SENTINEL   BUTTE   LIGNITE    FIELD,    N.    DAK.    AND    MONT. 
distance  above  the  Harmon  ranch,  at  the  southern  boundary  of  the 
field.  The  two  uppermost  members  of  the  Sentinel  Butte  group  are 
so  meager  in  distribution  that  their  outcrops  have  not  been  mapped. 
The  record  of  a  drill  hole  sunk  at  Medora  shows  that  at  least  800  feet 
of  lignite-bearing  rocks  lie  below  the  Harmon  bed. 
VARIABILITY. 
Heretofore  the  lignite  beds  of  this  general  region  have  been  described 
as  lenticular  in  form  and  impossible  of  correlation  for  distances 
exceeding  2  or  3  miles.  It  is  true  that  the  lignite  is  variable  in  thick- 
ness and  horizontal  extent,  but  some  of  the  more  important  beds 
have  been  traced  continuously  for  24  miles  and  have  been  correlated 
with  much  certainty  for  greater  distances.  So  far  as  this  area  is 
concerned,  there  is  a  more  or  less  definite  law  controlling  the  dis- 
appearance of  the  beds.  It  has  been  observed  that  the  two  lower 
members  of  the  Sentinel  Butte  group  become  thin  and  pinch  out 
toward  the  northwest.  Some  of  the  members  of  the  Medora  group 
follow  the  same  rule,  but  whether  this  law  has  a  general  application 
can  be  ascertained  only  by  further  detailed  work  in  the  surrounding 
territory.  The  clinker  zone  left  by  the  burning  of  the  two  lower 
members  of  the  Sentinel  Butte  group  bears  conclusive  evidence  that 
they  formerly  occupied  the  entire  southeast  corner  of  the  field  and 
that  they  extended  still  farther  to  the  east  and  southeast,  covering 
a  large  territory  beyond  the  limits  of  last  season's  investigations. 
CLINKER. 
A  noticeable  feature  which  has  had  a  great  influence  on  the  topog- 
raphy of  the  field  is  the  clinker  or  baked  and  partially  fused  sands 
and  clays,  due  to  the  burning  of  the  coal  beds.  The  prevailing  color 
of  this  material  is  red,  but  various  shades  of  yellow  and  green  were 
also  observed.  The  more  completely  fused  portions  are  usually 
dark  red  to  black.  At  places  these  clinker  zones  are  100  feet  thick,  but 
it  is  not  likely  that  so  great  a  thickness  has  resulted  from  the  burning 
of  a  single  bed.  The  distance  back  from  the  outcrop  of  a  bed  to 
which  burning  may  take  place  depends  on  the  amount  of  cover. 
Wherever  the  cover  is  great  enough  combustion  is  soon  smothered 
by  the  weight  of  the  overlying  material.  It  is  impossible  to  give 
exact  figures  in  this  connection,  as  no  prospecting  or  mining  has  ever 
been  done  where  a  lignite  bed  is  rimmed  by  clinker.  In  prospecting 
for  lignite  along  an  outcrop  which  has  been  burned  it  will  be  noted 
that  the  base  of  the  clinker,  which  is  usually  well  marked,  is  on  a 
level  with  the  base  of  the  bed  of  lignite. 
It  may  be  well  to  say  here  that,  as  a  general  rule,  the  lignite  beds 
that  have  been  burned  most  extensively  are  the  thicker  and  purer 
