THE  SENTINEL  BUTTE  LIGNITE  FIELD,  NORTH  DAKOI  \ 
ANI)  MONTANA. 
By  A.  G.  Leonard  and  Carl  D.  Smith. 
INTRODUCTION. 
As  shown  by  the  key  map  (fig.  1),  the  Sentinel  Butte  field  is  a  small 
area  near  the  center  of  the  Fort  Union  region  of  coal  and  lignite  bear- 
ing rocks  of  Tertiary  age,  which  cover  a  considerable  area  in  the  north- 
ern part  of  Wyoming,  the  eastern  quarter  of  Montana,  the  western 
M 
N 
W    Y    O     Mn 
or' 
Fig.  l.— Index  map  showing  location  of  the  Sentinel  Butte  lignite  field,  Norl  h  Dakota  and  Montana. 
half  of  North  Dakota,  and  the  northwest  corner  of  South  Dakota. 
The  field  examined  in  1907  comprises  approximately  25  townships  L9 
in  Billings  County,  N.  Dak.,  and  6  in  Dawson  County,  Mont.,  on 
either  side  of  the  Northern  Pacific  Railway. 
