18  CONTRIBUTIONS   TO   ECONOMIC    GEOLOGY,   1907,   PART   II. 
rendered  hard  and  resistant  by  the  heat  of  the  burning  lignite,  have  been 
an  important  factor  in  the  formation  of  the  picturesque  badlands. 
GEOLOGY. 
STRATIGRAPHY. 
FORT   UNION    FORMATION. 
Practically  all  of  the  strata  in  the  area  under  discussion  belong  to 
the  Fort  Union  formation  (early  Eocene).  In  common  with  the  rocks 
of  the  adjoining  regions  they  have  in  the  past  been  generally  referred 
to  the  Laramie,  but  according  to  F.  H.  Knowlton,  who  spent  some 
time  in  this  field  in  the  summer  of  1907  making  collections  of  plant 
remains,  their  Fort  Union  age  is  unmistakable. 
There  is  a  very  noticeable  difference  between  the  lower  Fort  Union 
beds,  which  outcrop  in  the  bluffs  bordering  Little  Missouri  River,  and 
the  upper  beds,  occurring  in  the  tops  of  the  higher  ridges,  divides,  and 
buttes,  usually  back  some  distance  from  the  river.  The  lower  mem- 
ber is  composed  of  buff  and  light  ash-gray  clays  and  sands  in  alternate 
layers.  The  upper  member  is  formed  of  strata  considerably  darker  in 
appearance,  mostly  dark  gray,  with  many  brown,  ferruginous,  sandy 
nodules  and  concretions.  The  contrast  between  these  members  is  so 
well  marked  and  their  contact  so  clearly  defined  that  it  can  be  readily 
distinguished  at  a  distance  and  traced  without  difficulty  wherever  it 
is  exposed.  Over  most  of  the  eastern  half  of  the  field  a  thick  bed  of 
lignite  or  a  layer  of  red  clay  formed  by  the  burning  of  the  lignite  occurs 
just  at  the  contact  of  the  upper  and  lower  members.  But  even  where 
the  coal  or  burnt-clay  bed  is  wanting,  the  line  of  separation  is  readily 
discernible.  The  lignite  beds  appear  equally  in  both  divisions,  though 
there  are  perhaps  more  in  the  lower.  Of  the  workable  lignite  beds 
shown  in  the  generalized  section  (PL  II)  the  lower  five  are  in  the  lower 
member,  the  next  separates  the  two  members,  and  the  other  three 
beds  are  in  the  upper  division. 
The  character  of  the  Fort  Union  formation  is  well  shown  by  the  fol- 
lowing sections : 
Section  in  river  bluff  about  one-fourth  mile  below  the  crossing  at  Short's  ranch,  in  the 
SE.  I  sec.  1,  T.  142  N.,  R.  102  W. 
Ft.    in. 
Clay  and  sand,  buff  and  gray,  on  which  rest  the  somber  beds  of 
the  upper  member 17 
Lignite 1    6 
Sand,  fine  grained;  contains  some  clay,  buff  and  gray 77 
Lignite 1 
Clay,  gray  and  yellow 16 
Lignite,  impure,  containing  two  thin  clay  seams 1 
Clay  and  sand 17 
Clay,  brown • 1 
Clay,  gray 7    6 
Lignite  and  some  brown  clay 6 
Clay,  blue  and  yellow 7 
