BULL   MOUNTAIN    COAL   FIELD,    MONTANA.  73 
feet  almost  everywhere.  In  the  northern  part  of  the  main  north  mesa 
it  is  exposed  at  few  places,  and  hence  its  thickness  here  is  questionable. 
The  Summit  coal  is  very  similar  to  the  Fattig  in  general  thickness,  but 
the  distribution  of  dimensions  is  different.  It  presents,  roughly,  less 
than  2  feet  of  coal  in  the  southern  portions  of  the  mesas  and  between 
2  and  3  feet  in  the  northern  portions.  In  the  northwest  mesa,  how- 
ever, it  is  generally  less  than  2  feet.  These  three  coal  beds  are  com- 
monly free  from  thick  partings.  Their  position,  however,  high  up  in 
the  mountains  makes  them  the  least  accessible  of  all  the  coal  beds  of 
the  region  surveyed. 
Near  the  crest  of  the  Bull  Mountains,  and  averaging  about  70  feet 
above  the  Summit  coal,  or  40  to  60  feet  below  the  tops  of  the  mesas, 
occurs  a  mass  of  clinkers  and  brecciated  rock  which  has  been  indu- 
rated and  reddened  for  a  vertical  distance  of  about  30  feet.  This 
doubtless  marks  a  bed  of  coal  of  considerable  thickness,  which,  owing 
to  the  thin  cover,  has  been  burned  through  its  entire  extent. 
PHYSICAL  CHARACTER. 
As  already  noted,  the  coals  of  this  region  are  soft  and  rather  easily 
reduced  to  a  fine  granular  mass  on  exposure  to  rain  and  sun.  This 
disintegration  is  accompanied  by  the  escape  of  gases,  which  may  be 
detected  for  some  distance  from  the  outcrop.  Though  no  instances 
of  resulting  spontaneous  combustion  were  observed,  it  is  believed  that 
this  is  perfectly  possible  and  has  been  the  cause  of  burning  of  some 
of  the  beds.  The  coals  on  fresh  surfaces  present  generally  a  lustrous 
black  appearance.  Still,  the  close  observer  may  detect  the  bedding 
by  minute  alternate  layers  of  dull  and  lustrous  coal.  These  are  seen 
to  best  advantage  on  clean  reflecting  joint  planes.  The  jointing  of 
the  coals  in  the  earlier  stages  of  disintegration  results  in  fragments 
having  a  more  or  less  cubical  form.  As  disintegration  progresses, 
however,  the  cubes  tend  to  break  down  generally  along  the  bedding 
into  small  square  plates.  Bedding  planes,  however,  are  not  always, 
or  perhaps  even  in  the  majority  of  cases,  the  lines  of  cleavage,  but 
rather  the  disintegration  of  the  cubes  is  by  conchoidal  fracture  roughly 
parallel  with  the  bedding  planes.  Indeed,  the  fracture  of  the  coals  is 
markedly  conchoidal.  If  the  coals  are  rubbed  against  unglazed  porce- 
lain the  resulting  streak  is  found  to  be  uniformly  grayish  or  blackish 
brown.  They  burn  with  a  yellowish  flame  of  moderate  length  and 
give  off  a  strong,  pungent  odor  of  sulphur.  The  remaining  ash  is  One, 
grayish,  and  not  inclined  to  be  clinkery  unless  the  coals  are  dirty. 
Some  of  the  coals  contain  small  globules  of  amber-colored  resin  and 
also  small  amounts  of  pyrite.  The  latter  is  in  places  distributed  in 
thin  flakes  along  the  joint  planes.  Even  the  best  coals  of  the  region 
contain  fragments  of  plants  that  are  still  brown  and  apparently 
unaffected  by  the  carbonization  which  the  coal  beds  have  undergone. 
