GRANITE-BIMETALLIC    AND    CABLE    MINES,    MONTANA.  35 
2  miles  of  the  Powell  mine.  Another  large  boss  of  granite,  not  con- 
nected on  the  surface  with  this  one;  occurs  jus!  west  of  Cable  and 
extends  northwestward  to  the  Southern  Cross  mine,  and  (hence  south- 
ward nearly  to  Silver  Lake.  An  area  much  larger  than  either  of 
of  these  overlaps  the  eastern  border  of  the  quadrangle,  its  western 
margin  extending  in  a  generally  north-south  direction  from  the  head 
of  Foster  Creek  to  the  head  of  Gold  Creek.  The  granite  of  this  mass 
forms  the  country  rock  of  the  Royal  mine. 
The  granite  prophyry  has  about  the  same  composition  as  the 
granite,  but  contains  more  quartz.  It  forms  several  small  masses  of 
which  the  areal  extent  is  insignificant  compared  with  that  of  the 
granite.  There  is  a  typical  occurrence  of  the  granite  porphyry  about 
a  mile  north  of  Philipsburg,  on  the  east  slope  of  Red  Hill. 
Where  the  sedimentary  rocks  are  in  contact  with  the  igneous 
intrusives,  the  limestones  and  shales  have  been  greatly  metamor- 
phosed. The  limestones  have  been  altered  to  marble,  which  in  many 
places  carries  more  or  less  tremolite  and  white  mica.  The  more  calca- 
reous shales  have  generally  been  changed  to  cryst  lline  aggregates, 
chiefly  of  garnet  and  pyroxene  accompanied  by  more  or  less  epidote, 
amphibole,  magnetite,  and  quartz,  while  others  have  been  changed 
to  rocks  rich  in  scapolite. 
DISTRIBUTION   AND   GENERAL  CHARACTER  OF  THE  ORE. 
The  most  important  mining  centers  are  those  near  Philipsburg, 
Cable,  Combination,  and  Princeton,  but  there  are  a  number  of  isolated 
mines  in  the  country  between  these  groups,  and  prospects  showing 
ore  are  widely  distributed  over  a  large  portion  of  the  quadrangle. 
They  are  most  numerous  in  the  intrusive  igneous  rocks  or  in  the  sedi- 
mentary rocks  near  their  contacts  with  the  intrusives. 
The  valuable  metals  of  the  ores  are  silver,  gold,  and  copper.  Some 
of  the  ores  carry  considerable  zinc  and  lead,  but  at  present  these 
metals  are  of  no  great  economic  importance  to  the  miner.  Magnetic 
iron  ore  is  mined  for  flux.  Some  manganese  oxide  has  been  shipped 
for  flux  and  to  a  less  extent  for  its  manganese  content. 
The  ore  deposits  occur  as  fissure  veins  cutting  both  the  granite  and 
sedimentary  rocks;  as  contact-metamorphic  replacement  deposits 
in  limestone  near  the  granite;  and  as  replacement  deposits  in  the 
sedimentary  rocks,  in  part  conforming  with  their  bedding  planes. 
From  the  standpoint  of  production  the  fissure  veins  are  the  most 
important.  In  the  granite  these  are  clear-cut  fissures  with  good  walls 
along  which  there  has  been  practically  no  replacement  of  the  country 
rock.  In  the  sedimentary  rocks  they  cut  across  the  bedding  at 
various  angles,  replacing  limestones  very  irregularly  and  making 
large  ore  shoots  at  favorable  places.  The  ore  bodies  of  the  Cable 
mine  are  the  only  known  replacement  deposits  of  contact-metamor- 
