98 
CONTRIBUTIONS   TO    ECONOMIC    GEOLOGY,   1907,   PART   II. 
coal  bed  No.  1  in  contact  with  coal  bed  No.  7.     The  location  of  the 
fault  and  its  known  extent  are  shown  on  PI.  VI,  and  its  effect  on  the 
strata  is  represented   in   fig.   2.     The   general 
\   fc  structural  features  from  Bridger  westward  past 
the  International  mine  to  the  base  of  the  moun- 
tain  slope  are  also  shown  in  the  same  figure. 
w 
h 
THE   COAL. 
GENERAL  CONDITIONS. 
The  coal  field  is  limited  on  the  south  by  the 
gradual  thinning  and  pinching  out  of  the  beds, 
on  the  east  and  north  by  the  line  of  outcrop  of 
the  lowest  bed,  and  on  the  southwest  by  a 
great  fault  along  the  foot  of  the  mountain 
slope.  Northwest  of  Red  Lodge  the  beds  pass 
beneath  the  gravel  covering  of  the  terrace  and 
continue  westward  beyond  the  boundary  of 
the  area  surveyed.  The  workable  coals  have  a 
vertical  range  of  about  800  feet,  as  shown  by 
the  above  section,  and  are  limited  both  above 
and  below  by  a  thick  mass  of  barren  strata. 
Greater  detail  of  the  coal-bearing  strata  may  be 
obtained  by  reference  to  the  section. 
This  section  shows  a  total  of  71  feet  of  coal 
in  beds  more  than  3  feet  thick.  From  point  to 
point  the  coal  beds  differ  somewhat  in  thick- 
ness and  character,  but  they  are  not  sufficiently 
variable  to  be  classed  as  lenticular  deposits.  In 
general  the  individual  beds  show  a  tendency  to 
decrease  in  thickness  from  northwest  to  south- 
east, a  condition  which  is  shown  by  a  compari- 
son of  the  sections  given  on  PI.  VI.  Another 
general  characteristic  is  the  predominance  of 
shale  as  a  roof  of  the  coal,  though  sandstones 
are  of  common  occurrence.  The  sandstone 
roofs  are  generally  wavy  and  contain  weak 
domelike  areas  from  1  to  15  feet  in  diameter, 
from  which  the  interior  easily  falls,  rendering 
mining  below  dangerous.  Very  little  gas  ac- 
companies the  coal  beds,  and  water  is  present 
only  in  small  quantities  except  where  mining 
is  carried  beneath  streams.  Outcrops  of  coal 
beds  may  be  traced  continuously  throughout 
the  field,  except  where  the  edge  of  the  bed  is  covered  for  short 
distances  by  gravel  or  talus.     The  coal  beds  are  easily  accessible  at 
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