COAL  IN  NORTHWESTERN  COLORADO  AND  NORTHEASTERN  UTAH.       291 
separate  in  the  fields  to  the  south  and  west  and  together  are  included 
in  the  middle  group.  The  upper  coal  group  is  probably  at  approxi- 
mately the  same  stratigraphic  position  in  the  two  fields,  although  no 
positive  evidence  can  be  offered  to  prove  this  point. 
STRUCTURE. 
General  outline. — The  sedimentary  strata  were  without  doubt 
originally  deposited  in  an  approximately  horizontal  position,  but  they 
have  later  been  tilted,  exposing  them  in  various  attitudes  and  result- 
ing in  much  irregularity  in  the  outline  of  their  outcrops.  This  dis- 
placement of  the  strata  and  subsequent  erosion  have  also  brought 
about  the  separation  of  some  fields  that  were  probably  once  continuous. 
It  also  seems  likely  that  some  of  the  deposits  were  laid  down  in  sepa- 
rated basins  and  so  may  never  have  been  continuous,  although 
apparently  separated  by  relatively  narrow  barriers. 
Major  structural  basins. — The  coal  fields  of  northwestern  Colorado 
lie  in  two  major  structural  basins,  surrounded  by  areas  of  older  rock 
formations  that  are  relatively  uplifted.  The  northern  of  these  two  is 
known  as  the  Green  River  Basin,  which  extends  into  Colorado  as  far 
as  the  upper  valley  of  Yampa  River.  The  Yampa  coal  field  occupies 
a  part  of  this  basin.  The  southern  of  the  two  major  structural  basins 
is  known  as  the  Uinta  Basin,  and  the  Colorado  portion  is  also  termed 
the  Grand  River  Basin.  This  is  separated  from  the  Green  River 
Basin  in  the  Yampa  coal  field  by  the  anticlinal  axis  of  the  Uinta 
Mountains,  which  extends  eastward  from  that  range  in  Utah  through 
Axial  Basin  and  joins  with  the  uplifts  of  the  White  River  Plateau  and 
the  western  spurs  of  the  Rocky  Mountain  system.  The  Grand  Hog- 
back, Danforth  Hills,  and  lower  White  River  coal  fields  are  a  part  of 
the  Uinta  or  Grand  River  Basin  of  Colorado. 
That  part  of  the  Uinta  Basin  which  was  examined  during  the  last 
year  extends  from  the  Danforth  Hills  westward  to  and  beyond  the 
Colorado-Utah  State  line.  The  Green  River  Basin  includes  the  west- 
ern part  of  the  Yampa  field  and  the  Henrys  Fork  field  of  Utah  and 
Wyoming,  which  are  also  described  in  this  report. 
Minor  folds. — Details  of  the  minor  structural  features  of  these  two 
major  basins  are  brought  out  by  the  structure  symbols  on  the  accom- 
panying maps.  It  is  evident  from  the  sinuosity  of  outcrop  of  the 
tilted  strata  that  their  positions  at  the  surface  are  the  direct  result  of 
the  depth  to  which  erosion  has  progressed  into  the  structurally  irregu- 
lar underlying  beds. 
Larger  folds  of  the  White  River  and  Vernal  fields. — The  structural 
irregularit}r  of  the  northeastern  margin  of  the  Grand  River  Basin  is 
pronounced,  forming  a  series  of  domes  and  anticlines  and  correlative 
synclinal  folds.  As  several  of  these  features  are  referred  to  in  the 
following  pages  in  relation  to  their  influence  on  the  trend  of  the  coal- 
bearing  rocks,  they  will  be  briefly  reviewed  here. 
