THE    CORDILLERAS.  849 
sedimentary  rocks  were  rendered  crystalline.  A  parallel  schistose 
structure  was  produced  both  in  the  sedimentary  rocks  and  in  the 
Archean.  On  the  west  side,  however,  where  the  shearing  was  less 
intense,  the  clastic  character  of  the  series  is  still  discoverable. 
If  the  above  interpretation  is  correct  the  Medicine  Bow  Range 
affords  another  locality  in  which  there  is  apparently  complete  grada- 
tion between  a  sedimentary  and  a  crystalline  series.  If  the  facts  of 
the  gradation  were  taken  by  themselves  they  would  be  explained  by 
some  geologists  as  proof  of  the  intrusive  character  of  the  gneissoid 
granite  in  the  Algonkian;  by  others  they  would  be  taken  as  evidence 
of  the  progressive  metamorphism  of  the  Algonkian  rocks  into  the 
Archean.  But  it  has  been  seen  that  the  more  probable  explanation 
is  that  the  two  are  really  unconformable,  but  have  been  mashed  and 
metamorphosed  until  there  is  gradation  between  the  two  and  conform- 
ity of  secondary  structures. 
In  the  Bighorn  Mountains  the  pre-Cambrian  rocks  are  granites, 
and  as  no  pre-Cambrian  fragmental  rocks  appear,  it  has  not  been 
possible  to  ascertain  whether  the  granites  are  Archean  or  Algonkian. 
SECTION  8.     IDAHO. « 
SUMMARY    OF   LITERATURE. 
Eldridge,131  in  1895,  gives  an  account  of  a  geological  reconnaissance 
across  Idaho,  on  a  northeast  line  through  Boise  and  Salmon  City. 
Rocks  are  found  which  are  provisionally  referred  to  the  Archean  and 
Algonkian.  To  the  Archean  are  referred  granite  and  gneiss,  which 
have  their  greatest  development  in  the  mountains  of  the  western  part 
of  the  State,  but  which  are  also  widely  exposed  elsewhere.  In  places 
in  the  granite  and  gneiss  are  included  bands  of  calcareo-micaceous 
or  quartzitic  slates,  and  in  these  cases  the  reference  of  the  rocks 
to  the  Archean,  instead  of  the  Algonkian,  is  questionable.  To  the 
Algonkian  is  provisionally  assigned  the  great  series  of  micaceous, 
quartzitic,  and  chloritic  schists  of  eastern  Idaho.  The  reference  is 
based  merely  upon  lithological  character  and  the  resemblance  to  other 
beds  in  the  Cordilleras  which  have  already  been  so  assigned.  The 
Algonkian  series  in  areas  of  strong  development  has  a  probable  thick- 
ness of  8,000  to  4.000  feet.  It  is  believed  to  be  unconformable  with 
the  underlying  granite. 
Ransome,132  in  1901,  in  the  course  of  a  reconnaissance  from 
Kootenai  River  westward  along  the  international  boundary,  found 
that  the  prevailing  rock  along  the  Kootenai  from  Bonners  Ferry 
northward  to  Porthill  is  a  coarse,  porphyritic,  granitic  gneiss.    This 
"Sim-  also  summary  of  literature  on  southwestern  Montana  and  adjacent  parts  of 
Idaho,  section  0,  pp.  853-863! 
55721— Bull.  360—09 54 
