SENTINEL    BUTTE    LIGNITE    FIELD,    N.    DAK.    AND    MONT.  33 
Bed  F  occurs  in  T.  142  N.,  R.  101  W.,  at  an  elevation  of  about  250 
feet  above  Little  Missouri  River.  Its  outcrop  is  extremely  sinuous 
and  is  everywhere  fringed  by  a  rim  of  clinkered  material.  In  the 
southeastern  part  of  the  township  the  lignite  is  so  concealed  that  no 
measurements  of  its  thickness  could  be  made,  but  it  is  probably 
about  as  thick  as  it  is  in  the  next  township  south,  or  about  15  feet. 
North  of  Ash  Creek  partings  develop  in  the  bed,  replacing  and  divid- 
ing it  so  that  it  is  rendered  unworkable.  Some  detailed  sections  of 
this  bed  follow: 
Section  of  bed^F in  NE.  {  sec.  4,  T.  142  N.,  R.  101  W. 
Ft.   in. 
Lignite 1     3 
Clay 7 
Lignite 3 
Total  lignite 4     3 
Section  of  bed  F  near  center  cf  N.  \  sec.  13,  T.  142  N.,  R.  101  W. 
Ft.  in. 
Lignite 2  7 
Clay 7 
Lignite 1  11 
Clay 1  1 
Lignite 1  5 
Total  lignite 5  11 
Section  of  bed  F  near  northeast  corner  sec.  15,  T.  142  N.,  R,  101  W. 
Ft.   in. 
Lignite 2     8 
Clay 8 
Lignite 4     2 
Total  lignite 6  10 
CHARACTER  AND   USES   OP^   THE   LIGNITE. 
The  lignite  is  brown,  tough,  and  woody  in  structure.  It  slacks 
rapidly  on  exposure  to  air  and  sunlight,  especially  when  much  han- 
dled or  subjected  to  long  hauls  by  rail.  As  a  rule  the  grain  of  the 
wood  and  the  compressed  trunks  of  trees  with  branches  are  plainly 
visible  in  the  coal,  but  pockets  and  layers  of  shiny,  black,  texture- 
less,  brittle  coal  are  scattered  through  many  beds.  These  pockets 
become  more  apparent  on  exposure  to  weathering  agencies,  as  they 
slack  more  rapidly  than  the  woody  parts. 
There  is  apparently  a  slight  change  in  the  character  of  the  lignite 
toward  the  west.  Near  Wibaux  less  woody  material  appears  in  it 
than  at  Medora  and  Sentinel  Butte.  -At  Glendive,  still  farther  west, 
the  lignite  is  almost  entirely  of  the  black,  brittle  variety.  This 
change  may  be  due  to  the  fact  that  the  coals  at  Glendive  are  much 
lower  iii  the  rock  section  than  those  farther  east,  and  hence  are  older 
and  have  suffered  a  greater  degree  of  metamorphism  than  the  higher 
coals. 
71407— Bull.  341—09 3 
