324         CONTRIBUTIONS   TO   ECONOMIC   GEOLOGY,    1907,    PART   II. 
STRUCTURE. 
The  Grand  Mesa  coal  field  is  a  part  of  the  southern  rim  of  the 
Uinta  coal  basin  of  northwestern  Colorado  and  eastern  Utah.  The 
rocks,  inclining  toward  the  lowest  part  of  the  basin,  dip  in  a  general 
northeasterly  direction  except  near  the  east  end  of  the  field,  where 
the  'dip  is  more  to  the  north  or  northwest.  The  degree  of  dip  varies 
locally,  but  the  average  is  about  2h°.  However,  it  is  usually  greater 
than  this  near  the  outcrop,  owing  to  local  upturning  of  the  exposed 
edges  of  the  strata.  Near  the  east  end  of  the  field  the  structure  has 
been  complicated  by  the  intrusion  of  great  masses  of  igneous  rock 
and  the  formation  of  laccolithic  mountains,  but  within  the  area 
mapped  the  coal  has  not  been  greatly  disturbed.  A  few  small  faults 
have  been  encountered  in  the  mines,  but  so  far  as  reported  nowhere 
has  the  mining  been  seriously  hindered  by  any  disturbed  condition  of 
the  beds.  The  only  displacements  of  consequence  that  have  been 
noted  are  at  the  mouth  of  Black  Canyon  of  the  Gunnison  east  of  Delta 
and  in  the  vicinity  of  Surface  Creek.  At  the  mouth  of  Black  Canyon 
a  local  fold  has  been  formed  and  for  a  short  distance  the  sedimentary 
rocks  are  vertical.  The  coal-bearing  beds  east  of  Surface  Creek  are 
apparently  about  400  feet  higher  than  the  same  beds  2  miles  farther 
west.  The  intervening  space  is  covered  with  stream  deposits  and  it 
could  not  be  ascertained  whether  the  displacement  is  due  to  faulting 
or  to  warping.  However,  the  disturbance  is  in  a  locality  where  the 
least  amount  of  coal  occurs  and  apparently  has  no  important  bearing 
on  the  development  of  the  field. 
THE  COAL. 
GEOGRAPHIC  DISTRIBUTION. 
Aside  from  the  coal  at  the  base  of  the  Mancos  formation,  which  is 
not  of  commercial  value,  the  outcrop  of  the  coal  in  the  Grand  Mesa 
field  forms  a  long,  sinuous  band  in  the  western  and  southern  slopes 
of  the  mesa,-  and  can  be  best  described  by  districts,  as  follows :  (1 )  The 
Gunnison  district  lies  between  Grand' Junction  and  Delta  and  includes 
only  the  Mancos  coal;  (2)  the  Palisades  district  extends  from  Grand 
River  to  the  southern  point  of  Grand  Mesa;  (3)  the  Rollins  district 
extends  from  the  southern  point  of  the  mesa  eastward  to  Paonia, 
and  (4)  the  Somerset  district  includes  the  coal  east  of  Paonia. 
GUNNISON   DISTRICT. 
The  coal  of  the  Gunnison  district  occurs  in  the  zone  of  carbonaceous 
shale  at  the  base  of  the  Mancos  formation  and  outcrops  in  the  bluffs 
north  of  Gunnison  River.  It  was  formerly  mined  to  some  extent 
near  Grand  Junction  (No.  la),  but  the  openings  are  now  abandoned. 
It  is  still  mined  for  local  use  in  Wells  Gulch  (No.  5),  and  has  been 
a  Numbers  refer  to  coal  sections  and  locations  on  PI.  XXI. 
