SHERIDAN    COAL   FIELD,    WYOMING.  133 
The  Ulm  coal  group  occupies  but  a  relatively  small  part  of  the 
Sheridan  district,  and  the  amount  of  coal  has  been  further  reduced 
by  burning  of  the  beds.  Each  of  the  coal  beds  of  the  intermediate 
coal  group,  as  estimated,  is  of  workable  thickness  in  only  a  small  part 
of  the  field.  Some  of  the  workable  beds  in  the  Tongue  River  coal 
group  are  known  to  thin  out,  and  others  doubtless  vary  in  thickness 
from  place  to  place.  It  can  not  be  assumed,  therefore,  that  the 
average  thickness  of  coal  for  the  whole  area  can  be  more  than  a  small 
fraction  of  the  total  average  of  the  known  thickness  of  the  separate 
beds. 
PHYSICAL  PROPERTIES   OF  THE  COAL. 
The  coal  beds  in  the  Tongue  River  group  are  so  nearly  uniform  in 
physical  character  that  select  specimens  from  one  bed  can  scarcely 
be  distinguished  from  like  specimens  from  other  beds.  All  these  coals 
are  distinctly  black  and  have  a  shiny  luster  when  fresh.  On  exposure 
in  dry  air  the  blackness  is  intensified  by  a  partial  loss  of  the  shiny 
luster  and  the  coal  assumes  a  dull,  dead  blackness.  At  the  same  time 
it  undergoes  a  rapid  change  by  the  loss  of  free  moisture,  which  causes 
it  to  check  or  crack  in  various  directions.  Though  weakened  by 
shrinkage  through  a  partial  loss  of  water  the  coal  will  adhere  together 
as  mined  for  an  indefinite  time  if  protected  from  the  weather.  When 
it  is  subjected  to  alternate  wetting  and  drying,  however,  it  breaks 
into  small  lumps  and  finally  is  reduced  to  a  powder-like  dust.  For 
these  reasons  the  coals  are  marketed  more  successfully  by  shipment 
in  box  cars.  The  streak  of  fine  dust  produced  by  abrading  the  coal  is 
distinctly  brown.  The  coals  in  the  thicker  beds  especially  are  not 
as  a  rule  distinctly  laminated,  but  are  for  the  most  part  massive.  In 
mining  they  break  into  angular  or  subcubical  blocks  that  present 
hackly  and  conchoidal  surfaces.  The  coal  in  this  district  is  not  reg- 
ularly or  distinctly  jointed  and  in  mining  is  undercut  by  pick  or 
machine  and  shot  from  the  solid. 
Though  the  coals  of  the  Tongue  River  group  contain  a  compara- 
tively high  percentage  of  water  and  disintegrate  on  exposure,  like  the 
lignites,  they  do  not  show  woody  structure.  Some  of  the  coal  beds 
in  the  upper  part  of  the  intermediate  group  and  in  the  Ulm  group 
show  the  texture  or  fiber  of  certain  plants  that  took  part  in  forming 
the  coal.  These  coals  are  homogenous,  however,  and  black  like  the 
coal  beds  lower  in  the  section,  but  their  water  content  is  a  little1 
greater  than  that  of  the  coals  on  Tongue  River.  Their  fuel  value  also 
is  a  little  lower,  as  indicated  by  the  calorific  determinations. 
