THE   CORDILLERAS.  859 
about  10,700  feet  in  thickness.  The  formations  are,  in  downward 
sequence :  Kintla  argillite,  Sheppard  quartzite,  Siyeh  limestone,  Grin- 
nell  argillite,  Appekunny  argillite,  and  Altyn  limestone.  There  is 
apparent  conformity  throughout.  The  series  is  so  situated  with 
reference  to  other  rocks  that  no  lower  or  upper  stratigraphic  limit 
could  be  determined.  G.  M.  Dawson  classified  the  strata  as  Cam- 
brian, Carboniferous,  and  Triassic,  but  it  is  believed  that  he  mistook 
certain  local  overthrust  faults  for  unconformities  and  was  misled  by 
lithological  resemblances. 
Igneous  rocks  occur  sparingly  in  the  Algonkian  series.  An  intru- 
sive sheet  of  diorite  is  extensive  in  the  upper  limestone  formation, 
and  an  extrusive  flow  of  diabase  caps  it. 
The  Algonkian  strata  form  a  syncline  whose  axis  trends  west  of 
north.  Southwestern  dips  vary  from  5°  to  30°.  Northeastern  dips 
are  generally  30°  to  40°,  and  locally  approach  or  pass  vertically. 
Minor  flexures  within  the  syncline  are  very  broad  and  low.  The 
northeastern  limit  of  the  fold  is  an  eroded  margin ;  the  southwestern 
is  an  anticlinal  axis  whose  western  limb  is  in  part  eroded,  in  part 
thrown  down  by  a  normal  fault  along  North  Fork  Valley.  Syncline 
and  anticline  are  closely  related  to  valley  and  ridge,  respectively, 
and  this  relation  extends  to  heights  of  peaks. 
Along  its  eastern  margin  the  oldest  Algonkian  formation  rests 
upon  Cretaceous  rocks.  The  outcrop  of  this  abnormal  contact  is 
deeply  sinuous  throughout  the  stretch  from  St.  Mary  Lake  to  Water- 
ton  Lake.  The  structure  is  described  as  an  overthrust  fault,  on 
which  the  Algonkian  series  has  moved  northeastward  relatively  over 
the  Cretaceous  rocks.  The  displacement  on  the  thrust  surface  is  7 
miles  or  more,  and  the  vertical  throw  is  estimated  at  3,400  feet  or 
more.  The  thrust  surface  dips  from  0°  to  10°  SW.,  and  strikes  vari- 
ously from  N.  to  N.  G0°  W.  Thus  it  is  warped,  and  this  warping  is 
found  to  determine  the  general  outline  of  the  eastern  face  of  the 
Rocky  Mountains,  particularly  the  prominence  of  Chief  Mountain, 
and  the  relative  position  of  the  Lewis  Range,  en  echelon  to  the 
Livingston. 
Lindgren,  in  1904,  reports  on  a  reconnaissance  of  the  Bitterroot 
and  Clearwater  mountains  of  Montana  and  Idaho.  See  summary  in 
section  8,  Idaho,  page  850. 
Weed,150  in  1905,  summarizes  the  development  of  knowledge  con- 
cerning the  pre-Cainbrian  geology  of  Montana. 
The  investigations  of  the  llavden  Survey,  covering  the  Yellow- 
stone Park  and  the  extreme  southwestern  part  of  the  State,  have 
led  to  a  grouping  of  the  rocks  as  Arehean  and  Paleozoic,  the  oldest 
beds  of  the  latter  being  assigned  to  Silurian.  No  attempt  was  made 
by  the  investigators  of  this  Survey  to  differentiate  the  Algonkian 
rocks  from  the  crystalline  complex.     At  a   later  period    (188*2-83) 
