MILES    CITY    COAL   FIELD,   MONTANA.  45 
regions  comparatively  little  coal  is  exposed  in  natural  outcrop,  but 
the  beds  present  are  more  uniformly  valuable  and  the  areas  which  can 
be  regarded  as  coal  land  are  larger  than  in  the  badlands. 
EVIDENCE  OF  BURNED   COAL. 
In  many  places,  especially  in  the  areas  underlain  by  the  upper  mem- 
ber, the  thicker  beds  of  lignite  have  burned  along  their  outcrop,  and 
the  adjacent  rocks,  together  with  the  clay  and  sand  partings  of  the 
lignite  beds,  have  been  metamorphosed  and  hardened.  The  resulting 
rock  has  a  general  red  color,  though  it  ranges  from  only  slightly 
altered  shale  and  sandstone  to  black  vesicular  slag  resembling  lava, 
for  which  it  has  often  been  mistaken.  The  prevailing  red  color  is 
doubtless  due  to  the  presence  of  iron,  which  has  been  partly  reduced, 
so  that  these  rocks  interfere  slightly  with  the  working  of  the  magnetic 
needle.  Where  the  cover  is  great  the  extent  of  burning  back  from 
the  outcrop  is  limited,  and  it  can  probably  be  safely  assumed  that  the 
lignite  is  intact  under  a  cover  of  100  feet.  There  are,  however,  large 
areas  fringed  by  outcrops  of  clinker  in  which  the  cover  is  thin  and 
where  the  burning  seems  to  have  extended  back  for  an  indefinite  dis- 
tance. Many  extensive  mesas  are  capped  with  clinker  under  a  slight 
cover  of  soil  and  in  such  places  it  is  difficult  to  distinguish  the 
unburned  from  the  burned  areas.  The  unburned  lignite,  where  it  out- 
crops on  grassy  slopes,  is  usually  covered  with  soil  and  is  hidden.  Its 
presence  is  here  and  there  revealed  by  the  burrowings  of  prairie 
dogs  or  by  the  cutting  of  streams,  but  such  evidences  are  rare.  That 
much  of  the  burning  occurred  as  early  as  Pleistocene  time  is  indicated 
by  clinker  pebbles  in  the  bench  gravels  of  some  of  the  canyons,  as  well 
as  by  the  extensive  erosion  and  dissection  of  the  clinker  beds.  The 
topography  produced  offers  an  important  line  of  evidence  as  to  the 
extent  of  the  burning.  Many  of  the  unburned  areas  occur  in  rounded 
hills,  which  are  fringed  with  clinker-covered  benches  and  low  ridges, 
separated  from  the  unburned  areas  by  shallow  valleys,  a  few  rods 
wide,  due  to  erosion.  Such  valleys  have  clinkers  on  one  side  and 
grassy  slopes  on  the  other.  There  are  many  variations  of  this  feature, 
which  depend  on  the  amount  of  erosion  since  the  burning  and  the 
relation  of  the  general  drainage.  Many  of  the  streams  flowing  from 
the  unburned  areas  across  the  clinker  rim  have  in  their  upper  parts 
wide  valleys  with  grassy  slopes,  which  contract  to  box  canyons  in 
crossing  the  burned  portions.  An  example  of  this  feature  can  be  seen 
at  the  edge  of  the  Pine  Hills,  10  miles  out  from  Miles  City  on  the 
Ekalaka  road.  This  road  crosses  a  clinker  rim  through  a  narrow  can- 
yon which  widens  into  a  small  valley  above  the  rim,  and  the  wash  in 
the  side  of  the  road  here  reveals  the  blossom  of  unburned  coal  at  the 
level  of  the  clinker. 
