124         CONTRIBUTIONS   TO   ECONOMIC   GEOLOGY,   1907,   PART  II. 
The  "writer  was  assisted  in  the  field  work  by  Albert  W.  Thompson 
and  Fred  II.  Kay,  and  in  the  office  by  Mr.  Kay.  The  larger  part  of 
the  results  obtained  in  the  season's  work  is  due  to  the  competent  serv- 
ices of  these  men.  The  officers  of  the  coal-mining  companies  located 
at  Dietz,  Carney ville.  Monarch,  and  Kooi,  together  "with  individual 
owners  of  coal  lands,  rendered  valuable  assistance. 
LOCATION  AND    GEOGRAPHIC   RELATIONS. 
The  Sheridan  coal  field  is  located  in  northeastern  Wyoming,  to  the 
east  of  and  near  the  Bighorn  Mountains.  The  northern  edge  of  the 
field  is  the  Wyoming-Montana  State  line,  and  is  arbitrarily  chosen 
>"ig.  3. — Index  map  showing  location  of  Sheridan  coal  field,  Wyoming. 
without  reference  to  the  boundary  of  coal-bearing  rocks.  Likewise 
the  eastern  and  southern  borders  are  conveniently  situated  on  range 
and  township  lines  and  have  no  particular  geologic  significance,  being 
chosen  to  mark  the  limits  of  the  surveys  of  the  season.  The  western 
boundary  of  the  coal  field,  however,  is  the  western  boundary  of  the 
rocks  that  contain  workable  coal  beds.  The  field  thus  defined  is  a 
part  of  the  Fort  Union  coal  region,  which  extends  from  northeastern 
Wyoming  into  eastern  Montana  and  North  Dakota.  The  Wyoming 
part  of  this  larger  region  is  in  the  wide,  flat  trough  or  basin  between 
the  Black  Hills  and  the  Bighorn  Mountains. 
