12  CONTRIBUTIONS   TO   ECONOMIC    GEOLOGY,   1907,   PART   II. 
COAL.. 
In  attempting  a  general  investigation  of  the  western  coal  fields 
close  attention  was  given  to  the  study  of  the  coal  itself,  as  to  its 
physical,  chemical,  and  commercial  properties.  Such  a  study  can  be 
more  profitably  undertaken  in  these  States  than  in  the  older  and 
better-known  fields  of  the  East,  for  the  reason  that  many  single  fields 
contain  several  grades  of  coal,  and  in  the  Rocky  Mountain  province 
as  a  whole  all  grades  from  lignite  to  anthracite  are  represented. 
The  study  of  the  various  kinds  of  coal  has  not  progressed  far 
enough  to  say  what  shall  be  the  exact  limits  assigned  to  these  grades, 
but  the  grades  themselves  are  fairly  well  established  and  have  been 
definitely  adopted  by  the  Geological  Survey  for  use  in  its  publications. 
They  are  as  follows:  (1)  Anthracite,  (2)  semianthracite,  (3)  semibi- 
tuminous,  (4)  bituminous,  (5)  subbituminous,  (6)  lignite.  The  first 
three  are  not  common  in  the  western  fields,  and  their  definition  will 
not  be  attempted  at  the  present  time,  further  than  to  say  that  the 
common  trade  distinctions  will  be  followed.  The  distinction  between 
bituminous,  subbituminous,  and  lignite  is  much  more  important  in 
the  West,  and  provisional  definitions  are  given  as  follows : 
Lignite:  Brown,  woody,  or  earthy  coals  which  slack  quickly  on 
exposure  to  the  atmosphere.  Their  moisture  content  is  usually  over 
30  per  cent,  and  their  heating  value  is  generally  less  than  8,000  B.  t.  u. 
Subbituminous  coal:  Intermediate  in  quality  between  lignite  and 
bituminous  coal.  It  is  generally  glossy  black,  is  relatively  free  from 
joints,  and  slacks  readily  on  exposure  to  the  atmosphere.  Coal  of 
this  grade  is  usually  called  "  black  lignite,"  but  it  is  not  woody  in 
appearance,  and  more  nearly  resembles  bituminous  coal  than  lignite. 
Its  moisture  content  is  usually  over  10  per  cent,  and  its  heating  value 
ranges  from  8,000  to  11,000  B.  t.  u. 
Bituminous  coal :  Bituminous  coal  hardly  needs  description,  since 
it  is  the  coal  with  which  most  persons  are  familiar.  In  a  commercial 
way  it  is  distinguished  from  subbituminous  coal  by  its  ability  to 
withstand  the  weather. 
CHEMICAL  WORK. 
For  the  purpose  of  classifying  the  coals  and  determining  the  values 
of  the  coal  land,  a  great  many  samples  were  collected  for  analysis. 
In  order  to  make  comparisons  with  the  analyses  of  coals  from  different 
fields  and  different  regions  and  provinces,  the  sampling  was  done 
according  to  a  uniform  method,  as  follows : 
1.  Select  a  fresh  face  of  un weathered  coal  at  the  point  where  the  sample  is  to  be 
obtained,  and  clean  it  of  all  powder  stains  and  other  impurities. 
2.  Spread  a  piece  of  oilcloth  or  rubber  cloth  upon  the  floor  so  as  to  catch  the  particles 
of  coal  as  they  are  cut  and  to  keep  out  impurities  and  excessive  moisture  where  the  floor 
is  wet.  Such  a  cloth  should  be  about  1|  by  2  yards  in  size  and  spread  so  as  to  catch  all 
the  material  composing  the  sample 
