RED   LODGE   COAL   FIELD,    MONTANA.  97 
The  upper  barren  member  of  the  Fort  Union  formation  is  com- 
posed, like  the  lower  member,  of  sandstone  and  shale,  with  a  very 
little  carbonaceous  material.  Shale  predominates  in  the  lower  por- 
tion and  sandstone  in  the  upper  portion.  As  previously  stated,  no 
workable  coals  are  present  in  this  member. 
Since  the  Fort  Union  sediments  were  deposited  they  have  been 
tLxd  and  eroded  and  in  places  covered  by  gravel  and  alluvium. 
The  terrace  deposits,  which  consist  mostly  of  coarse  sand  and  gravel 
with  bowlders  here  and  there,  are  spread  as  a  thin  covering  over  the 
older  rocks.  They  conceal  the  coal  on  the  flat-topped  divide  between 
Bear  Creek  and  Ked  Lodge,  and  cover  the  beds  on  the  west  side  of 
Rock  Creek.  The  alluvial  deposits  are  confined  to  the  Rock  Creek 
valley,  where  they  cover  the  coal  beds  to  a  slight  extent  about  Red 
Lodge. 
Small  dikes  cut  the  sedimentary  rocks  and  outside  of  the  gravel- 
covered  area  form  low  dark-colored  ridges  extending  in  a  northwest- 
southeast  direction  across  the  field.  The  rocks  composing  these  dikes 
have  been  examined  by  Albert  Johannsen,  of  the  United  States 
Geological  Survey,  and  found  to  be  camptonite.  The  dikes  are  so 
thoroughly  disintegrated  that  they  interfere  very  little  with  coal 
mining  where  they  have  been  encountered.  They  seem  to  have 
caused  no  displacement  of  the  strata  and  to  have  had  little  meta- 
morphic  effect  on  the  surrounding  rocks.  Where  the  beds  are  cut 
the  coal  is  affected  only  for  a  very  short  distance  from  the  dike,  and 
anthracitization  was  not  observed  at  any  place,  though  charred  coal 
was  found  near  the  contact. 
STRUCTURE. 
Structurally  the  rocks  form  part  of  an  eroded  monocline  which  dips 
southwestward  from  the  Pryor  Mountains  to  the  Beartooth  Range, 
where  it  is  terminated  by  a  fault  having  a  throw  of  several  thousand 
feet.  The  dip  of  the  beds,  which  is  18°  at  Red  Lodge,  decreases  south- 
eastward to  9°  in  the  western  part  of  the  Bear  Creek  region  and  to  3° 
a  short  distance  farther  south.  Near  the  southern  limit  of  the  field 
the  beds  are  horizontal.  They  show  minor  undulations  in  many 
parts  of  the  field,  and  between  the  International  and  the  Smokeless 
and  Sootless  mines  are  lifted  into  a  small  dome,  which  is  slightly 
elongated  in  outline  and  is  less  than  a  quarter  of  a  square  mile  in  area. 
The  beds  are  steeply  tilted  on  the  flanks  of  the  dome  and  arched 
sharply  over  the  top.  A  fault  breaks  the  beds  on  the  northwest 
side  near  the  south  line  of  the  SE.  i  sec.  31,  T.  7  S.,  R.  21  E.,  and 
extends  irregularly  N.  70°  W.  into  sec.  30,  where  it  passes  beneath  a 
deposit  of  gravel.  The  fault  seems  to  be  nearly  vertical,  with  the 
downthrow  on  the  east.  The  displacement  of  the  strata  on  the 
north  side  of  sec.  31  is  between  500  and  600  feet,  or  sufficient  to  bring 
71497— Bull.  341—09 7 
