132        CONTRIBUTIONS   TO   ECONOMIC   GEOLOGY,   1907,   PART  II. 
STRUCTURE. 
The  rocks  in  the  western  part  of  the  Sheridan  field  are  tilted  at 
low  angles  in  a  generally  eastward  direction.  From  the  Tongue 
River  valley  southward  on  the  strike  of  the  lowest  coal  outcrop  the 
rocks  dip  about  4°  E.  Northward  from  Tongue  River  the  dip  is 
toward  the  southeast  at  about  the  same  angle.  These  dips  produce 
a  shallow  eastward-pitching  trough  or  basin.  East  of  the  mouth  of 
Goose  Creek  this  basin  structure  is  scarcely  perceptible.  Similar 
structure,  if  it  is  not  the  same  shallow  trough,  can  be  seen  in  the 
western  slopes  of  the  Badger  Hills,  in  T.  58  N.,  R.  82  W.,  when 
viewed  from  a  distance.  The  warping  of  the  strata  here  is  so  slight  that 
it  is  not  perceived  unless  a  considerable  part  of  the  broad  fold  is  in 
view.  North  of  the  mouth  of  Goose  Creek  the  northeasterly  dip, 
though  slight,  is  resumed.  The  tilt  of  the  rocks  eastward  grows 
.gradually  less  on  the  whole,  but  with  local  variations  in  degree.  On 
the  east  side  of  the  Goose  Creek  valley  the  dips  average  about  3°. 
In  the  Prairie  Dog  Creek  valley  it  is  only  1°.  Farther  east  and 
continuing  to  the  Powder  River  valley,  beyond  the  area  mapped, 
the  rock  strata  are  essentially  horizontal. 
THE  COAL. 
GROUPING  OF  COAL  BEDS. 
As  noted  previously,  the  coal  beds  of  this  field  are  divided  into 
three  groups,  each  group  depending  on  the  association  of  certain 
beds  and  their  seeming  continuity  in  lateral  extent. 
Order  and  thickness  of  the  coal  beds  in  the  Sheridan  field. 
ULM    COAL    GROUP. 
Feet. 
Upper  Ulm  coal  bed  (variable) 12 
Lower  Ulm  coal  bed  (variable) 16 
INTERMEDIATE    COAL    GROUP. 
Several  workable  coal  beds  of  local  extent,  probably  eight  in 
number,  varying  in  thickness  up  to  9  feet;  total  maximum  thick- 
ness about  30  feet. 
TONGUE    RIVER    COAL    GROUP. 
Roland  coal  bed  (variable) 13 
Smith  coal  bed  (variable) 5 
Dietz  coal  bed  No.  1 8 
Dietz  coal  bed  No.  2 8 
Dietz  coal  bed  No.  3  (variable) 12 
Monarch  coal  bed 18-32 
Carney  coal  bed 12-17 
Masters  upper  coal  bed 6 
Masters  lower  coal  bed 6 
Estimated  average  of  known  exposures 149 
