ECONOMIC  FEATURES  OF  NORTHERN   IDAHO  AND 
NORTHWESTERN  MONTANA. 
By  D.  F.  MacDonald. 
INTRODUCTION. 
After  the  detailed  study  of  the  Coeur  d'Alene  mining  district  by  Messrs.  Ransomca  and 
Calkins  in  1903  and  1904,  there  remained  an  extent  of  country  stretching  a  hundred  miles 
to  the  north,  the  geology  of  which  was  comparatively  little  known.  In  order  to  investigate 
this  field  and  correlate  its  rocks  with  the  Cceur  d'Alene  sediments  and  the  known  rocks 
along  the  boundary  and  in  northwestern  Montana,  a  geological  reconnaissance  was  ordered. 
This  examination  was  undertaken  by  Mr.  F.  C.  Calkins,  assisted  by  the  writer,  in  July,  1905. 
A  good  idea  of  the  location  and  extent  of  country  covered  can  be  had  by  following  the 
route  taken  as  it  is  traced  on  the  accompanying  map  (fig.  2).  Obviously  only  the  main  geo- 
logical features  could  be  observed  in  passing  over  so  large  an  area  within  the  time  allotted. 
An  endeavor  was  made,  however,  to  visit  mines  and  prospects  wherever  possible,  although 
economic  investigation  was  merely  incidental. 
PHYSICAL   FEATURES. 
The  region  investigated  is  a  great  succession  of  mountain  peaks  and  ridges  having  an 
average  altitude  of  5,000  to  6,000  feet.  A  wide  valley  forms  its  western  boundary  and  two 
large  rivers,  Clark  Fork  of  the  Columbia  and  the  Kootenai,  have  trisected  this  extensive 
mountain  region  into  three  northwest-southeast  mountain  systems.  A  glance  at  the 
accompanying  map  will  show  the  location  of  these  systems,  which  are  the  Coeur  dAlene 
Mountains  on  the  south,  the  Cabinet  Range  in  the  center,  and  the  Loop  Mountains  on  the 
north,  the  latter  being  subdivided  into  the  Mooyie,  Yaak,  and  Purcell  ranges. 
The  Cabinet  Mountains  is  the  only  one  of  the  ranges  in  which  rugged  saw-tooth  ridges, 
at  some  points  over  8,000  feet  high,  occur.  Here  on  the  north  slopes  are  still  found  a  few 
small  ice  fields,  remnants  of  a  once  extensive  system  of  alpine  glaciers. 
GEOLOGY. 
INTRODUCTORY   STATEMENT. 
Algonkian  rocks  prevail  over  a  large  area  in  northern  Idaho,  northwestern  Montana,  and 
southern  British  Columbia.  The  best  known  Algonkian  rocks  of  this  particular  area  are 
the  mineral-bearing  formations  of  the  "Coeur  d'Alene  series,"  which  forms  perhaps  the  major 
portion  of  the  Algonkian  province.  Within  this  area  were  also  found  some  limy  sediments 
of  doubtful  age. 
Ripple  marks  and  sun  cracks  show  that  these  rocks  were  laid  down  in  shallow  water. 
No  unconformity  was  seen  throughout  the  20,000  feet  of  their  thickness. 
Bounding  this  great  Algonkian  rock  province  on  the  west  are  Archean  schists  and  gneisses. 
On  the  east  are  the  middle  Cambrian,  Carboniferous,  and  Upper  Cretaceous  sediments  of 
western-central  Montana.  The  southern  boundary  is  formed  by  the  extensive  body  of  intru- 
sive rocks  of  central  and  southern  Idaho.  Toward  the  southwest  lie  the  great  lava  beds  of 
the  Columbia  plain. 
aliansonu',  F.  L.,  Bull.  U.  S.  Geol.  Survey  No.  260,  1904,  pp.  274-303. 
