burchard.]     LIGNITES  OF  MIDDLE  AND  UPPER  MISSOURI  VALLEY.     285 
Butte,  on  Government  land,  in  wee.  20,  T.  133  N.,  R.  104  W.,  are  exten- 
sive seams  25  feet  thick,  while  seams  15  feet  thick  are  not  uncommonly 
exposed  along  Little  Missouri  River,  and  more  rarely  to  the  north  and 
east.  A  large  number  of  seams  6  to  10  feet  thick  are  described  in  that 
part  of  Doctor  Winder's  report  which  treats  of  the  deposits  by  counties, 
and  it  is  stated  that  seams  2  feet  or  less  in  thickness  were  so  abundant 
in  the  western  part  of  the  lignite  area  that  in  the  preliminary  work 
being  done  it  was  not  practicable  to  minutely  note  them  all. 
Especial  attention  was  given  to  the  correlation  of  seams  in  one 
locality  with  those  of  another,  but  in  general  it  was  not  found  practi- 
cable to  correlate  seams  in  exposures  more  than  1  or  5  miles  apart. 
The  nature  of  the  seams  hardly  admits  of  close  correlation  without 
much  detailed  work,  on  account  of  great  variations  in  thickness  and 
elevation.  In  some  instances  the  lateral  persistence  of  seams  is  con- 
siderable and  can  be  demonstrated,  but  "while  a  single  seam  was 
often  traced  for  6  miles  or  more  along  the  banks  of  the  Little  Missouri, 
others  seen  at  the  same  time  thinned  out  and  gave  place  to  new  ones  a 
little  above  or  belowT  them."  An  interrupted  seam  often  is  seen  to  be 
present  after  a  break  of  a  half  mile  where  the  lignite  has  been  replaced 
by  bituminous  or  nearly  pure  clay.  Neither  can  it  be  shown  that 
certain  horizons  in  the  Laramie  are  notably  richer  in  lignite  than 
others,  except  in  given  localities.  The  thicker  seams  appear  to  exist  as 
lens-shaped  masses,  some  of  which  are  of  considerable  areal  extent, 
and  of  the  thinner  seams  often  two  or  more  lie  so  close  together, 
separated  by  a  foot  or  so  of  clay,  that  they  may  be  mined  as  one. 
Methods  of  mining. — The  methods  of  mining  may  be  summarized  as 
follows : 
Practically  all  the  lignite  of  North  Dakota  is  mined  by  three  well-known  systems — 
the  strip  pit,  drifting  in  on  the  seam,  and  shafts,  vertical  or  sloping.  The  system 
that  is  in  use  at  a  given  locality  depends  on  two  factors — the  stage  of  development 
that  the  seam  has  reached  and  the  nature  of  the  seam.  The  strip-pit  system  is  the 
simplest,  but  a  coal  bank  begun  in  this  way  generally  passes  with  time  into  a 
drift  mine.  Later  it  may  be  found  desirable  to  abandon  the  drift  and  sink  a  shaft. 
Local  conditions,  however,  may  interfere  with  this  natural  development,  arresting  it 
before  the  second  and  third  stages  are  reached.  Where  capital  is  sufficient  and  other 
conditions  are  favorable,  the  preliminary  stages  are  omitted  and  a  shaft  is  sunk  at 
once. 
The  overlying  clay  has  not  been  found  satisfactory  for  a  roof,  and  it 
-is  generally  necessary  to  leave  6  inches  to  1  foot  of  the  lignite  at  the 
top  of  the  seam.  Consequently  this  amount  must  be  deducted  from 
the  thickness  of  a  seam  in  measuring  its  available  thickness.  With 
part  of  the  lignite  thus  left  for  a  roof,  timbering  is  generally  unnec- 
essary. 
Lying  between  clays,  the  lignite  usually  carries  some  water,  and 
springs  often  issue  from  its  outcrops.  The  water  is  not  excessive, 
requiring  a  moderate  amount  of  pumping  in  some  mines,  and,  being 
