CONTKIBUTIONS   TO    ECONOMIC    GEOLOGY,   1907,   PAKT   II. 
beds,  although  here  and  there  dirty  beds  burn,  making  considerable 
clinker.  Moreover,  the  burning  of  thicker  beds  causes  more  com- 
plete fusion  of  the  overlying  material  than  the  burning  of  thin  or 
impure  beds. 
Little  is  known  as  to  the  manner  in  which  the  lignite  may  have 
been  ignited,  except  in  a  few  places  where  it  is  known  to  have  been 
set  on  fire  by  human  agencies  and  is  still  burning.  Other  explana- 
tions as  to  the  origin  of  the  fire  are  more  or  less  theoretical  and 
attribute  it  to  lightning,  to  prairie  fires,  and  to  spontaneous  ignition. 
All  of  these  causes  have  doubtless  been  instrumental,  each  becoming 
active  under  favorable  conditions. 
EXPLANATION   OF  MAP. 
The  outcrops  of  seven  different  beds  of  lignite  have  been  mapped 
in  various  parts  of  the  field,  and  in  order  to  distinguish  these  beds,  a 
columnar  section  showing  their  relative  positions  has  been  placed  on 
the  map,  with  a  letter  opposite  each,  corresponding  to  the  same  letter 
written  along  its  line  of  exposure  on  the  map  (PL  II). 
DETAILED  DESCRIPTION  OF  LIGNITE  BEDS. 
A  few  representative  detailed  sections,  beginning  with  the  lowest 
lignite  exposed  in  the  area  covered  by  this  report,  are  given  in  the 
following  pages,  in  the  order  of  their  occurrence. 
HARMON    LIGNITE    BED. 
The  Harmon  lignite,  A  in  the  columnar  section,  is  the  lowest  bed 
of  consequence  within  the  limits  of  the  area  investigated.  It  is 
partially  exposed  in  the  east  bank  of  Little  Missouri  River,  one-half 
mile  south  of  the  south  boundary  of  the  Sentinel  Butte  field.  Here 
the  section  is  as  follows: 
Section  of  Harmon  lignite  one-half  mile  south  of  south  boundary  of  Sentinel  Butte  field, 
on  Little  Missouri  River. 
Clay,  sandy.  Ft.    in. 
Lignite 4 
Clay,  sandy 5 
Lignite  (with  four  thin  shale  partings) 4    2 
Lignite 7     g 
Lignite  (with  three  thin  shale  partings) 1 
Lignite  (base  concealed  by  water) 2    6 
Total  lignite 15    6 
North  of  this  exposure  the  bed  dips  beneath  the  level  of  the  river 
and  does  not  appear  again  in  the  eastern  part  of  the  field.  Lignite, 
which  is  without  doubt  the  Harmon  bed,  was  penetrated  in  a  well 
sunk  by  the  Northern  Pacific  Railway  Company  at  Medora.     The 
