SENTINEL   BUTTE   LIGNITE    FIELD,    N.    DAK.    AND    MONT.  31 
The  second  member  of  the  group,  bed  G,  from  25  to  50  feet  above 
the  lowest  member,  shows  to  better  advantage  in  the  south-central 
part  of  the  surveyed  area,  in  the  base  of  the  higher  buttes,  where  its 
outcrop  is  marked  by  a  fringe  of  clinker.  Both  the  lower  members 
become  thin  and  disappear  toward  the  northwest.  These  beds  have 
been  so  generally  burned  that  few  exposures  showing  their  whole 
thickness  can  be  found.  Other  members  ranging  from  100  to  250 
feet  above  the  base  of  the  group  are  so  limited  in  distribution  in  the 
region  examined  that  they  will  not  be  considered  further  than  to 
say  that  they  occur  in  Sentinel,  Flat  Top,  and  Camels  Hump  buttes, 
and  in  a  small  area  in  the  extreme  northeastern  part  of  the  region. 
The  maximum  thickness  of  the  members  of  this  group  and  their  posi- 
tions in  the  geologic  column  are  shown  in  the  graphic  section  on  the 
map  (PL  II).  Some  detailed  sections  of  the  two  lower  members  of 
the  group  are  given  below. 
A  few  exposures  of  parts  of  the  two  lower  members  of  the  Sentinel 
Butte  group  were  noted  in  the  southeast  corner  of  the  field,  where 
small  patches  of  somber-colored  beds  are  underlain  by  them.  The 
coal  is  about  350  feet  above  the  level  of  Little  Missouri  River  at  the 
southern  boundary  of  the  field.  The  outcrop  of  the  lignite  is  so  con- 
cealed by  a  fringe  of  clinker  that  it  is  difficult  to  find  the  whole  of 
the  lower  bed  exposed,  but  it  is  about  17  feet  thick,  apparently  with- 
out partings.  These  beds  are  unimportant  here,  as  they  are  detached 
bodies  of  small  extent. 
Section  of  bed  G  in  the  northeast  base  of  Sentinel  Butte,  in  the  SE.  \  sec.  5,  T.  139  N., 
R.  104  W. 
Clay,  sandy,  somber  colored.  Ft.  in. 
Lignite 14 
Clay 3 
Lignite 6  11 
Clay. 
Total  lignite 20  11 
This  bed  has  been  mined  here  for  local  use.  The  same  bed  has 
been  mined  in  the  NE.  |  NW.  i  sec.  7,  1  \  miles  west  of  this  exposure, 
and  is  there  20  feet  11  inches  thick.  It  has  also  been  mined  in  the 
SW.  \  sec.  5,  but  its  whole  thickness  could  not  be  seen. 
A  number  of  small  areas  underlain  by  the  lower  members  of  the 
Sentinel  Butte  group  remain  in  the  southeast  corner  of  T.  139  N., 
R.  105  W.  Near  the  center  of  sec.  25  of  this  township,  lignite  is 
mined  for  neighborhood  use  by  stripping.     A  section  follows: 
Section  near  center  of  sec.  25,  T.  139  N.,  R.  105  W. 
Clay,  white.  Ft.  in. 
Lignite,  weathered 8 
( 'lay 10 
Lignite  (base  concealed) 7 
Total  lignite  exposed lo 
