72  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1906,  PART    I. 
Sheep  Creek,  a  tributary  to  John  Days  River,  where  the  most  rugge< 
topography  was  encountered,  there  is  a  change  in  the  elevation  o 
3,000  feet  in  less  than  half  a  mile.     The  area  is  traversed  from  wes  I 
of  north  to  east  of  south  by  two  parallel  mountain  ranges — Salt  Rive: 
Range  and  Wyoming  Range— and  is  bounded  on  the  northeast  b~\ 
the  Gros  Ventre  Mountains,  which  extend  in  an  east-southeast  anc 
west-northwest  direction.     Near  the  south  end  of  the  area  the  Sal' 
River  and  Wyoming  ranges  become  lower,  passing  into  Absaroks 
Ridge  and  Meridian  Fold,  respectively,  and  they  finally  lose  theii 
topographic  importance  south  of  the  area,  where  the  rocks  of  whicl . 
they  are  composed  are  deeply  buried  by  Tertiary  deposits.     Then 
are  numerous  minor  ridges  and  folds,  in  general  more  or  less  paralle 
to  the  main  ridges,  the  most   important  being  the  Hoback  Range 
which  connects  the  Wyoming  Range  and  the  Gros  Ventre  Mountains 
To  the  east  of  the  Wyoming  Range  is  the  great  plain-like  basin  ot 
Green  River,  whose  mean  elevation  is  about  7,500  feet.     The  entire 
ana,  except  a  narrow  strip  along  the  eastern  margin,  is  a  rugged  suc- 
cession of  mountain  peaks  and  ridges  cut  by  numerous  gorges  and! 
canyons  whose  walls  often  are  in  places  nearly  perpendicular.     Many 
of  the  hills,  valleys,  and  slopes  are  well  timbered  with  pine,  fir,  and 
spruce. 
The  drainage  belongs  to  three  great  systems— Snake  River,  whose 
waters  flow  into  the  Pacific;  Green  River,  whose  waters  flow  into  the1 
Gulf  of  California;  and  Bear  River,  whose  waters  enter  the  Greati 
Basin.  The  greater  part  of  the  drainage  area  is  almost  equally 
divided  between  Snake  and  Green  rivers,  with  a  margin  in  favor  of1 
Snake  River,  only  a  small  area  in  the  southwestern  portion  beings 
drained  by  the  tributaries  of  Bear  River. 
STRATIGRAPHY. 
The  survey  of  last  summer  was  carried  on  with  special  reference  to 
coal,  nearly  all  of  the  work  being  restricted  to  the  coal-bearing  forma- 
tions. Some  time  was  devoted  to  the  formations  contiguous  to  the 
coal-bearing  rocks,  and  sufficient  informal  ion  regarding  their  age  was 
obtained  i<>  correlate  them  with  similar  beds  in  other  parts  of  the 
country.  The  fossils  collected  were  studied  by  Dr.  T.  W.  Stanton  and 
indicate  that  the  formations  studied  and  mapped  in  the  field  (PI.  I) 
are  of  Jurassic  age  and  form  the  upper  portion  of  the  Jurassic  in  this 
area.a 
a  For  the  geologic  time  values  of  the  rocks  other  than  those  mapped  in  I'i.  T  see  Veatch,  A.  ('..  Coal 
and  oil  in  southern  Uinta  County,  Wyo. :  Bull.  U.  S,  Geol.  Survey  No.  285, 1906,  pp.  332-334;  or  the  report 
l>y  th    writer  on  coal  in  central  Uinta  County.  Wyo.    now  in  preparation. 
