70  CONTRIBUTIONS   TO   ECONOMIC   GEOLOGY,  1907,   PART  II. 
The  Rehder  coal  bed  (J  coal,  PL  IV)  is  unimportant  in  the  area 
surveyed.  It  occurs  about  25  feet  above  the  Mammoth  bed,  from 
which  it  is  separated  by  a  massive  sandstone,  and  its  best  develop- 
ment is  at  the  head  of  Half  breed  Creek,  in  the  northeast  corner  of 
T.  6  N.,  R.  26  E.  It  is  very  lenticular,  and  at  this  point  only  does 
it  reach  a  thickness  of  more  than  2  feet.  It  is  probably  nowhere 
more  than  3  feet  thick. 
About  25  feet  above  the  Rehder  coal  there  is  a  more  persistent  bed 
named  the  Rock  Mesa  coal  (I  coal,  PI.  IV).  This  bed  is  almost 
everywhere  present,  and  follows  closely  the  outcrop  of  the  Mam- 
moth bed.  It  is,  however,  erratic  in  that  its  persistent  partings  here 
and  there  thicken  to  proportions  which  destroy  the  value  of  the  coal 
bed  for  commercial  purposes.  In  T.  6  N.,  R.  26  E.,  this  coal  locally 
develops  a  thickness  between  2  and  3  feet.  It  attains  a  like  thick- 
ness in  the  southwest  corner  of  T.  6  N.,  R.  27  E.,  but  from  this  point 
eastward  the  bed  is  usually  less  than  2  feet  thick.  North  of  the  Bull 
Mountains,  however,  especially  in  the  northeast  corner  of  T.  6  N., 
R.  27  E.,  the  bed  is  remarkably  uniform  in  character  and  thickness, 
presenting  almost  everywhere  2  to  3  feet  of  good  coal.  In  this 
vicinity,  too,  the  coal  is  particularly  free  from  partings,  whereas  on 
the  southwest  side  of  the  mountains  partings  are  more  numerous. 
The  following  section  is  typical  of  the  latter  locality: 
Section  of  Rock  Mesa  coal  bed  in  sec.  SO,  T.  6  N.,  R.  27  E. 
Ft.  in. 
Shale,  bituminous 6 
Coal 11 
Shale 2 
Coal 7 
Shale 1 
Coal 8 
2  11 
For  170  feet  above  the  Rock  Mesa  coal  the  rocks  are  destitute  of 
coal  beds  of  any  value,  but  at  this  distance  appears  the  Matt  coal, 
which  is  very  persistent  throughout  the  area.  On  the  south  side  of 
the  mountains  its  usual  position  is  about  20  or  30  feet  above  the 
massive  sandstone  which  skirts  the  foot  of  the  bluffs.  Generally 
it  is  less  than  2  feet  thick,  except  at  the  head  of  Dorrity  Coulee,  in 
the  northwestern  portion  of  T.  6  N.,  R.  27  E.,  where  it  is  best  devel- 
oped on  the  southeast  branches  and  northeast  side  of  the  main 
coulee.  In  the  former,  especially  those  locally  known  as  Strait 
and  John  Matt  coulees,  it  is  between  4  and  5  feet  thick,  though 
generally  the  lower  measurement  prevails.  Toward  the  northwest, 
along  the  northeast  side  of  Dorrity  Coulee,  the  coal  is  thinner,  rang- 
ing between  2  and  3  feet.  This  coal  bed  is  generally  free  from  part- 
ings, but  in  places  has  commonly  one  thin  parting,  as  shown  by  the 
following  section: 
