ROCK    SPRINGS    COAL    FIELD,    WYOMING.  269 
of  red-white  or  reddish  ash.  Samples  taken  from  surface  prospects 
and  placed  in  air-tight  cans  soon  lose  their  bright  luster,  and  the  sur- 
face becomes  covered  with  a  velvety  brown  coating,  which  is  no  doubt 
due  to  the  alteration  of  the  weathered  coal.  The  chief  impurities 
of  the  coal  are  the  sulphur  balls  and  small  lenses  of  pyrite  that  are 
scattered  somewhat  irregularly  through  the  bed. 
The  higher  coals  are  also  distinctly  black,  with  a  bright  luster  as 
they  come  from  the  mine.  They  show  more  traces  of  iron  stain  and 
contain  considerable  gypsum  and  salt  flakes  in  the  joints  or  bedding 
planes.  On  exposure  to  the  air  they  alter  more  readily  than  the 
lower  Mesaverde  coals,  lose  their  bright  luster,  and  become  a  dull 
black.  -  In  breaking  down  cracks  form  along  the  bedding  planes  and 
at  right  angles  to  them,  producing  somewhat  regular  blocks  instead 
of  the  irregular  pieces  resulting  from  conchoidal  fracture.  The  coals 
resemble  somewhat  the  Adaville  coals  of  Uinta  County,  but  seem  to  be 
affected  less  on  exposure  to  the  air.  The  fractures  and  joints  are 
more  regular  and  cubical,  and  are  quite  different  from  the  con- 
choidal fracture  of  the  Adaville  coals. 
CHEMICAL  PROPERTIES. 
METHOD    OF    SAMPLING. 
Representative  samples  of  coal  were  collected  throughout  the  field 
from  the  four  coal  groups,  wherever  good  coal  could  be  obtained,  and 
were  sent  in  air-tight  cans  to  the  chemical  laboratory  of  the  fuel- 
testing  plant  at  Pittsburg,  Pa.,  where  they  were  analyzed  under  the 
direction  of  F.  M.  Stanton. 
In  order  that  the  results  from  the  samples,  collected  might  be 
entirely  comparable,  all  sampling  was  done  in  accordance  with  the 
general  plan  adopted  by  the  fuel -testing  plant  and  described  on 
pages  12-13  of  this  report. 
ANALYSES    OF    COALS. 
The  accompanying  table  gives  the  result  of  analysis  of  samples  as 
received  in  the  laboratory,  containing  all  the  moisture,  and  the 
result  expressed  in  terms  of  the  air-dried  sample — that  is,  one  lacking 
the  moisture  which  in  that  particular  sample — under  the  conditions 
of  the  analysis,  was  lost  in  air  drying.  The  analyses  are  grouped 
according  to  the  geologic  age  of  the  bed  sampled  and  arranged 
according  to  the  calorific  values,  the  highest  coals  heading  the  list 
for  each  group. 
