PITTSBURG  COAL  IN  THE  BURGETTSTOWN  QUADRANGLE, 
PENNSYLVANIA. 
By  W.  T.  Griswold. 
METHODS  OF  WORK. 
The  geologic  work  on  the  Burgettstown  quadrangle  has  been  car- 
ried on  in  what  might  be  called  an  instrumental  way.  The  combina- 
tion of  topographic  and  geologic  work  by  one  party  made  it  possible 
to  do  the  geologic  work  with  a  much  greater  degree  of  accuracy  than 
is  ordinarily  attained  in  such  work.  Important  outcrops  were  located 
on  the  map  with  the  same  accuracy  as  the  topographic  features  by 
the  traverse  party  when  the  map  of  the  roads  was  made.  Spirit-level 
lines  were  run  over  all  the  roads  of  the  quadrangle  with  a  degree  of 
precision  that  insured  the  closing  of  circuits  within  an  error  of  1 
foot.  As  this  work  proceeded  the  elevation  of  each  outcrop  that  could 
be  identified  was  determined,  and  the  name  of  the  formation  Avas 
marked  upon  the  map.  Profiles  were  then  made  of  all  the  roads 
which  gave  sections  of  the  geologic  formations.  Above  and  beloAV 
the  outcrop  of  a  stratum  of  economic  importance,  such  as  the  Pitts- 
burg coal,  the  hillsides  were  examined  for  good  reference  strata,  such 
as  solid  limestone  ledges  and  coal  seams,  and  the  elevations  of  these 
determined  by  spirit  level.  The  profiles  of  the  roads  and  the  special 
sections  were  then  critically  examined  for  the  purpose  of  determining 
the  interval  between  easily  recognized  beds,  which  might  serve  as  key 
rocks.  Only  those  outcrops  were  selected  that  were  horizontally  near 
together  or  were  in  such  a  position  that  the  elevation  of  the  outcrop  of 
one  stratum  could  be  compared  with  the  elevations  of  two  outcrops  of 
the  other  occurring  upon  different  sides  of  the  first  outcrop.  This 
method  was  used  to  eliminate  any  error  which  might  enter  into  the 
results  from  the  dip  of  the  rocks.  By  thus  determining  the  distance 
between  the  Pittsburg  coal  and  other  well-known  beds  in  different 
parts  of  the  quadrangle  the  relation  which  this  well-known  bed  bears 
to  others  was  determined.  Throughout  the  quadrangle  some  bed! 
were  found  to  be  in  almost  all  places  parallel  with  the  Pittsburg 
coal,  and  were  therefore  adopted  as  key  horizons.  The  most  impor- 
tant of  these  are  a  thin  bed  of  limestone  near  the  center  of  the  Ben- 
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