NORTH    DAKOTA-MONTANA    LIGNITE    AREA.  321 
absent  wherever  there  are  extensive  outcrops  of  the  bods.  This  red  clay  has  been  pro- 
duced by  the  heat  generated  by  the  burning  of  the  seams  of  lignite,  which  lias  doubtless 
been  going  on  ever  since  the  formation  of  the  seams.  Bands  of  this  material  may  be 
traced  for  many  miles  along  the  bluffs  of  the  stream  valleys  and  in  the  buttes  representing 
the  horizons  of  burned-out  lignite  seams,  portions  of  which  are  often  found  intact  at  some 
points. 
The  Laramie  beds  have  a  thickness  of  at  least  1,800  feet  in  the  western  part  of  North 
Dakota.  The  Medora  well,  941  feet  deep,  did  not  pass  through  them,  and  Sentinel  Butte 
rises  860  feet  above  the  curb  of  the  well.  This  is  perhaps  the  maximum  thickness  of  the 
formation  in  North  Dakota  and  it  grows  thinner  toward  the  east  until  it  is  only  a  few 
hundred  feet  and  less  beyond  Missouri  River. 
Tertiary. — The  youngest  strata  occurring  in  the  lignite  area  are  certain  beds  which  are 
referred  to  the  Tertiary.  These  form  the  upper  strata  of  the  Slim  Buttes  and  East  and 
West  Short  Pine  Hills  of  South  Dakota,  the  Long  Pine  Hills  of  Montana,  and  the  low 
mound  on  top  of  Sentinel  Butte,  North  Dakota.  The  beds  consist  mostly  of  fine-grained, 
white,  soft  sandstone,  with  some  calcareous  clay  and  limestone.  Their  thickness  in  the 
Slim  Buttes  is  200  to  240  feet,  and  in  the  East  Short  Pine  Hills  they  attain  nearly  300  feet. 
The  white  sandstone  forms  conspicuous  cliffs  toward  the  top  of  the  buttes,  which  can  be 
seen  from  a  great  distance. 
LIGNITE. 
Interstratified  with  the  sands  and  clays  of  the  Laramie  are  seams  of  lignite.  These 
are  not  confined  to  any  particular  horizon  or  horizons,  but  are  distributed  quite  uniformly 
from  top  to  bottom  of  the  formation.  In  the  Slim  Buttes  of  South  Dakota  a  5-foot  seam 
occurs  95  feet  from  the  top  of  the  Laramie.  The  21^-foot  seam  in  Sentinel  Butte  lies 
340  feet  below  the  top,  and  there  is  an  18-inch  seam  142  feet  below  the  same  horizon.  On 
the  other  hand,  the  records  of  deep  wells  at  Medora  and  Dickinson  show  that  the  lignite 
is  found  toward  the  base  of  the  formation.  The  seams  thus  have  a  vertical  range  of  1,500 
feet  or  more. 
The  number  of  seams  varies  considerably  in  different  parts  of  the  field.  At  some  points 
but  two  or  three  seams  appear,  while  in  other  localities  there  are  ten  or  fifteen  in  a  vertical 
distance  of  from  250  to  500  feet,  though  many  of  them  are  only  a  few  inches  in  thickness. 
Rarely  are  several  hundred  feet  of  Laramie  strata  exposed  without  one  or  more  lignite 
beds.  In  the  section  already  given  (p.  319)  appearing  in  the  bluffs  of  the  Little  Missouri 
near  the  mouth  of  Cherry  Creek,  there  are  no  less  than  fifteen  seams  within  a  vertical  range 
of  480  feet,  having  an  aggregate  thickness  of  25  feet.  In  the  Third  Creek  district,  8  miles 
north  of  Glendive,  Mont.,  four  lignite  seams,  7,  8,  10,  and  14  feet  thick,  respectively,  occur 
in  a  vertical  distance  of  200  feet,  their  aggregate  thickness  being  39  feet.  In  the  180  feet 
of  Laramie  clays  exposed  in  the  bluffs  on  the  south  side  of  Missouri  River  3  miles  below 
Williston  the  thickest  of  the  six  seams  present  is  only  2  feet  and  the  aggregate  thickness 
is  less  than  3  feet.  The  Medora  well  passed  through  941  feet  of  Laramie  strata  without 
reaching  their  base  and  encountered  seventeen  seams  with  a  total  thickness  of  more  than 
60  feet.  The  Dickinson  well,  with  a  depth  of  about  1,075  feet,  penetrated  sixteen  seams, 
their  aggregate  thickness  being  about  55  feet. a 
The  lignite  seams  vary  in  thickness  from  an  inch  and  less  to  33  feet.  Seams  6,  8,  or  10 
feet  thick  are  common;  seams  from  10  to  20  feet  thick  are  not  rare,  but  those  over  20  feet 
thick  seldom  occur. 
The  seams  are  usually  not  continuous  over  very  large  areas.  A  particular  seam  will 
pinch  out  within  a  mile  or  two  and  perhaps  be  replaced  by  another  at  about  the  same  or 
a  different  horizon.     Two  seams  may  overlap  each  other  and,  while  both  appear  at  one 
a  Darton,  N.  H.,  Preliminary  report  on  artesian  waters  of  a  portion  of  the  Dakotas:  Seventeenth 
Ann.  Rept.  U.  S.  Geol.  Survey,  pt.  2,  1896,  p.  663. 
