4()()  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1903.  [bull.225. 
that  there  may  be  a  number  of  sandstones  at  this  horizon,  any  one  of 
which  may  be  taken  to  be  the  Gas  sand.  The  average  depth  of  the 
sand  below  the  Pittsburg  coal  is  765  feet. 
This  sandstone  usually  is  regarded  as  the  Homewood  sandstone,  the 
uppermost  member  of  the  Pottsville  formation,  but  in  some  records 
the  Gas  sand  is  recognized  at  the  Clarion  horizon  and  in  others  it  is 
placed  as  high  as  the  Kittanning  coal.  Its  thickness  ranges  from  15 
to  1-H>  feet,  the  variations  being  due  in  part  to  undoubted  changes  in 
the  amount  of  sandstone  present  at  this  horizon  and  in  some  instances 
to  a  tendency  on  the  part  of  the  driller  to  class  everything  as  sandstone 
for  a  considerable  distance,  when  in  reality  it  may  be  much  broken  by 
shale  beds.  For  these  reasons  the  average  distance  below  the  Pitts- 
burg coal  can  not  be  depended  on  for  anything  like  correctness. 
Salt  sand. — At  an  average  distance  of  932  feet  below  the  Pittsburg 
coal  there  is  encountered  a  sandstone,  which,  according  to  the  records 
in  hand,  varies  in  thickness  from  15  to  175  feet.  This  is  commonly 
known  among  the  drillers  as  the  Salt  sand,  and  probably  in  most  cases 
is  the  lower  member  of  the  Pottsville  formation,  or  Connoquenessing 
sandstone.  It  should  be  noted  at  this  point  that  those  records  which 
give  a  thickness  of  over  100  feet  for  either  the  Gas  or  the  Salt  sand 
omit  mention  of  the  other,  which  would  seem  to  point  to  a  difficulty 
on  the  part  of  the  drillers  in  determining  which  one  is  encountered. 
Red  shales  and  limestone. — At  a  depth  of  from  1,100  to  1,200  feet 
below  the  horizon  of  the  Pittsburg  coal  there  generally  occurs  a  bed 
of  red  shales.  The  thickness  of  these  red  shales  is  a  point  of  interest. 
In  Monongalia  and  Marion  counties,  W.  Va.,  thicknesses  of  250  feet 
have  been  recorded,  while  in  Washington  County  some  records  show 
no  red  shales  at  this  horizon.  In  fact,  it  seems  to  be  well  proved  that 
these  shales,  which  are  a  portion  of  the  Mauch  Chunk  formation,  thin 
out  in  a  general  way  toward  the  northwest  and  disappear  in  Washing 
ton  County,  so  that  the  overlying  shales  and  sandstones  of  the  Potts 
ville  formation  lie  unconformably  upon  the  Greenbrier  or  Mountain 
limestone. 
The  Greenbrier  limestone,  commonly  known  to  Greene  County 
drillers  as  the  Big  lime  or  Mountain  lime,  is  persistent  throughout 
this  area,  and  shows  almost  as  great  thickness  in  Washington  County 
as  it  does  in  West  Virginia.  Its  reported  thickness  varies,  however, 
because  the  upper  portion  of  the  Pocono  formation  is  sometimes 
decidedly  calcareous  and  the  driller  is  unable  to  distinguish  between 
the  limestone  of  the  Mauch  Chunk  and  the  calcareous  sandstone  at  the 
top  of  the  Pocono. 
Big  Injun  sand.— A  sand  rock  which  the  drillers  always  recognize! 
from  its  thickness  and  position  is  the  Big  Injun,  which  is  the  term 
commonly  applied  to  the  Pocono  formation  in  these  well  records.  The 
top  is  about  1,225  feet  below  the  Pittsburg  coal,  and  the  bed  is  usually 
