346  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1903.  [bull. 225. 
encircling  the  basin,  the  maximum  width  being  found  along  Goose- 
berry Creek  and  the  minimum  in  the  vicinity  of  Clarks  Fork  Canyon. 
Along  the  eastern  edge  of  the  basin  a  uniform  width  of  4  to  5  miles  is 
maintained.  The  dips  vary  from  1°  to  60°,  the  strike  circling  around 
parallel  to  the  periphery  of  the  basin  syncline,  the  average  inclination 
of  the  beds  rarely  exceeding  25°.  The  base  of  the  formation  is  dis- 
tinctly marked  by  an  escarpment  of  sandstone  overlooking  the  soft 
shales  of  the  underlying  formation. 
Owing  to  the  changeable  character  of  the  Laramie  beds  no  distin- 
guishing stratigraphic  subdivisions  exist,  so  there  is  no  means  for 
determining  the  horizon  of  a  particular  coal  bed  except  by  direct  meas- 
urement from  the  base,  or  by  reference  to  some  of  the  broad  general 
divisions  of  the  formation.  Accordingly  it  has  been  practicable  in 
the  present  reconnaissance  to  ascertain  only  the  approximate  position 
of  the  various  beds.  The  entire  series  of  the  Laramie  beds  in  the 
Bighorn  Basin  is  segregated  for  reference  into  three  general  divisions, 
the  lower  300  to  400  feet,  chiefly  sandstones;  the  middle  300  to  700  ; 
feet,  shales  and  clays,  and  an  overlying  series  of  sandstone  and  shale 
of  undetermined  thickness. 
The  coal  deposits  of  the  Laramie  formation  within  the  Bighorn 
Basin  exhibit  the  usual  variation  in  character  encountered  in  the, coal 
measures  of  this  formation  elsewhere.  The  most  common  variation  is 
due  to  the  presence  of  shale  and  sandstone  partings,  separating  the 
beds  into  ""benches."  Sometimes  these  partings  so  increase  in  num- 
ber and  extent  over  a  limited  area  as  to  render  a  part  of  the  deposit 
worthless.  Not  infrequently  numerous  slaty  layers  contained  in  the 
coal  bed  at  the  outcrop  will  be  seen  to  disappear  entirely  within  a  few 
hundred  feet  of  the  entrance  to  the  tunnel.  In  one  localit}^  examined 
the  lower  layer  of  a  coal  horizon  was  represented  at  the  outcrop  by 
brown  carbonaceous  shale,  with  minute  streaks  or  threads  of  coal.  At 
a  distance  of  100  feet  down  the  main  entry  the  proportion  of  shale  to 
coal  changed  to  such  an  extent  as  to  render  the  deposit  workable. 
While  the  coal  deposits  of  the  Laramie  formation  in  the  Bighorn  Basin 
are  usually  of  a  variable  nature,  there  are  exceptions.  Thus,  the  lower 
coal  deposit  of  the  basal  sandy  series,  with  a  characteristic  bluish- 
gray,  slaty  parting  near  the  middle,  presents  considerable  uniformity 
all  along  the  west  side  of  the  basin.  Many  of  the  smaller  beds  in  the 
middle  shaly  division  also  exhibit  a  marked  degree  of  persistence,  but 
are  too  thin  to  be  worked  separately. 
In  the  following  discussion  reference  will  be  made  to  the  above 
divisions  as  the  " basal  sandy  series,"  the  "middle  shaly  series,"  and 
the  "upper  sandy  series." 
The  basal  sandy  series  contains  two  workable  beds,  the  lower  and 
more  important  occurring  about  100  feet  above  the  base  of  the  forma- 
