852  PRE-CAMBRIAN    GEOLOGY   OF    NORTH    AMERICA. 
the  Coeur  d'Alene  district  (the  Belt  series)  extends  south  through  the 
St.  Joe  River  basin  with  all  the  formations  which  have  been  distin- 
guished in  the  Coeur  d'Alene  district.  The  rocks  are  intensely  folded, 
and  intruded  by  granite  and  diabase. 
Calkins,137  in  1906,  extends  the  observations  on  the  Algonkian 
rocks  of  the  Coeur  d'Alene  district  by  a  reconnaissance  northward 
through  the  Coeur  d'Alene,  Cabinet,  and  Loop  mountains  to  the  inter- 
national boundary,  and  eastward  to  the  vicinity  of  Ravalli  and 
Lothrop,  in  western  Montana.  In  both  directions  a  sequence  generally 
similar  to  that  in  the  Coeur  d'Alene  district  (see  pp.  850-851)  persists, 
but  in  the  eastern  part  of  the  zone  rocks  stratigraphically  higher 
occur,  whose  succession  was  not  satisfactorily  worked  out.  These 
later  rocks  include  some  limestones,  probably  Paleozoic.  The  sequence 
found  by  Walcott  (p.  862)  in  northwestern  Montana  is  almost  dupli- 
cated in  this  region,  though  with  variations  in  detail,  but  a  great 
thickness  of  rocks  stratigraphically  beneath  Walcott's  section  is 
found. 
The  western  boundary  of  the  great  zone  of  comparatively  unaltered 
sediments  coincides  roughly  in  northern  Idaho  with  a  topographic 
depression  extending  from  the  southern  end  of  Lake  Coeur  d'Alene 
northward  across  the  international  boundary  at  Porthill.  West  of 
this  valley  are  hills  carved  from  granites,  gneisses,  and  schists.  The 
schists  and  some  of  the  gneisses  are  in  all  probability  Archean.  The 
granites  are  partly  intrusive  in  the  Algonkian  as  well  as  the  Archean 
rocks.  The  actual  contact  of  Algonkian  and  Archean  rocks  was  not 
observed,  being  covered  by  basalt  and  alluvial  deposits,  but  there  are 
reasons  for  believing  it  to  be,  at  least  in  part,  a  fault  contact. 
Ransome  and  Calkins,137*1  in  1908,  discuss  the  geology  of  the 
Coeur  d'Alene  district  of  Idaho  and  of  the  adjacent  territory.  They 
cover  no  essential  features  not  already  covered  by  the  preceding  re- 
views b}^  Ransome  and  Calkins  (Nos.  134  and  137).  However, 
instead  of  regarding  the  ancient  schists  and  gneisses  west  of  the 
Purcell  trench  as  probably  Archean,  they  state :  "  It  is  by  no  means 
certain  that  the  rocks  immediately  west  of  the  zone  are  Archean  or 
that  they  are  a  part  of  the  ancient  floor  upon  which  the  Belt  sedi- 
ments were  laid  down." 
SUMMARY  OF  PRESENT  KNOWLEDGE. 
The  work  of  Lindgren,  Ransome,  and  Calkins,  summarized  in  the 
immediately  preceding  pages,  gives  the  present  state  of  knowledge  so 
clearly  that  further  summary  is  not  necessary.  A  striking  feature 
of  recent  developments  has  been  the  wide  extension  of  the  known  area 
of  the  Belt  sediments.  Eldridge's  account  is  vitiated  by  his  assump- 
tion of  an  Archean  age  for  the  great  Mesozoic  batholith  of  central 
Idaho. 
