THE   CORDILLERAS.  863 
northern  part  of  the  area.  Toward  the  south  the  overlying  greenish 
Empire  shale  is  found,  transitional  into  the  bull'  to  blue  impure  Hel- 
ena limestone.  Finally,  in  the  extreme  southeast  corner,  the  deep-red 
Marsh  shale  overlies  the  Helena  limestone,  constituting  the  uppermost 
formation  of  the  Belt  group  and  lying  unconformably  beneath  the 
middle  Cambrian  quartzite.  The  latter,  however,  outcrops  beyond 
the  limits  of  the  district,  and  therefore  is  not  shown  on  the  map. 
Intrusive  into  the  Belt  group  is  a  great  batholith  of  granite  2J  miles 
long  and  of  variable  width,  which  occupies  the  center  of  the  district 
and  around  the  borders  of  which  is  concentrated  the  mineral  wealth 
that  has  given  the  district  prominence  as  a  mining  region.  One  set 
of  dikes  and  sheets — the  microdiorite  of  Bald  Butte — is  of  earlier  ori- 
gin than  the  invasion  of  the  batholith  and  the  formation  of  the  zone 
of  contact  metamorphism.  Another  set  of  dikes — the  Belmont  cliorite 
porphyry — is  closely  connected  with  the  batholithic  invasion  in  point 
of  time,  but  whether  immediately  before  or  after  it  is  not  settled. 
The  batholith  itself  possesses  granitic  dikes  and  sheets  as  outliers, 
mostly  of  more  acidic  composition.  Later  than  the  batholith  are 
pegmatites  cutting  the  granite,  the  Drumlummon  porphyry,  and  a 
few  rare  basic  dikes. 
Walcott,152*  in  1908,  examined  the  area  of  pre-Cambrian  rocks 
bounded  by  the  North  Fork  of  Flathead  River  on  the  west,  the  great 
east-west  fault  between  the  Carboniferous  and  pre-Cambrian  south  of 
Crows  Nest  Pass,  British  Columbia,  on  the  north,  and  the  Cretaceous 
of  the  plains  to  the  east.  South  of  the  international  boundary  he 
made  a  reconnaissance  for  nearly  200  miles  in  order  to  correlate  the 
pre-Cambrian  formations  that  he  studied  in  1905  with  those  described 
by  Willis  and  Daly. 
The  most  important  result  of  the  work  was  the  proving  that  the 
Siyeh  limestone  of  Willis  is  identical  with  the  Holland  and  Black- 
foot  limestones  of  Walcott.  This  carries  with  it  the  correlation  of  the 
Helena  limestone  with  the  Siyeh.  Walcott  also  found  the  upper  por- 
tion of  the  lower  Cambrian  resting  unconformably  upon  the  Camp 
Creek  series  (Walcott)  several  thousand  feet  above  the  Siyeh  lime- 
stone. This  controverts  Daly's  theory  that  the  Siyeh  limestone  is  of 
Cambrian  or  Ordovician  age. 
SUMMARY  OF  PRESENT  KNOWLEDGE. 
The  preceding  summary  of  the  pre-Cambrian  geology  of  Montana 
by  Weed  is  largely  historical  in  treatment.  For  this  reason  there  is 
added  here  only  a  brief  general  statement  of  the  essential  points  in 
the  present  knowledge  of  the  pre-Cambrian. 
In  southwestern  Montana  the  predominant  rocks  of  the  pre-Cam- 
brian are  granites,  gneisses,  and  schists.  The  gneissoid  granite  of 
this  system  is  magnificently  exposed  in  the  canyon  of  the  Madison. 
