena  and  zinc  blende.  Sparingly  scattered  through  this  ore 
are  small  specks  of  pyrargyrite,  proustite,  and  here  and  there  realgar 
and  orpiment.  This  ore  carries  from  20  to  30  ounces  of  silver  and 
from  $1.50  to  S3  in  gold.     Commonly  broad  bands  of  quartz  alternate 
