COAL    NEAR    CRAZY    MOUNTAINS,    MONTANA.  83 
are  the  masses  of  theralite  and  theralite  porphyry  which  cap  Coffin 
Butte ;  the  6,400-foot  hill  2  miles  to  the  northeast  of  it ;  a  small  butte 
2  miles  west  of  the  Miller  ranch,  on  the  west  range  line  of  T.  7  N., 
R.  12  E.,  and  Gordon  Butte.  These  igneous  masses  are  believed 
to  be  the  remnants  of  laccoliths.  Near  Lennep,  which  is  close  to 
the  Crazy  Mountains,  dikes  and  sheets  of  andesite  porphyry  and 
theralite  are  particularly  numerous.  On  the  west  side  of  Comb  Creek 
sheets  of  igneous  rock  in  upturned  shaly  beds  weather  as  parallel  walls 
and  resemble  dikes  in  appearance. 
STRUCTURE. 
As  mentioned  above,  the  Crazy  Mountains  are  composed  of  granitic 
rocks  intruded  through  sediments  that  dip  gently  toward  the  moun- 
tains. Coincident  with  the  intrusion  of  the  great  masses  of  igneous 
material  which  form  the  mountains,  the  region  was  subjected  to 
strains  which  produced  folds  and  domes,  the  former  in  some  places 
many  miles  in  extent. 
The  upturned  edges  of  the  higher  rocks  down  to  and  including  the 
Judith  River  formation  describe  a  fairly  regular  concentric  belt  around 
the  base  of  the  mountains,  toward  which  these  rocks  dip.  Out  in  the 
valley,  where  these  beds  have  been  worn  away  and  the  Claggett,  Eagle, 
and  Colorado  formations  underlie  the  surface,  the  structure  is  less 
regular,  consisting  of  gentle  folds  and  low  domes.  Erosion  has 
reduced  these  forms,  cutting  away  the  less  resistant  beds  and  leaving 
the  harder  ones  standing  out  as  ridges.  The  outcrop  of  the  Eagle 
sandstone  shown  on  the  accompanying  map  (PI.  V)  suggests  the  com- 
plexity of  the  structure  in  the  upper  valley  of  the  Musselshell.  The 
southern  half  of  an  eroded  anticline  having  a  northeast-southwe.  t 
axis  is  indicated  by  the  outcrop  of  the  Eagle  sandstone  winch  crosses 
the  river  near  the  mouth  of  American  Fork  and  again  8  miles  below 
Shawmut.  South  of  Twodot  and  Martinsdale  the  structural  features 
are  round  domes,  but  farther  west,  toward  the  north  end  of  the  Crazy 
Mountains,  folds  are  most  pronounced.  On  the  west  side  of  the  Crazy 
Mountains,  however,  the  structure  is  simpler.  Close  to  the  moun- 
tains a  number  of  parallel  folds  form  the  ridges  between  Shields  River 
and  Potter  Creek,  but  the  Potter  Creek  valley  is  a  syncline,  the  rockt 
on  the  east  side  dipping  west,  those  on  the  west  side  dipping  east,  and 
those  lying  in  the  middle  of  the  valley  being  practically  flat. 
THE   COAL. 
GENERAL  DESCRIPTION. 
Although  the  area  included  in  the  Little  Belt  Mountains  quadrangle 
has  been  reported"  to  contain  several  coal  beds  of  workable  thick- 
ness, none  was  found.     Coal  has  been  discovered  and  prospected  a! 
a  Op.  cit. 
