66  CONTRIBUTIONS   TO   ECONOMIC    GEOLOGY,   1907,   PART   II. 
Though  the  coals  occur  in  lenses,  an  effort  was  made  to  locate  and 
define  the  workable  portions  of  the  coal  beds  from  a  study  of  the 
outcrop  alone.  The  method  of  work  may  be  outlined  briefly  as 
follows:  A  careful  search  was  made  in  coulees  and  ravines  over  the 
whole  area  for  outcrops  of  the  coal  beds.  All  coals  thus  discovered 
were  traced  throughout  the  area,  but  only  those  having  a  thickness 
of  2  feet  or  more  were  mapped.  The  ordinary  difficulties  of  mapping 
presented  by  the  rough  country,  deeply  cut  by  canyons,  were  further 
aggravated  by  grassed  slopes,  forested  ridge,  and  slumping  talus, 
which  tend  to  obscure  outcrops  of  the  coal.  There  is  a  marked 
difference,  however,  in  this  respect  between  the  two  sides  of  the  Bull 
Mountains,  the  coals  being  much  more  obscure  on  the  north  side  than 
on  the  south  side,  where  erosion  seems  to  be  more  active.  Wherever 
the  outcrop  was  thus  covered  the  horizon  was  traced  by  following 
adjacent  rocks.  The  traverses  of  the  coal  outcrops  were  made  by 
compass  and  pacing,  on  foot  in  rough  country  and  on  horseback  in 
open  areas,  each  traverse  being  tied  to  section  and  quarter-section 
corners.  The  error  in  such  work  was  found  rarely  to  exceed  550  feet 
per  mile,  and  in  nearly  every  case  it  was  within  270  feet  per  mile. 
It  was  impossible  to  find  corner  stones  at  every  desired  point, 
especially  in  townships  that  were  surveyed  fifteen  or  twenty  years 
ago,  for  the  reason  that  many  of  the  monuments  then  set  have  been 
obliterated  by  the  elements.  The  map  (PI.  IV)  shows  the  number  of 
corners  found.  It  was  found  that  traverses  of  3  or  4  miles  could  be 
made  within  a  reasonable  closing  error,  which  was  distributed  along 
the  traverse.  The  traverses  of  the  outcrops  of  the  most  important 
coal  beds  are  plotted  on  PL  IV  by  means  of  solid,  broken,  and  dotted 
lines,  which  represent  within  certain  rough  limits  the  thicknesses  of 
the  coal  beds  and  the  position  of  the  outcrops  with  reference  to 
section  corners. 
DETAILED    DESCRIPTIONS. 
In  this  discussion  the  coals  will  be  taken  up  in  order,  beginning  with 
the  lowest  workable  bed  and  continuing  upward  to  the  highest  bed  in 
the  Bull  Mountains  proper.  Named  in  this  order  the  coal  beds  are 
as  follows:  Glendive,  Wildhorse,  Buckey,  Dorrity,  Pompey,  Mam- 
moth, Render,  Rock  Mesa,  Matt,  Bull  Mountain,  Wescott,  Strait, 
Red  Butte,  Fattig,  and  Summit. 
The  Glendive  coal  is  of  very  poor  quality,  though  the  bed  reaches  a 
general  thickness  of  3  to  6  feet.  It  occurs  in  the  southern  portions  of 
T.  5  N.,  Rs.  26  and  27  E.  (P  coal,  PI.  IV),  and  at  one  point  extends 
about  three-fourths  of  a  mile  south  of  the  southern  boundary  of  the 
latter  township.  It  forms  on  weathered  exposures  a  conspicuous 
black  band  in  the  soil,  which  is  well  shown  along  the  Billings  road, 
about  3  miles  south  of  Buckey  post-office,     Like  many  of  the  higher 
