THE   CORDILLERAS.  861 
degree  of  metamorphism  between  the  unaltered  Cambrian  above  and 
the  crystalline  complex  below. 
The  Belt  series,  described  as  the  Belt  terrane  by  Walcott,  covers  a 
large  part  of  the  region  adjacent  to  the  Missouri  River,  and  has  been 
traced  northward  by  Chapman;  there  seems  no  reason  to  doubt,  from 
the  lithological  similarity,  close  proximity,  and  stratigraphic  sequence, 
that  the  rocks  exposed  over  so  large  a  part  of  northwestern  Montana 
arc  identical  with  the  rocks  of  this  formation  exposed  in  the  Big  Belt 
and  Little  Belt  ranges  and  in  the  main  Front  Range  of  the  Rocky 
Mountain  area  of  the  Missouri  River.  The  series  embraces  several 
(list i net  formations  separated  by  at  least  two  well-marked  uncon- 
formities. In  a  general  way  it  may  be  described  as  a  series  of  argil- 
laceous beds  varying  into  quartzites  and  into  impure  limestones,  the 
lower  rocks  showing  tilting  before  the  deposition  of  the  later  beds. 
The  series  as  exposed  near  Neihart,  Mont.,  exhibits  a  cycle  of  deposi- 
tion, beginning  with  the  Neihart  quartzites  and  passing  through  the 
Chamberlin  shales,  Newland  limestones,  Greyson  shales,  and  Spokane 
quartzites;  beginning  again,  after  a  period  of  erosion,  with  the 
Empire  shales,  and  passing  through  the  Helena  limestones  and  the 
higher  beds  exposed  near  Marysville,  Mont.,  and  mapped  as  the 
Greenhorn  shales.  These  beds  form  the  central  core  not  only  of  the 
Big  Belt  and  Little  Belt  mountains,  which  are  anticlinal  uplifts,  but 
also  of  the  Big  Valley  at  Helena,  Mont.,  which  is  a  dome-shaped 
uplift  eroded  into  the  present  basin.  Followed  westward,  the  rocks 
are  found  to  be  covered  by  various  later  formations,  but  appear  at 
intervals  as  far  west  as  the  South  Fork  of  Bitterroot  River,  where 
they  rest  discordantly  upon  the  underlying  Archean  (?)  gneisses. 
Along  the  line  of  the  Northern  Pacific  Railway  the  rocks  seen  be- 
tween Townsend  and  Helena  are  covered  by  later  sediments,  across 
the  Continental  Divide,  but  the  Algonkian  rocks  reappear  in  Hell 
Gate  Canyon  and  over  the  broad  expanse  of  the  Missoula  Valley  near 
the  city  of  that  name,  continuing  westward,  with  occasional  patches 
of  Tertiary  and  of  Paleozoic  rocks,  to  the  State  line. 
Along  the  Great  Northern  Railway  these  Algonkian  rocks  of  the 
Belt  terrane  form  the  high  and  truly  alpine  peaks  lying  between 
Flathead  River  and  the  Front  Range.  These  Belt  rocks  extend  west- 
ward beyond  the  State  line,  forming  all  the  mountainous  country  and 
being  covered  in  the  valleys. 
The  region  between  the  Great  Northern  Railway  and  the  Canadian 
boundary  was  studied  by  Walcotf,  Weeks,  and  Weed  in  L894,  and  by 
Weed  in  181)8.  This  work,  which  formed  the  basis  for  the  geologic 
mapping  of  the  State,  seen  in  recent  maps  of  the  United  States, 
accords  in  general  with  the  conclusions  reached  by  Willis.  The  latter 
finds  the  Lewis  and  Livingston  ranges  of  the  northern  Rocky  Mountain 
front  of  Montana  to  consist  of  stratified  rocks  of  the  Belt  series,  the 
lowest  limestone  carrying  Beltina  danai  of  the  Greyson  shale.    The 
