64  CONTKIBUTIONS   TO   ECONOMIC    GEOLOGY,   1907,   PAKT   II. 
south  is  nearly  25  miles  and  from  east  to  west  35  or  40  miles,  its  area 
being  about  750  square  miles.  The  rocks  in  which  the  coal  beds 
occur  extend  as  far  west  as  Dean  Creek,  but  the  coals  thin  out  in 
that  direction,  and  hence  the  area  underlain  by  workable  coal  beds 
may  be  regarded  as  bounded  on  the  west  by  the  divide  between 
Golden  and  Wildhorse  creeks.  The  area  treated  in  this  report  covers 
about  140  square  miles  in  the  southwestern  portion  of  the  field, 
including  Tps.  5  and  6  N.,  Us.  26  and  27  E.,  with  parts  of  bordering 
townships.     (See  PL  IV.) 
TOPOGRAPHY. 
The  topography  of  this  field  is  peculiar  and  distinct  from  that  of 
the  surrounding  country.  The  divide  between  Musselshell  and  Yel- 
lowstone rivers  extends  through  the  Bull  Mountains  with  a  general 
north  of  east  trend.  Prominently  rising  above  the  general  level  of 
this  divide  stands  an  isolated  group  of  flat-topped  mesas  whose  blue 
bluffs  and  reddened  caps  are  conspicuous  features  of  the  landscape; 
these  are  the  Bull  Mountains  proper.  To  the  north  and  to  the  south 
intermittent  streams  have  carved  radiating  valleys  that  are  canyon- 
like and  sparsely  tree  clad.  North  of  the  mountains  the  rough 
country  extends  to  Musselshell  River,  but  on  the  south  it  shortly 
breaks  into  rolling  hills,  which  extend  to  the  Yellowstone.  The 
mesas  of  the  Bull  Mountains  consist  of  arborescent  flat-topped  ridges 
whose  general  axis  has  a  west  of  north  direction.  There  are  three 
main  ridges  or  mesas,  with  a  few  bordering  buttes.  The  southern- 
most mesa  has  an  extent  of  nearly  4  miles  north  and  south  and 
about  2  miles  east  and  west.  The  two  irregular  mesas  which  make 
up  the  northern  part  of  the  mountains  have  general  dimensions  of  3 
miles  from  north  to  south  and  a  similar  distance  from  east  to  west. 
These  mesas  rise  from  600  to  700  feet  above  the  surrounding  country 
and  attain  an  altitude  of  about  4,700  feet  above  the  sea.  The  effect 
of  the  topography  on  the  accessibility  of  the  coals  is  discussed  under 
"Development,"  page  75. 
GEOLOGY. 
Throughout  the  Bull  Mountain  field  coal  of  value  occurs  chiefly 
in  the  Fort  Union  formation.  These  rocks  form  the  Bull  Mountains 
and  the  surrounding  country  for  a  distance  of  6  to  12  miles  in  three 
directions — that  is,  to  Musselshell  River  on  the  north,  Dean  Creek  on 
the  west,  and  T.  5  N.,  inclusive,  on  the  south.  Their  extent  toward 
the  east  is  not  yet  known,  as  that  part  of  the  field  was  not  surveyed 
during  the  last  season.  In  these  rocks  coal  beds  are  most  numerous 
in  the  upper  part,  that  is,  in  the  Bull  Mountain  mesas,  where  they 
occur  at  intervals  of  50  feet  or  less;  in  the  lower  part  the  intervals 
are  100  feet  or  more. 
