296  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1903.  [bull.225. 
GENERAL  GEOLOGY. 
Stratigraphy. — The  rocks  exposed  at  the  surface  of  this  field  are  all 
of  Carboniferous  age  (Pennsylvanian  series)  and  belong  to  three  forma- 
tions, Pottsville,  Allegheny,  and  Conemaugh.  In  this  district  the 
only  exposure  of  Pottsville  sandstone,  which  is  the  lowest  of  the 
formations,  occurs  at  Thomas  Mill,  about  2  miles  north  of  Patton 
on  Chest  Creek,  where  it  is  shown  in  the  wagon  road  50  feet  above 
railroad  grade.  Immediately  above  it  the  "A"  coal  outcrops  in  the 
road.  Just  above  the  Pottsville  formation  occurs  the  Allegheny, 
which  is  the  coal-bearing  formation. 
The  coal  beds  of  this  region  were  named  by  early  workers  in  the 
the  field,  UA,"  "B,"  "C,"  etc,  extending  from  coal  "A"  at  the  base  to 
coal  "  E"  at  the  top  of  the  Allegheny  formation.  Later  the  geologists 
of  the  second  survey  of  the  State  correlated  these  beds  with  those  of  the 
type  locality  in  the  Allegheny  Valley.  This  correlation  has  not  defi- 
nitely been  established,  but  as  commonly  used  the  terms  for  the 
workable  beds  are  as  follows:  " E"  =  Upper  Freeport;  uD"  =  Lower 
Freeport; . "  C  "= Upper  Kittanning;  "  B  "—Lower  Kittanning. 
The  rocks  of  this  region  are  so  similar  in  composition  that  it  is 
extremely  difficult  to  use  them  in  tracing  and  correlating  the  coal  beds. 
The  Mahoning  sandstone,  which  normally  closely  overlies  the  Upper 
Freeport  coal,  is  well  developed  about  Patton,  where  it  caps  the  hills 
and  makes  well-defined  dip  slopes,  but  over  the  major  part  of  the  terri- 
tory it  is  inconspicuous  and  probably  is  absent  in  many  localities. 
The  most  conspicuous  sandstone  of  the  region  lies  from  200  to  250 
feet  above  the  Upper  Freeport  coal.  This  seems  to  correspond  with 
the  heavy  cliff-making  sandstone  along  the  Conemaugh  River  below 
Blairsville,  which  was  named  by  Stevenson  a  the  Saltsburg  sandstone. 
This  bed  is  particularly  prominent  on  the  plateau  about  Nicktown,  on 
the  ridge  about  Plattsville,  and  in  the  syncline  west  of  Cherry  tree. 
Structure. — -The  Barnesboro-Patton  field  occupies  a  roughly  ellip- 
soidal area,  with  its  major  diameter  at  right  angles  to  the  great  struc- 
tural features  of  the  region.  As  a  consequence  of  this  fact  the  coal 
beds  vary  greatly  in  altitude  in  different  parts  of  the  field,  and  the 
dips  are  in  directions  depending  on  the  relation  of  the  point  of 
observation  to  the  structural  features  of  the  region.  Thus  the  UD" 
or  Lower  Freeport  coal,  which  is  the  most  extensively  developed  bed 
in  the  field,  rises  from  an  altitude  of  about  1,800  feet  at  Patton  to 
2,050  feet  near  Carrolltown,  on  the  crest  of  the  Laurel  Hill  anticline. 
From  this  axis  the  coal  bed  descends  to  the  northwest,  reaching  an 
altitude  of  1,500  feet  in  the  center  of  the  syncline  at  Barnesboro. 
Beyond  the  axis  of  this  basin  the  coal  rises  slightly  to  an  altitude  of 
aStevenson,  J.  J.,  Report  of  progress  in  the  Fayette  and  Westmoreland  district  of  the  bituminous 
coal  helds  of  western  Pennsylvania:  Second  Geol.  Survey  of  Pennsylvania,  Rept.,  K3,  1878,  p.  22. 
