MILES   CITY   COAL  FIELD,   MONTANA.  43 
Coal  beds  in  this  member  are  not  so  numerous  near  Miles  City  as  in 
the  lower  member,  but  are  more  constant  in  thickness  and  quality. 
The  workable  coal  is  confined  to  two  beds,  160  and  300  feet  above  the 
base  of  the  member. 
In  some  places  these  rocks  give  rise  to  badland  topography,  but  in 
general  the  features  are  larger  than  those  produced  in  the  somber- 
colored  beds  below;  the  hills  and  buttes  are  higher  and  the  level 
stretches  more  extensive.  Calcareous  layers  such  as  occur  in  the 
lower  beds  are  not  expressed  in  the  topography,  but  many  of  the 
buttes  are  capped  with  the  clinkers  of  burned  coal.  The  character- 
istic features  of  these  two  members  of  the  Fort  Union  formation 
indicate  conclusively  that  the  conditions  which  prevailed  during  the 
deposition  of  the  lower  member  were  much  more  changeable  than 
those  during  the  deposition  of  the  upper.  When  the  lower  beds  were 
deposited  shallow- water  conditions  evidently  prevailed,  with  shifting 
currents  and  changes  of  elevation  too  frequent  to  permit  the  forma- 
tion of  extensive  beds  of  coal.  The  carbonaceous  materials  in  many 
of  the  coal  beds  are  thoroughly  macerated,  and  no  doubt  in  some 
places  were  partially  decomposed  by  exposure  to  air  before  the  over- 
lying strata  were  laid  down.  During  the  deposition  of  the  upper  beds 
general  conditions  were  more  stable,  and  consequently  the  different 
layers  of  this  part  of  the  formation  are  more  uniform  both  in  thickness 
and  distribution.  Though  there  is  some  false  bedding,  it  is  neither 
so  common  nor  so  well  marked  as  in  the  lower  beds.  The  coals  retain 
more  of  their  original  woody  texture  and  are  distinctly  lignitic  in 
appearance. 
STRUCTURE. 
Throughout  the  area  under  discussion  the  strata,  except  where  they 
are  locally  displaced  by  surface  influences,  lie  perfectly  flat.  No 
deep-seated  faults  have  broken  them,  though  in  some  localities  dis- 
placements of  a  few  feet,  attributable  to  purely  local  conditions,  have 
been  observed.  As  a  result  of  this  simple  structure  no  rocks  lying 
below  the  level  of  Yellowstone  River  are  exposed.  Individual  coal 
beds  may  be  traced  and  correlated  with  certainty  throughout  the 
field,  and  their  outcrops  coincide  with  contour  lines.  The  map 
(PI.  Ill)  showing  in  detail  the  outcrops  of  the  more  important  coal 
beds  has  the  appearance,  therefore,  of  a  contour  map  with  irregular 
contour  interval. 
Some  facts,  however,  indicate  that  either  all  or  a  large  part  of  the 
field  lies  in  a  shallow  syncline  or  basin.  A  number  of  wells  drilled  at 
Miles  City  yield  a  fair  flow  of  artesian  water  which,  in  order  to  reach 
the  surface,  must  have  descended  along  bedding  planes  from  some 
higher  levels.  It  is  thought  that  the  source  of  this  water  is  to  the 
west  of  the  field,  where  there  is  some  evidence  that  underlying  Cre- 
taceous rocks  outcrop  at  Howard,  on  the  Yellowstone.     The  rise  of 
