MONTANA. 
In  this  locality  the  Fort  Union  formation  presents  a  thickness  of 
1,400  feet,  but  this  probably  does  not  represent  the  whole  formation, 
a  portion  having  been  removed  by  erosion,  for  it  reaches  a  thickness 
of  several  thousand  feet  in  neighboring  localities,  such  as  the  Bighorn 
Basin  on  the  south.  It  is  composed  chiefly  of  gray  to  buff  sandstone 
alternating  with  gray  shale.  The  sandstone,  though  extremely 
variable,  is  commonly  massive  and  evenly  distributed  throughout 
the  section.  The  base  of  the  formation  is  strongly  marked  by 
contrast  with  a  band  of  olive-green  clay  shale,  which  belongs  to  the 
next  lower  formation.  This  shale  is  well  exposed  along  the  Billings- 
Roundup  road,  2  or  3  miles  south  of  Buckey  post-office,  and  may  be 
traced  westward  across  the  various  branches  of  Razor  Creek,  through 
Fratt's  ranch,  and  down  Dean  Creek  to  Musselshell  River.  Some  of 
the  best  exposures  are  found  on  the  east  side  of  Dean  Creek,  and  the 
same  greenish  shale  may  be  seen  a  short  distance  north  of  the  river 
at  Roundup.  In  a  general  way  this  shale  outlines  the  extent  of  the 
rocks  bearing  important  coal  beds,  though  it  must  not  be  supposed 
that  everywhere  the  Fort  Union  formation  contains  coal  beds  of 
workable  dimensions. 
.  A  general  section  of  the  Fort  Union  rocks  and  the  underlying 
olive-green  shale  given  on  Pi.  IV  shows  the  proportions  of  sandstone 
and  shale  and  the  relative  positions  of  the  various  coal  beds. 
The  geologic  structure  of  this  region  is  simple.  In  general,  the  rocks 
lie  flat.  Careful  observations,  however,  disclose  a  slight  and  con- 
sistent dip  of  1°  to  3°  in  a  northeasterly  direction  on  the  southwest 
margin  of  the  field,  but  this  dip  flattens  toward  the  mountains  and 
turns  northward  at  their  eastern  extremity.  On  the  northeast  side 
of  the  mountains  near  the  head  of  Fattig  Creek,  the  rocks  dip  slightly 
towrard  the  southwest,  and  near  Roundup  the  dip  in  this  direction 
is  very  pronounced.  Thus  the  structure  of  the  field  so  far  as  surveyed 
suggests  a  gentle  syncline  plunging  slightly  toward  the  northwest. 
COAL. 
GENERAL   STATEMENT. 
All  the  coals  in  the  Bull  Mountain  field,  so  far  as  studied,  may  be 
classed  as  a  good  grade  of  subbituminous a  coal  (" black  lignite"). 
Most  of  the  coal  beds  are  lenticular  in  shape,  showing  a  wide  variation 
in  thickness  at  different  points  on  the  outcrop.  Still,  this  habit  is 
more  common  with  some  coal  beds  than  with  others.  In  general  it 
may  be  stated  that  the  coal  beds  appear  to  thin  toward  the  west,  so 
that  beds  of  considerable  thickness  in  Rs.  26  and  27  E.  present  fit  tie  or 
no  coal  west  of  R.  26  E.  The  average  thickness,  however,  of  all  the 
coal  beds  in  the  portion  of  the  field  studied  would  probably  reach  a 
total  of  about  35  feet. 
a  For  the  sense  in  which  the  term  subbituminous  Is  used  in  this  papei  Bee  p.  12. 
71497— Bull.  341—09 5 
