398  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1903.  [bull.  225. 
tion  is  about  800  feet.  The  Dunkard  contains  a  number  of  small  coal 
scams,  none  of  which  are  of  economic  importance  in  this  area.  The 
Wavnesburg  sandstone  and  the  Upper  Washington  limestone  are  con- 
spicuous and  fairly  persistent  members.  The  greater  part  of  the 
formation  is  composed  of  soft,  thin  beds  of  sandstone  and  shale. 
DEEP-WELL   KECORDS. 
The  gas  and  oil  companies  and  private  individuals  operating  in  this 
county  usually  require  the  drillers  to  keep  a  record  of  the  thickness 
of  strata  passed  through  in  sinking  wells.  It  is  from  this  source  that 
information  is  obtained  concerning  the  underground  structure  and 
stratigraphy.  The  contractors  and  drillers,  however,  in  too  many 
cases  do  not  realize  the  value  of  accurate  and  carefully  kept  logs,  and 
furnish  meager,  incomplete  records. 
The  accumulation  of  gas  and  oil  in  this  region  is  dependent  upon 
the  structure  of  the  rocks,  and  the  precise  location  of  anticlinal  and 
synclinal  axes  can  be  determined  only  by  the  aid  of  good  logs.  It 
would  seem  at  first  thought  that  the  depth  of  the  Pittsburg  coal  and 
two  or  three  sand  rocks  would  be  sufficient,  and  this  is  all  that  is  noted 
by  some  drillers,  but  the  omission  of  intermediate  details  leaves  the 
geologist  little  basis  for  correlating  the  records  of  wells.  As  might 
be  expected,  drillers  make  mistakes  in  recognizing  sand  rocks,  and 
their  errors  are  not  easy  to  confirm  when  records  are  incomplete. 
In  comparing  the  logs  of  deep  wells  it  is  necessary  to  bear  in  mind 
the  changes  in  the  character  of  beds  which  are  likely  to  occur  even  in 
short  distances.  The  possibility  of  error  on  the  part  of  the  driller  in 
recognizing  any  particular  bed,  and  inaccuracyr  of  measurements  made 
b}r  manila  cable,  which  probably  stretches  several  feet  when  support- 
ing two  tons  of  tools  at  a  depth  of  2,000  or  3,000  feet,  have  to  be 
considered. 
The  logs  of  most  of  the  wells  in  this  territory  have  been  obtained 
and  studied.  The  measurements  in  about  100  of  them  have  been 
compiled  and  the  averages  determined  in  order  to  construct  a  general- 
ized section  for  the  region.  The  Pittsburg  coal,  which  underlies  the 
entire  county,  is  several  feet  thick  and  easily  recognized,  so  the 
drillers  use  it  as  a  starting  point  in  figuring  the  depths  at  which  the 
producing  sands  will  be  found.  This  coal  varies  in  depth  from  10  to 
1,060  feet  below  the  well  mouth,  according  to  the  location  of  the  well. 
The  intervals  between  the  coal  and  the  tops  of  ten  well-known  sands 
are  shown  in  the  following  table,  together  with  the  number  of  well 
records  from  which  the  average  of  each  interval  was  computed: 
