THE    CORDILLERAS.  839 
tinct  series  of  sedimentary  rocks  is  exposed  in  this  district,  and  it  is 
therefore  not  easy  to  refer  any  of  the  eruptives  to  one  division  or  the 
other  of  the  pre-Cambrian.  On  the  ground  of  alteration  or  lithologic 
features,  however,  the  oldest  schistose  rocks  and  the  gneissic  granites 
by  which  they  are  invaded  may  be  tentatively  referred  to  the  Archean. 
The  granites  and  other  intrusives  may  well  be  of  distinctly  later  age. 
BIGHORN    MOUNTAINS. 
Hayden,111  in  18G1,  states  that  red  feldspathic  granites,  with  meta- 
morphic  slates  and  schists,  constitute  the  nucleus  of  the  Bighorn 
Mountains.  As  these  are  surrounded  by  strata  as  recent  as  the  Cre- 
taceous, this  uplift  is  subsequent  to  this  time. 
Hayden,112  in  1868,  states  that  the  unconformity  between  the  crys- 
talline and  unmetamorphosed  strata  at  the  Bighorn  Mountains  is 
very  apparent. 
Carpenter,113  in  1878,  describes  the  Bighorn  Range  as  composed  at 
the  base  of  thick  masses  of  Primordial  sandstone  resembling  the  Pots- 
dam sandstone  of  the  Black  Hills,  although  the  heat  coeval  with  the 
upheaval  of  the  mountains  has  probably  obliterated  the  fossils  which 
are  so  abundant  in  that  region.  The  sandstone  rests  unconformably 
against  the  Archean,  is  inclined  from  the  flanks,  is  folded,  and  in 
many  places  is  upturned  as  in  the  Black  Hills  and  Colorado  Moun- 
tains. Above  the  sandstone  is  a  limestone  containing  numerous  casts 
of  Spirifer  cameratus.  The  crystalline  rocks  appear  at  an  elevation 
of  about  9,000  feet  and  compose  the  higher  parts  of  the  range.  Near 
the  summit  fine-grained  grayish  granite  predominates,  occasionally 
varied  by. patches  of  mica  schist.  The  Owl  Creek  Mountains  are 
composed  of  porphyritic  granite  rich  in  feldspar,  which  gives  place 
at  higher  elevations  to  a  gneissoid  granite.  They  connect  the  south- 
ern end  of  the  Bighorn  Mountains  with  the  northern  part  of  the 
Wind  River  Range. 
Darton,114  in  1900,  describes  the  granite  in  the  northern  central  por- 
tion of  the  Bighorn  uplift  as  occupying  an  area  of  about  1,200  square 
miles.  Its  form  is  elliptical,  extending  about  (>2  miles  north-north-; 
west  and  south-southeast  and  having  its  greatest  breadth,  about  30 
miles,  near  Cloud  Peak.  To  the  north  and  south,  as  well  as  on  the 
sides,  the  granite  passes  beneath  the  sandstone  at  the  base  of  the  Dead- 
wood  (Cambrian)  formation.  To  the  northwest  it  reappears  again 
in  the  center  of  the  uplift  in  an  irregular,  narrow  zone  extending 
from  the  foot  of  Hunt  Mountain  pasi  Bald  Mountain  nearly  to  the 
Montana  line.  A  small  exposure  appears  in  the  crest  of  the  anticline 
in  the  middle  of  the  range  near  Point  Lookout  in  Montana  and  in 
the  anticline  of  Dry  Fork  Ridge  where  it  is  crossed  by  the  canyon  of 
Little  Horn  River. 
