134         CONTRIBUTIONS   TO   ECONOMIC   GEOLOGY,   1907,   PART  II. 
QUALITY   OF  THE  COAL. 
The  coals  that  have  been  tested  chemically  are  considered  to  be 
representative,  with  few  exceptions,  of  the  several  beds  that  have  been 
examined.  The  samples  believed  not  to  be  truly  representative  were 
obtained  from  shallow  workings  and  from  mines  near  the  outcrop,  and 
are  thought  to  have  suffered  a  partial  loss  of  volatile  matter,  due  to 
the  more  or  less  free  access  of  air.  Such  samples  seem  to  show  a  higher 
percentage  of  moisture  than  the  average  coal  of  the  same  beds,  though 
the  coal  at  the  time  the  samples  were  collected  did  not  contain  an 
apparent  excess  of  water.  Samples  Nos.  5  and  6  in  the  table  of 
analyses  were  taken  from  the  same  mine,  one  under  a  thick  and  the 
other  under  a  thin  covering  of  rock.  Nos.  2  and  4  represent  samples 
from  the  same  bed,  one  in  a  deep  mine  and  the  other  under  shallow 
cover.  It  is  probable,  therefore,  that  coals  represented  by  samples 
Nos.  4,  6,  7,  8,  20,  21,  and  22  contained  reduced  components  of  volatile 
matter  and  a  slight  excess  of  moisture,  so  that  the  samples  as  received 
and  in  the  air-dried  state  are  not  truly  representative  of  the  beds 
from  which  they  were  taken.  However,  they  were  the  best  that  could 
be  obtained  at  the  time  examinations  were  made  and  are  approximate 
within  a  small  percentage. 
All  the  coal  samples  were  collected  in  conformity,  as  nearly  as  pos- 
sible, with  the  regulations  of  the  fuel-testing  plant.  Each  sample  is 
first  subjected  to  warm,  dry  air  in  a  protected  vessel  until  the  free 
moisture  is  evaporated  and  the  coal  has  attained  practically  a  con- 
stant weight.  In  this  state  it  is  said  to  be  air  dried  and  is  analyzed. 
The  analysis  is  then  recalculated  back  to  the  condition  of  the  sample 
as  received.  A  comparison  of  these  two  analyses  indicates  the 
approximate  relative  fuel  values  of  the  coal  in  its  moist  and  dry  state. 
The  chemical  analyses  show  that  the  coals  of  the  Sheridan  district 
as  a  whole  have  higher  heat  values  and  are  therefore  more  efficient 
fuels  than  the  better  grades  of  lignites.  These  coals  are  black  and  in 
that  respect  are  also  unlike  the  lignites,  which  are  brown.  Because 
of  their  general  homogeneous  texture,  blackness,  and  fuel  efficiencies 
the  Sheridan  coals  are  distinctly  separate  in  quality  from  lignites. 
On  the  other  hand,  they  are  distinguished  from  bituminous  coals 
chiefly  on  account  of  their  relatively  high  percentage  of  moisture, 
conchoidal  fracture,  varying  luster,  and  inability  to  withstand  weath- 
ering agencies.  The  average  bituminous  coal  contains  considerably 
less  than  10  per  cent  of  moisture,  has  angular  or  cubical  as  distin- 
guished from  conchoidal  fracture,  retains  luster,  and  will  not  check 
or  slack  on  exposure  to  the  weather.  The  coal  of  the  Sheridan  dis- 
trict stands  between  bituminous  coal  and  lignite,  and  is  to  be  classed 
as  subbituminous. 
The  chemical  analyses  here  given  were  made  at  the  laboratory  of 
the  Geological  Survey  fuel-testing  plant,  Pittsburg,  Pa. 
