CONTRIBUTIONS   TO   ECONOMIC    GEOLOGY,   1907,   PART   II. 
A  number  of  lignite  beds  lower  in  the  section  than  bed  C  of  the 
Medora  group  are  exposed  along  the  east  bank  of  Beaver  Creek,  in 
the  southeastern  part  of  T.  16  N.,  R.  60  E.  In  a  bluff  on  the  east 
bank  of  Beaver  Creek,  in  the  NE.  \  sec.  27,  the  following  section  was 
measured.  The  three  upper  beds  probably  represent  bed  C,  with 
partings  greatly  increased. 
Section  in  the  NE.  \  sec.  27,  T.  16  N,  R.  60  E.,  Montana. 
Ft.    in. 
Sandstone,  soft,  yellow-white 22 
Lignite 2 
Sandstone,  soft,  dark 5     6 
Lignite 1 
Sand,  clayey,  gray 4    9 
Lignite ._ 3    6 
Sandstone,  soft,  white 20 
Lignite 5     6 
Sandstone,  white,  clayey 10    3 
Lignite 4 
Clay,  dark  gray 30 
Lignite 5 
Level  of  Beaver  Creek. 
Total  lignite 16    5 
An  exposure  of  bed  C  in  the  NE.  \  sec.  12,  T.  15  N.,  R.  59  E., 
measures  12  feet,  but  the  upper  part  of  the  bed  contains  many  thin 
partings  of  clay.  Northward  from  this  point  the  bed  becomes  thin, 
as  shown  by  a  measurement  in  the  southwest  corner  of  T.  16  N., 
R.  60  E.,  where  it  is  only  5  feet  thick.  Toward  the  southwest  the 
bed  rises  gradually  and  appears  in  small  isolated  areas  near  the  tops 
of  the  highest  hills. 
SENTINEL    BUTTE    GROUP   OF   LIGNITE    BEDS. 
The  Sentinel  Butte  group  of  lignite  beds  is  so  named  on  account 
of  its  occurrence  in  Sentinel  Butte.  Although  the  distribution  of  its 
members  is  now  restricted  to  small  detached  areas,  hundreds  of 
clinker-capped  buttes  at  the  horizon  of  the  two  lower  members  of  the 
group,  dotting  almost  the  entire  eastern  half  of  the  area  surveyed, 
bear  evidence  of  the  fact  that  these  members  covered  at  some  earlier 
time  a  much  larger  area  than  at  present,  and  that  they  have  been 
burned  over  nearly  all  of  the  area. 
The  lowest  member  of  the  group,  bed  F  of  the  columnar  section, 
has  its  greatest  development  in  the  eastern  part  of  the  field,  where  it 
is  from  10  to  16  feet  thick.  It  is  found  at  the  contact  of  the  buff 
sandstone  and  gray  shale  with  the  somber-colored  member  above. 
This  contact  is  well  marked  on  account  of  the  abrupt  change  in  the 
color  and  character  of  the  material,  and  can  be  readily  followed 
throughout  the  district  where  it  occurs. 
