132  CONTRIBUTIONS    TO   ECONOMIC    GEOLOGY,  1905. 
Lindgren  a  has  mapped  a  great  batholith  of  granite,  covering  an  area  as  great  as  5,000  squai 
miles,  which  forms  the  whole  mass  of  the  Bitterroot  Range  south  of  this  district.  Th 
increasing  schistosity  of  these  rocks  to  the  south  is  probably  to  be  accounted  for  by  th 
proximity  of  this  great  intrusive  mass,  the  northern  limits  of  which  have  not  been  detei 
mined. 
Igneous  rocks  occur  as  dikes,  sills,  and  stocks  intruded  in  these  sediments  at  variou 
places.  They  fall  into  the  two  groups,  diabases  and  granites,  which  are  of  about  equa 
importance  as  regards  distribution.  Diabases  occur  as  dikes  and  sills  cutting  the  sedi 
mentary  rocks,  more  commonly  in  the  northeastern  part  of  the  district,  though  som 
smaller  dikes  occur  in  the  southern  and  western  parts.  Some  of  the  larger  areas  of  diabas 
are  shown  on  the  map,  PI.  IV.  The  largest  mass  observed  is  cut  by  St.  Joe  River  at  th 
upper  box  canyon.  Several  smaller  masses,  which  were  observed  near  the  Coeur  d'Alens 
divide,  between  Stevens  Peak  and  Wards  Peak,  appear  sometimes  as  dikes,  hut  nion 
often  as  sills.  As  far  as  observed  their  outcrops  either  extend  north  and  south  or  north 
west  and  southeast.  This  rock  has  a  granitic  texture;  it  consists  essentially  of  plagioclasi 
and  augite,  with  quartz  as  an  accessory  mineral  and  magnetite  and  ilmenite  as  import  an 
constituents. 
The  granites  are  more  local  in  their  distribution  and  have  been  observed  by  the  write 
only  along  St.  Joe  River  in  T.  45  N.,  Rs.  2  and  3  E.  Mr.  Thomas  Cooney,  a  mining  engi 
neer  employed  by  the  Northern  Pacific  Railway,  reports  that  they  also  occur  in  the  E.  j 
of  T.  43  N.,  R.  5  E.  Although  examined  only  cursorily  these  rocks  seem  to  he  intrusiv< 
masses  in  quartzites  of  the  Coeur  d'Alene  series,  which  near  the  contacts  are  changed  te 
hornfels.  They  consist  essentially  of  quartz,  orthoclase  and  plagioclase  feldspars,  anc. 
biotite.  All  of  the  minerals  show  more  or  less  crushing  and  distortion,  indicating  dis- 
turbances  subsequent  to  the  intrusion  of  the  granite. 
In  the  Coeur  d'Alene  district  two  types  of  igneous  rock  have  been  recognized. 6  Tin 
most  important  of  these  is  a  coarse-grained  syenite,  which  has  a  tendency  toward  porphy- 
ritic  development  of  the  dominant  feldspar.  These  rocks  occur  as  irregular  stocks  anc 
dikes  and  find  their  greatest  development  in  the  vicinity  of  the  most,  productive  group  oft 
lead  and  silver  mines,  though  no  causal  relation  between  them  and  the  ore  deposits  hai 
been  established.  The  rocks  of  the  second  type  occur  in  dark-colored  dikes  near  the 
southern  edge  of  the  Coeur  d'Alene  area,  and  are  of  minor  importance  in  that  district.  They 
are  diabases  consisting  essentially  of  plagioclase  feldspar  and  augite,  with  quartz  as  am 
important  accessory.  The  granites  of  the  great  batholith  of  the  Bitterroot  Range  are 
described  by  Lindgren  c  as  quartz-monzonite,  a  variety  of  gianite  containing  both  ortho- 
clase and  plagioclase  feldspars. 
By  a  comparison  of  the  igneous  rocks  found  in  this  region  with  (hose  described  from 
the  Cceur  d'Alene  district  on  the  north  and  the  Bitterroot  Range  on  the  south,  it  seems^ 
evident  that  the  granites  of  the  St.  Joe  Valley  differ-  considerably  from  the  syenites  of 
the  Coeur  d'Alene  district  and  are  more  nearly  like  the  quartz-monzonites  of  the  Bitter- 
root batholith.  The  diabases  are  probably  of  almost  identical  character  with  those  from 
the  Coeur  d'Alene  district,  but  they  occur  in  much  larger  masses  and  are  therefore  of  more 
importance. 
ECONOMIC   GEOLOGY. 
Lead-silver,  copper,  and  gold  ores  have  been  found,  and  some  attempts  to  develop 
valuable  deposits  of  these  minerals  have  been  made  at  many  places  within  the  area  under 
investigation,  but  most  of  the  prospecting  and  development  have  been  unsystematic  and 
desultory.  The  region  is  difficult  of  access  and  devoid  of  means  of  transportation.  Most 
of  the.  reported  mineral  discoveries  and  prospects  are  situated  along  the  northern  and 
eastern  boundaries,  which  are  fairly  accessible  from  the  line  of  the  Northern  Pacific  Rail- 
o  Lindgren,  Walclemar,  A  geological  reconnaissance  across  the  Bitterroot  Range  and  Clearwat« 
Mountains  in  Montana  and  Idaho:   Prof.  Paper  U.  S.  Geol.  Survey  No.  27,  1904,  p.  18. 
l>  Ransome,  F.  L.,  op.  cit.,  p.  282. 
c  Lingren,  Waldemar,  op.  cit.,  p.  18. 
