840  PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
In  the  southern  portion  of  the  Bighorn  Mountains  the  granites 
appear  along  a  fault  northeast  of  Bigtrails  and  in  an  area  of  about 
100  square  miles  near  the  southern  end  of  the  mountains,  due  to  local 
increase  in  the  amount  of  uplift.  The  latter  area  is  in  an  elevated 
region  along  the  upper  portions  of  Badwater,  Clear,  Lonetree,  Lost, 
Deep,  and  Trout  creeks  and  the  west  fork  of  Powder  River. 
In  the  Bridger  uplift  the  pre-Cambrian  rocks  occupy  an  area  of 
about  65  square  miles,  constituting  the  high  central  summits  and  the 
long,  rugged  southern  slopes.  They  also  are  exposed  by  deep  erosion 
in  the  upper  canyon  of  Bighorn  River  at  the  west  end  of  the  Bridger 
Range. 
RATTLESNAKE     (  BRIDGER  )     MOUNTAINS. 
Engelmann,108  in  1876,  states  that  granites  and  granitic  syenites 
which  are  regarded  as  igneous  rocks  form  a  large  part  of  the  Rattle- 
snake Mountains. 
SWEETWATER    AND    ADJACENT    MOUNTAINS. 
Ball,115  in  1835,  notes  granitic  rocks  along  the  Sweetwater. 
Hayden,112  in  1868,  mentions  granites  and  syenites  as  occurring  in 
the  Sweetwater  Mountains. 
Engelmann,108  in  1876,  places  the  crystalline  schists  between  the 
three  crossings  of  the  Sweetwater  and  South  Pass,  and  those  on  the 
eastern  slope  of  South  Pass,  as  metamorphics.  They  include  gneiss, 
mica  schist,  argillaceous  and  siliceous  schist,  and  hornblendic  rocks. 
Endlich,109  in  1879,  describes  Prozoic  and  metamorphic  rocks  in 
the  Sweetwater  Valley  and  adjacent  regions.  In  ■  the  Sweetwater 
Hills  are  Prozoic  rocks,  coarse-grained,  structureless  granites,  like 
those  west  of  the  Wind  River  Mountains,  which  are  cut  by  basaltic 
dikes  that  never  penetrate  the  overhung  younger  rocks.  The  meta- 
morphic granite  of  the  Sweetwater  and  Seminole  hills  is  regarded  as 
a  continuation  of  the  youngest  granite  of  the  eastern  slope  of  the 
Wind  River  Range.  East  of  Elkhorn  Gap  is  found  a  series  of  folded 
sedimentary  beds,  upon  both  sides  of  which  is  granite  apparently  of 
the  same  character.  The  northern  and  northwestern  portions  of  the 
granite  hills,  instead  of  being  composed  of  Prozoic  granite,  are  formed 
of  stratified  granites  with  hornblende  schists.  Toward  the  eastern 
termination  the  stratification  is  so  apparent  that  from  a  short  dis- 
tance the  rocks  were  supposed  to  be  unchanged  sedimentary  ones,  and 
the  suspicion  presented  itself  that  a  portion  of  these  are  metamor- 
phosed Silurian  beds.  The  Potsdam  quartzite  with  an  easterly  dip 
is  found  to  rest  upon  the  schists,  and  at  the  western  end  of  the  Sweet- 
water hills  sub-Carboniferous  dolomites  rest  directly  upon  the  Pro- 
zoic granites.  In  the  Sweetwater  region  the  younger  metamorphics 
occupy   a   more   conspicuous   position   than   the   older  metalliferous 
