322 
CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,    L905. 
point,  at  another  half  a  mile  or  so  distant  but  one  of  them  is  present .  It  is  frequently  : 
possible  to  correlate  seams  on  the  opposite  sides  of  the  same  valley  and  only  1  or  2  m  - 
apart.  On  the  other  hand,  some  scams  can  be  traced  .'i  or  4  miles  in  the  river  bluffs  ;. 
less  commonly  twice  that  distance,  but  sooner  or  later  I  hey  thin  out  and  disappear.  T 
lack  of  persistence  makes  it  unsafe  to  correlate  lignite  beds  whose  outcrops  are  separai 
by  more  than  a  few  miles. 
The  lignite  differs  considerably  in  appearance  and  physical  properties  in  different  pa 
of  the  region,  though  it  is  remarkably  uniform  over  extensive  areas.  In  North  and  Soi 
Dakota  it  is  usually  brown  in  color  and  more  or  less  woody  in  character,  exhibiting  clea 
the  grain  of  the  wood  and  having  the  toughness  of  the  same  material.  It  is  difficult 
break  this  lignite  except  along  the  grain,  but  it  splits  or  breaks  readily  in  that  di recti 
Often  portions  of  flattened  t  ree  t  ranks  or  branches  are  found  in  the  seam,  resembling  nu 
the  original  wood  excepl  for  their  brown  color.  The  same  seam  is  frequently  more  woo 
in  some  poit  ions  than  others,  being  made  up  of  alternating  bands  of  tough  brown  W^n 
and  black,  lustrous,  more  brittle  material.  The  lignite  changes  in  character  toward  t 
wesl  and  in  Montana  much  of  it  is  black  and  lustrous,  with  little  of  the  woody  texti 
and  appearance  characterizing  it  in  North  Dakota,  while  it  is  almost  as  brittle  as  bitui 
nous  coal.  This  is  much  easier  to  mine  than  the  tough  brown  variety,  since  it  breaks 
readily  with  the  pick  or  in  blasting.  The  lignite  in  the  9-foot  seam  at  Medora  and  t 
'211-foot  seam  in  Sentinel  Butte  is  woody  and  brown  from  top  to  bottom;  that  in  i 
12-foot  seam  on  Beaver  Creek,  near  Wibaux,  Mont.,  is  mostly  black  and  lustrous,  w 
only  .i  few  brown  layers;  while  the  lignite  in  the  vicinity  of  ( Hem  live  is  wholly  of  the  bla< 
brittle  variety. 
When  exposed  to  the  air  fora  short  time  the  lignite  loses  much  of  its  moisture  and  cru 
bles  into  small  fragments.  For  this  reason  it  is  very  desirable  thai  the  coal  be  used 
promptly  as  possible  after  it  is  mined,  before  it  has  time  to  disintegrate,  or  "slack." 
The  chemical  composition  of  the  ugnite  is  shown  in  the  following  analyses.  The  fi 
was  made  ai  the  United  States  Geological  Survey  coal-testing  plant  at  the  Louisiana  Pi 
chase  Exposition  and  reported  in  Bulletin  No.  261.  The  other  analyses  are  taken  fn 
the  Second  Biennial  Reporl  of  the  North  Dakota  Geological  Survey. 
Analyses  ofligniU  from  North  <tu<l  South  Dakota. 
Moisture 
le  ni.ii  ter 
Fixed  carbon. . 
Ash 
Sulphur 
39.  56 
27.  78 
26.30 
(i.  36 
.  93 
2. 
3. 
43.  78 
40.  25 
26.07 
25.  21 
26.  33 
27.17 
:;.  82 
7.  37 
.r.l 
.50 
1.  From  mine  of  (  edar  Coulee  '  oal  <  iompany,  near  fl  illiston,  \.  Dak. 
2.  From  33-fool  seam  on  Sand  Creek,  ai  Russell  ranch,  Billings  County,  X.  Dak. 
:;.  From  9  fool  seam  on  Riley  ranch,  a1  the  base  of  the  Cave  Hills.  Butte  County,  S.  Dak. 
4.  From  7-foot  seam  near  Cartwright,  A llred  County,  X.  Dak. 
37, 
