LEWISTOWN    COAL    FIELD,    MONTANA.  Ill 
The  Colorado  shale  decreases  in  thickness  northward  from  the  Big 
Snowy  Mountains,  for  a  section  of  that  formation  to  the  south  of  the 
uplift  shows  that  the  interval  between  the  Kootenai  red  shale  and 
the  Eagle  sandstone  is  2,400  feet,  whereas  the  same  interval  15  miles 
northeast  of  Lewistown  is  less  than  1 ,600  feet. 
Throughout  the  field  the  Kootenai  is  fairly  uniform  in  thickness, 
although  the  individual  members  comprising  the  formation  vary 
locally.  As  a  formation  it  is  readily  distinguished  by  the  bright, 
maroon-colored  shales  and  by  the  coarsely  cross-bedded  sandstone 
just  above  the  coal.  This  sandstone  in  many  places  produces 
marked  topographic  features,  standing  out  in  bold  ridges,  which  are 
usually  timbered  with  pines.  So  characteristic  is  the  association  of 
pines  and  other  vegetation  with  this  sandstone,  especially  where  it 
is  exposed  over  a  considerable  area,  that  the  growth  serves  as  a 
valuable  guide  in  the  search  for  coal. 
In  the  vicinity  of  the  Moccasin  Mountains  and  to  the  south  of  the 
Judith  uplift,  along  the  McDonald  Creek  divide,  certain  areas  are 
overlain  by  a  massive  white  limestone  from  50  to  250  feet  thick, 
which  locally  resembles  the  upper  member  of  the  Madison  limestone. 
Its  structural  relations,  however,  as  well  as  the  character  of  the  rock, 
preclude  the  possibility  that  it  belongs  to  the  Carboniferous  system; 
it  represents  instead  remnants  of  a  formerly  extensive  hot-spring 
deposit.  The  fact  that  hitherto  it  has  been  considered  as  Madison 
limestone  has  delayed  to  some  extent  the  development  of  coal  in 
certain  localities.  This  is  especially  true  with  reference  to  Flat 
Mountain,  the  southernmost  extension  of  the  Judith  uplift,  where  it 
was  believed  that  the  coal  bed  would  be  cut  off  by  the  limestone 
within  a  short  distance  from  the  outcrop.  Wherever  the  spring 
deposit  overlies  the  coal-bearing  formation,  however,  there  seems  no 
reason  to  conclude  that  mining  conditions  are  rendered  especially 
unfavorable  thereby. 
STRUCTURE. 
MAJOR    FEATURES. 
The  structure  in  the  western  part  of  the  Lewistown  coal  field  is 
relatively  simple,  the  beds  dipping  at  a  slight  angle  away  from  the 
Little  Belt  and  Big  Snowy  mountains.  In  general  this  relation  pre- 
vails throughout  the  area  west  of  Judith  River  and  Ross  Fork  up  to 
the  very  base  of  the  Little  Belt  Range,  where  the  strata  stand  prac- 
tically vertical  or  even  dip  at  a  high  angle  to  the  south.  The  struc- 
ture of  the  east  end  of  this  range,  therefore,  is  that  of  an  anticline 
overturned  to  the  north.  The  Big  Snowy  Mountains  are  also  anti- 
clinal in  structure  and  are  apparently  slightly  overturned  to  the 
south  near  their  west  end.  North  of  these  mountains  the  strata  in 
general  dip  away  from  the  uplift  at  a  low  angle,     The  ends  of  the 
