104         CONTRIBUTIONS   TO    ECONOMIC    GEOLOGY,   1907,   PART   II. 
sizes  with  only  a  small  percentage  of  fine  coal  or  slack.  The  coals 
are  about  equal  in  hardness  to  the  average  subbituminous  coals. 
They  are  about  the  same  in  this  respect  as  the  coals  mined  at  Bridger, 
Mont.,  harder  than  those  at  Sheridan,  Wyo.,  and  softer  than  the 
bituminous  coals  of  the  East.  They  are  indistinctly  banded  with 
layers  of  dull  coal.  Small  accretions  of  clear  brown  resin  are  dis- 
tributed irregularly  through  the  coal.  They  appear  generally  in 
globular  form  less  than  one-fourth  inch  in  diameter,  though  some 
exceptionally  large  ones  measure  as  much  as  1J  inches.  When 
broken  they  appear  as  brown  spots  in  the  black  coal. 
The  coal  is  free  from  pyrite  nodules,  but  contains  other  impurities 
which  produce  ash  on  burning.  Dirt  occurs  as  partings  too  thin  to 
be  separated  in  mining  and  mixed  with  coaly  matter  in  thin  dull- 
colored  lenses  in  the  good  coal.  Some  of  these  impurities  can  be 
taken  out  by  washing,  and  in  that  way  coals  which  are  not  mined 
at  present  could  be  put  on  the  market.  The  Red  Lodge  mine  is  the 
only  one  in  the  field  equipped  with  a  washing  plant,  and  there,  as  at 
other  mines,  it  is  found  to  be  more  economical  to  leave  untouched 
the  beds  containing  much  dirt  than  to  mine  and  clean  the  coals 
before  shipment. 
According  to  the  classification  of  coal  adopted  by  the  United 
States  Geological  Survey,  these  coals  are  high-grade  subbituminous/ 
closely  approaching  the  bituminous  class.  The  coal  usually  cleaves 
in  small  blocks  and  at  some  places  breaks  into  prisms,  but  weathering 
generally  develops  the  platy  structure  along  the  bedding  planes. 
They  do  not  stock  well  and  are  therefore  placed  below  the  bitu- 
minous class,  though  in  heat  value  they  approach  some  of  the 
bituminous  coals  of  the  Mississippi  Valley. 
CHEMICAL    PROPERTIES. 
The  composition  of  the  coal  of  the  Red  Lodge  field  is  shown  by  the 
following  analyses,  made  under  the  direction  of  F.  M.  Stanton,  of 
the  United  States  Geological  Survey.  Samples  were  taken  accord- 
ing to  the  method  described  by  M.  R.  Campbell  in  the  introduction 
to  this  volume  (pp.  12-13).  The  table  shows  the  analysis  of  each  sam- 
ple as  received  and  also  after  air  drying.  The  former  represents  the 
coal  as  it  comes  from  the  mine,  and  the  latter  more  nearly  indicates 
the  condition  of  the  coal  in  the  market  and  as  it  is  burned.  For  the 
sake  of  comparison,  therefore,  the  analyses  of  the  air-dried  samples 
should  be  used. 
a  The  name  subbituminous  has  recently  been  adopted  by  the  United  States  Geological  Survey  for 
the  class  of  coal  above  the  brown  lignites  and  below  the  bituminous  coals— the  class  generally  called 
"black  lignite." 
