GRAND    MESA   COAL   FIELD,    COLORADO.  323 
known  and  is  separable  into  three  members  of  economic  importana 
These  will  be  fully  described  in  the  detailed  report  on  this  region, 
which  is  practically  completed  and  will  soon  appear  as  a  bulletin  of 
the  United  States  Geological  Survey. 
The  low^er  member  is  a  white,  massive,  cliff-making  sandstone  that 
is  persistent  throughout  the  field  and  marks  the  base  of  the  coal- 
bearing  rocks.  The  name  Rollins  sandstone,  derived  from  the 
Rollins  mine,  north  of  Delta,  where  the  sandstone  forms  a  conspicuous 
cliff,  has  been  adopted  for  this  member. 
Above  this  basal  sandstone  at  either  end  of  the  field  is  a  coal-bearing 
member  that  consists  mainly  of  shale  and  beds  of  relatively  hard  and 
high-grade  coal  and  contains  shells  of  marine  and  brackish-water 
invertebrates.  The  name  Bowie  shale,  from  Bowie,  a  mining  town 
on  the  North  Fork  of  the  Gunnison,  where  the  shale  is  typically 
exposed,  has  been  adopted  for  this  member. 
The  third  member  of  economic  importance  is  the  Paonia  shale,  so 
named  from  Paonia,  a  town  on  the  North  Fork  of  the  Gunnison.  It 
consists  principally  of  shale  with  beds  of  relatively  soft  and  low-grade 
coal  and  is  characterized  by  plant  remains  and  shells  of  fresh-water 
mollusks.  The  Paonia  shale  member  rests  upon  the  Bowie  shale 
member  with  apparent  conformity  at  the  east  end  of  the  field,  but 
unconformably  upon  the  Rollins  sandstone  member  in  the  central 
part.  At  the  west  end  the  Bowie  shale  member  again  appears,  but 
there  is  an  erosional  unconformity  between  it  and  the  overlying 
Paonia  member. 
These  two  coal-bearing  members  of  the  Mesaverde  formation  differ 
notably  in  physical  character  at  the  west  end  of  the  field.  The  Bowie 
member  there  consists  principally  of  massive  sandstone  and  thick 
beds  of  shale,  with  one  bed  of  coal  at  the  base.  The  Paonia  member 
consists  mainly  of  carbonaceous  shale  and  thin  beds  of  sandstone, 
with  one  thick  bed  of  coal  at  the  base  and  several  thin  beds  above. 
At  the  east  end  of  the  field  the  lithologic  distinction  between  these 
two  members  is  not  so  pronounced,  as  both  contain  carbonaceous 
shale  and  sandstone  in  alternating  layers,  with  beds  of  coal  at  many 
horizons.  However,  the  two  are  usually  separated  by  a  sandstone 
that  contains  marine  shells.  The  line  of  separation  bet  ween  1  he  Bowie 
member  and  the  Paonia  member,  although  in  places  dimcull  to 
determine  in  the  field,  is  economically  important  because  of  the  dif- 
ference in  the  quality  of  the  coal,  the  upper  coals  being  soft  and  lig- 
nitic  and  the  lower  coals  bituminous  and,  locally,  of  coking  quality. 
No  coal  of  commercial  importance  is  known  above  the  Paonia 
shale,  and  the  overlying  rocks  are  only  of  negative  importance  in  the 
present  consideration,  as  they  cover  the  coal  to  depths  of  several 
thousand  feet, 
