444  CONTRIBUTIONS    TO   ECONOMIC    GEOLOGY,  1904.        [bull.  260, 
sippian  is  found  to  vary  between  200  and  300  feet.  According  to 
this  interpretation  of  the  well  records,  the  Muscogee  oil  sand  occurs 
mterbedded  with  or  near  the  Mississippian  limestones. 
The  Mississippian  limestones,  as  exposed  in  the  hilly  country  of 
the  western  Ozark  region  east  of  Neosho  River,  in  the  Cherokee 
Nation,  are  locally  folded  and  faulted,  and  are  inclined  at  low 
angles  toward  the  west.  Above  the  Mississippian  limestones  conies 
the  Pennsylvanian  series,  consisting  of  shales,  thin  sandstones,  and 
limestones.  All  the  rocks  of  northern  Indian  Territory  above  the 
Mississippian  limestones  west  of  Neosho  River  belong  to  this  series. 
In  these  Pennsylvanian  deposits  are  found  the  local  oil  pools  or  oil- 
bearing  sands,  which  occur  in  the  Chelsea,  Bartlesville,  and  Redfork 
fields.  Like  the  Mississippian  rocks,  they  are  tilted  toward  the  west 
without  appreciable  local  folding,  at  inclinations  of  60  to  100  feet 
per  mile.  The  edges  of  these  rock  beds  outcrop  west  of  the  Neosho 
River  Valley. 
The  geologic   conditions  governing  the  occurrence  of  oil  in  the 
Pennsylvanian  rocks  in  northern  Indian  Territory  and  Kansas  have 
become  comparatively  well  known  by  means  of  geologic  surveys  andi 
extensive  well  drilling.     The  oil  is  apparently  retained  in  local  de- 
posits of  porous  rocks,  inclosed  by  impervious  deposits  of  shale. 
Structure. — The  structure  of  the  Mississippian  rocks  and  of  the 
strata  both  above  and  below  them  in  eastern  Cherokee  Nation  is  com- 
plicated. A  survey  of  the  region  between  Muscogee  and  the  Arkan- 
sas-Indian Territory  line  supports  the  following  statements,  which 
may  have  bearing  upon  the  occurrence  of  oil :  1.  There  is  an  un- 
conformity between  the  Mississippian  limestone  and  the  overlying 
Pennsylvanian  deposits.  2.  There  are  indications  that  folds  were 
developed  in  the  Mississippian  rocks  before,  and  probably  during,  the 
earlier  part  of  the  deposition  of  the  Pennsylvanian.  3.  The  Missis- 
sipian  limestones  lie  across  the  folded  and  eroded  surface  of  Silurian 
and  Ordovician  limestones,  sandstones,  and  shales. 
In  no  instance,  however,  have  the  older  rocks  been  very  steeply 
tilted.  Fig.  21  shows  approximately  the  outcrop  of  the  Mississip- 
pian rocks  and  the  strike  of  a  sandstone  bed  which  lies  in  the 
Pennsylvanian  series,  1,200  to  1,500  feet  above  the  Mississippian. 
It  shows  that,  instead  of  having  even  westerly  dips,  the  Missis- 
sippian and  lower  Pennsylvanian  rocks  east  of  Neosho  and  Arkan- 
sas rivers  have  been  thrown  into  low  folds,  with  which  faults 
are  usually  associated.  The  folds  and  faults  bear  in  a  southwest^ 
erly  direction.  The  faults  do  not  extend  far  westward  into  the 
rocks  above  the  Mississippian.  The  folds  with  which  they  are  asso- 
ciated continue  farther  westward,  but  become  rapidly  flatter  until 
they  are  lost.     It  is  presumed  that  if  oil  occurs  in  the  region  south 
