332         CONTRIBUTIONS   TO   ECONOMIC   GEOLOGY,    1907,   PART   II. 
The  coal  is  hard  and  dull  in  luster,  and  is  virtually  a  natural  coke 
changed  by  the  heat  of  igneous  rocks,  though  without  vesicular 
structure,  probably  because  of  the  pressure  of  the  overlying  strata. 
This  coal  occurs  in  comparatively  small  blocks  of  the  coal-bearing  forma- 
tion, surrounded  by  and  included  in  the  igneous  rock  of  the  mountains, 
and  the  coal  is  so  disturbed  and  so  uncertain  in  occurrence  that  it  has 
not  beenincluded  within  the  area  here  regarded  as  productive  coal  land. 
Although  of  excellent  quality,  it  is  scanty  in  quantity  and  difficult  of 
access,  and  its  value  from  a  commercial  standpoint  is  doubtful. 
Four  mines  of  the  Somerset  district  were  producing  at  the  time  of 
the  investigation.  Two  of  these,  the  Conine  and  Cooperative,  were 
not  equipped  with  machinery  and  the  mining  was  done  by  hand. 
The  King  mine  (No.  38)  was  well  equipped  for  mining  operations.  It 
is  located  in  the  side  of  the  canyon  and  the  coal  is  lowered  600  feet 
by  means  of  a  gravity  incline  to  the  foot  of  the  cliff,  where  it  is 
screened  and  loaded  directly  into  cars  for  shipment.  The  Utah 
Fuel  Company's  mine  at  Somerset  is  the  largest  producer  in  the  dis- 
trict, and  is  the  objective  point  of  the  North  Fork  branch  of  the 
Denver  and  Rio  Grande  Railroad.  No  information  in  regard  to 
this  mine  could  be  obtained  from  the  company. 
QUANTITY  AND  VALUE  OF  COAL. 
No  close  estimate  can  be  made  of  the  amount  of  coal  in  this  field 
until  more  is  known  of  the  number  of  the  coal  beds  and  their  varia- 
tions in  thickness.  The  average  thickness  of  workable  coal,  based 
on  available  information,  is  11  feet  for  the  Palisades  district,  15  feet 
for  the  Rollins  district,  and  65  feet  for  the  Somerset  district.  It  is 
probable  that  these  figures  will  be  increased  when  the  beds  are  all 
prospected.  If  the  practical  limit  of  available  coal  is  assumed  to 
be  6  miles  back  from  the  outcrop,  there  are  about  120  square  miles 
of  coal  land  in  the  Palisades  district,  about  235  square  miles  in  the 
Rollins  district,  and  about  195  square  miles  in  the  Somerset  district- 
Coal  of  1.3  specific  gravity  weighs  81.25  pounds  per  cubic  foot,  and 
a  square  mile  of  coal  1  foot  thick  contains  1,132,544  short  tons.  If 
the  estimated  thicknesses  are  correct,  the  Grand  Mesa  coal  field  con- 
tains 19,842,270,880  short  tons  of  coal  in  workable  beds.  After  de- 
ducting 25  per  cent  for  waste  in  mining,  there  remain  14,881,703,160 
short  tons  of  available  coal,  mainly  on  Government  land. 
COMPOSITION   OF  COAL. 
The  following  analyses  show  that  good  coal  occurs  throughout  the 
field,  but  that  the  coal  of  the  Bowie,  or  marine  Mesaverde,  is  superior 
to  that  of  the  Paonia,  or  nonmarine  beds.  In  general,  Paonia  coals 
have  large  percentages  of  moisture  and  low  heating  values,  are  com- 
paratively soft,  and  slack  more  or  less  readily  on  exposure.  The 
Bowie  coals  have  low  percentages  of  moisture  and  comparatively 
high  heating  values,  are  relatively  hard,  and  do  not  slack  easily. 
There  is  a  gradual  improvement  in  quality  from  west  to  east,  or 
