46  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1907,   PART   II. 
There  are  probably  many  such  small  areas  of  unburned  coal  in  the 
Pine  Hills  country,  and  some  of  them  may  be  of  economic  value  but 
could  not  be  differentiated  in  the  field  for  lack  of  evidence  as  to  their 
extent. 
EXPLANATION  OF  THE  MAP. 
On  the  accompanying  map  (PL  III)  the  areas  occupied  by  the 
lower  and  upper  members  are  differentiated,  the  former  being  left 
blank  and  the  latter  ruled  with  parallel  lines.  The  contact  of  these 
two  members  coincides  with  a  contour  line  2,850  feet  above  the  sea,  or 
550  feet  above  the  level  of  the  Yellowstone.  Owing  to  the  intricate 
detail  of  topography  this  line  is  somewhat  generalized.  The  out- 
crops of  five  of  the  more  important  coal  beds  designated  by  the  letters 
A,  C,  D,  E,  and  F  are  traced  on  the  map  and  coincide  with  the  con- 
tour lines  at  various  levels  indicated  in  the  columnar  section  at  the 
right.  The  stratigraphic  relations  are  also  shown  in  the  profile  at 
the  bottom  of  the  map.  The  character  of  the  coal  bed  along  the  out- 
crop is  indicated  by  the  weight  of  the  line.  Wherever  the  coal  bed 
has  been  found  to  be  workable  its  outcrop  is  traced  in  a  heavy  line, 
and  the  detail  is  drawn  with  as  great  accuracy  as  possible,  but  where 
the  bed  is  not  known  to  be  workable  its  outcrop  is  shown  by  a  light 
line,  which  has  been  somewhat  generalized.  In  the  upper  member 
the  lignite  in  many  places  has  been  burned  for  an  unknown  distance 
back  of  the  outcrop,  and  the  limits  of  unburned  and  therefore  work- 
able lignite  have  been  placed  at  a  safe  distance  back  of  the  outcrop 
indicated  by  clinkers.  The  edges  or  outcrops  of  the  clinker  due  to 
the  burning  of  the  bed  lettered  E  are  shown  by  lines  of  dots,  and  a 
critical  examination  of  the  map  will  give  an  idea  of  the  quantity  of 
burned  lignite  in  this  bed  as  compared  with  that  remaining  in  the 
district. 
COALS  OF  THE   LOWER   MEMBER. 
GENERAL    OUTLINE. 
The  coals  of  the  lower  member  are  black  and  usually  show  little  of 
the  woody  texture  common  to  the  lignite  higher  in  the  section. 
They  are  fairly  brittle  and  have  an  even  fracture.  Joints  and  slips 
transverse  to  the  bedding  are  not  prominent,  but  such  features  have 
been  observed  in  some  of  the  mines  and  openings.  When  freshly 
mined  the  coal  has  a  bright  luster,  but  this  is  soon  lost  in  the  process 
of  slacking,  which  begins  as  soon  as  the  coal  is  exposed  to  the  air. 
It  is  estimated  that  there  is  an  average  of  at  least  one  bed  of  coal  or 
carbonaceous  shale  for  every  50  feet  of  thickness  in  this  part  of  the 
Fort  Union  formation,  but  less  than  1  per  cent  of  the  coal  exposed 
in  outcrop  is  of  workable  thickness  and  purity,  if  no  coal  beds  under 
