34  CONTRIBUTIONS   TO   ECONOMIC    GEOLOGY,  1907,   PART   II. 
The  impurities  in  the  lignite  consist  mainly  of  sand  and  clay  part- 
ings, which  are  difficult  to  separate  in  mining.  Intimate  mixtures  of 
fine  sand  or  clay  and  lignite,  of  irregular  shape  and  extent,  are  rather 
common  in  the  beds,  and  are  not  easily  detected  without  close  exami- 
nation, as  they  have  the  same  color  as  the  lignite.  Crystals  of  selenite 
or  gypsum  are  also  found. 
The  fact  that  the  woody  parts  of  the  lignite  are  freer  from  impuri- 
ties than  the  textureless  parts  is  recognized  by  ranchmen  and  farmers, 
who  usually  throw  aside  the  less  desirable  lignite  in  mining. 
Heretofore  the  lignite  has  been  little  used  except  as  a  local  fuel 
supply,  but  the  fact  that  lignites  similar  in  every  respect  are  mined 
on  a  commercial  scale  at  Lehigh,  N.  Dak.,  40  miles  to  the  east,  and 
near  Glendive,  Mont.,  25  miles  to  the  west,  seems  to  justify  the  state- 
ment that  the  field  is  one  of  growing  importance. 
Briquetting  tests  of  North  Dakota  lignite  have  been  made  by  the 
United  States  Geological  Survey  fuel-testing  plant  and  by  individuals. 
No  binder  is  used,  but  the  product  is  firm  and  gives  excellent  results, 
except  when  subjected  to  forced  draft,  which  has  the  effect  of  disin- 
tegrating the  briquets  and  carrying  the  unburned  particles  out 
through  the  smokestack. 
Another  consideration  that  adds  materially  to  the  value  of  the 
brown  lignite  is  its  surprising  success  in  the  producer-gas  plant.  The 
following  statement  has  been  made  concerning  the  value  of  North 
Dakota  lignite  in  the  gas  producer  and  gas  engine:" 
The  result  of  the  steam  test  was  so  unsatisfactory  that  there  is  nothing  by  which  a 
direct  comparison  can  be  made  of  the  efficiency  of  the  fuel  used  in  the  producer-gas 
plant  as  compared  with  the  efficiency  developed  in  the  steam  plant.  Nevertheless 
a  comparison  of  the  results  obtained  on  other  coals  under  the  steam  boiler  is  instruc- 
tive. The  table  shows  that  to  produce  one  electrical  horsepower  hour  in  the  producer- 
gas  plant  required  2.29  pounds  of  dry  North  Dakota  lignite,  whereas  to  produce  the 
same  result  in  the  steam  plant  required  3.39  pounds  of  the  best  West  Virginia  coal. 
This  means  that  North  Dakota  lignite,  with  the  moisture  eliminated,  will  do  more 
work  when  used  in  a  producer-gas  plant  than  the  best  coal  of  the  country  will  do  in  a 
steam  plant. 
Many  detached  terraces  or  flats  along  Little  Missouri  River  are  too 
small  to  justify  the  construction  of  expensive  canal  systems  for  the 
purpose  of  irrigation.  In  other  parts  of  this  general  region  such  ter- 
races, from  10  to  30  feet  above  water  level,  are  being  watered  by 
pumping  from  adjacent  streams,  lignite  being  used  as  fuel.  Lignite 
in  abundance  is  at  hand  or  within  easy  reach  of  most  of  the  probable 
locations  of  such  plants  on  the  Little  Missouri,  and  it  seems  likely 
that  with  the  settlement  of  the  country  more  use  will  be  made  of  it 
for  this  purpose. 
The  samples  for  the  following  analyses  were  all  taken  from  open 
pits  where  the  coal  was  more  or  less  weathered.     It  is  believed  that 
a  Prof.  Paper  U.  S.  Geol.  Survey  No.  48,  pt.  1,  19u0,  p  111. 
