862  PRE-CAMBRIAN    GEOLOGY   OF    NORTH    AMERICA. 
rocks  form  a  north-south  syncline,  with  gentle  southwest  and  steeper 
(30°-40°)  northeast  dips,  and  broad  minor  flexures,  mountain  and 
valley  agreeing  with  saddle  and  syncline.  The  rocks  are  overthrust 
upon  Cretaceous  shales  from  St.  Mary  to  Waterton  Lake  on  the 
boundary,  the  thrust  having  a  low  dip  (0°-10°)  southwest,  a  side 
shove  of  7  miles,  and  a  vertical  throw  of  3,400  or  more  feet.  Igneous 
rocks  are  rare,  consisting  of  a  dioritic  sheet  in  the  upper  limestone 
and  an  extrusive  flow  of  diabase  capping  it. 
Walcott,151  in  1906,  discusses  the  Algonkian  formations  of  north- 
western Montana  and  their  correlation  with  those  of  northern  Idaho 
and  southern  British  Columbia.  Sedimentary  beds,  correlative  with 
the  Belt  terrane  of  the  Belt  Mountains,  extend  north  of  Helena  along 
the  Rocky  Mountains  to  the  boundary  area,  where  they  have  been 
described  by  Willis.  Westward  Walcott  has  followed  them  as  far  as 
Ravalli,  in  northwestern  Montana,  and  they  have  been  found  from 
here  to  the  Coeur  d'Alene  district  in  western  Idaho  by  Calkins  and 
Ransome.  A  similar  series  is  found  by  Daly  in  the  section  along  the 
international  boundary  between  Kootenay  River  and  the  eastern  edge 
of  the  Tobacco  Plains.  The  Belt  series  is  correlated  also  with  most 
of  Dawson's  Adams  Lake  (Selkirk)  series  and  all  of  his  Nisconlith 
series.  In  general  fine-grained  rocks,  including  limestones,  shales, 
and  sandstones,  predominate  to  the  southeast,  in  the  Belt  Mountains, 
the  Rocky  Mountain  front,  and  westward  to  Idaho.  Coarse  conglom- 
erate, grits,  and  sandstones  predominate  to  the  northwest,  especially 
west  of  Kootenai  Valley.  The  great  source  of  the  sediments  must 
have  been  to  the  west  and  northwest  of  the  Kootenai  Valley.  The 
eastern  shore  of  the  shallow  sea  afforded  very  little  coarse  material. 
The  only  place  at  which  the  base  of  the  Belt  series  is  seen  is  in  the 
Belt  Mountains,  where  it  comes  in  contact  with  the  Archean.  The 
Belt  series,  which  is  extended  broadly  to  cover  all  the  rocks  in  these 
several  sections,  is  overlain  unconformable  by  Middle  Cambrian 
sediments  for  all  of  the  area  except  the  Lewis  Range  and  the  Coeur 
d'Alene  and  Kootenai  districts.  The  Bow  River  series,  described 
by  McConnell,  seems  to  be  Lower  Cambrian  and  to  represent,  in  part 
at  least,  the  erosion  interval  between  the  Algonkian  and  the  Lower 
Cambrian. 
Calkins,  in  1906,  discusses  reconnaissance  observations  in  northern 
Idaho  and  northwestern  Montana.  See  summary  in  section  8, 
page  852. 
Barrell,152  in  1907,  maps  and  describes  the  geology  of  the  Marys- 
ville  mining  district,  Montana.  All  of  the  sedimentary  rocks  belong 
to  the  Belt  group  of  the  Algonkian,  which  in  this  area  includes  five 
formations  grading  into  one  another,  including,  in  order  of  age: 
Sandstones  and  shales  of  the  Greyson  formation  and  the  red  shales 
and  sandstones  of  the  Spokane  formation,  both  occurring  in  the 
