fenneman.]  OIL    FIELDS    OF    TEXAS-LOUISIANA    COAST.  465 
therefore  to  be  expected  that  there  are  other  similar  occurrences  of 
the  peculiarly  associated  materials,  including  oil,  whose  positions 
have  never  been  marked  by  any  topographic  features.  On  the  per- 
fectly uneroded  parts  of  the  Coastal  Plain  far  the  largest  significance 
has  very  justly  been  attached  to  these  mounds  as  suggestions  of  the 
possible  presence  of  oil.  The  failure  to  distinguish  these  from  hills 
of  erosion  is  a  very  common  error. 
Structure  of  the  mounds. — For  the  first  TOO  to  1,000  feet  below  the 
surface  at  Spindletop  the  material  consists  of  unconsolidated  Coastal 
Plain  deposits,  most  largely  clay,  but  to  a  less  extent  sand  and  gravel. 
There  are  occasional  thin  beds  of  hard  limestone.  At  the  higher 
mounds  these  beds  are  thinner.  On  the  Salt  Islands  they  are  at 
places  all  but  absent.  At  Damon  Mound  the  next  lower  formation 
appears  locally  at  the  surface.  Beneath  these  characteristic  deposits 
of  the  Coastal  Plain  there  is,  at  Spindletop,  a  highly  porous  and  crys- 
talline limestone.  The  pores  may  be  small,  giving  to  the  rock  a 
sugary  appearance,  or  they  may  be  large  caverns.  Those  seen  in 
hand  specimens  are  sometimes  as  large  as  walnuts,  but  there  seems 
no  reason  to  doubt  that  the  caverns  of  this  kind  reach  a  very  much 
larger  size.  A  corresponding  porous  limestone  occurs  at  some  of  the 
other  mounds  and  in  some  other  oil  fields  (e.  g.,  Sour  Lake  and  Bat- 
son),  where  a  surficial  mound  has  either  never  existed  or  been 
obscured  by  erosion. 
The  porous  limestone  here  referred  to  is  in  some  fields  the  principal 
reservoir  of  the  oil.  Even  at  Spindletop,  however,  some  of  the  oil 
seems  to  have  worked  its  way  upward  from  this  reservoir  and  was 
found  to  be  contained  in  sandy  beds  inclosed  between  dense  clays. 
While  at  Spindletop  far  the  largest  part  of  the  oil  was  found  in  the 
limestone,  the  proportion  of  that  contained  in  limestone  to  that  con- 
tained in  the  overlying  sands  varies  greatly  in  the  different  fields. 
At  Sour  Lake  perhaps  one-half  of  the  wells  derived  their  oil  from  the 
limestone;  at  Batson,  much  less  than  one-half;  at  Saratoga  and  at 
Jennings  all  the  oil  thus  far  obtained  has  come  from  the  sandy  beds. 
A  considerable  amount  of  sulphur  has  been  found  at  various  depths 
and  in  various  relations.  The  reports  of  "  solid  sulphur,"  however, 
must  be  received  with  caution.  In  so  far  as  pure  sulphur  has  been 
brought  to  the  surface  it  has  been  found  to  be  contained  within  the 
same  caverns  described  above  as  containing  the  oil.  There  arc  also 
sulphurous  clays  and  sands.  Whether  the  sulphur-filled  caverns 
exist  for  the  most  part  above  the  horizon  of  the  oil  or  below  remains 
to  be  determined.  Naturally  the  larger  part  of  the  deposits  dis- 
covered was  above  the  oil,  but  there  is  some  reason  for  supposing  that 
if  the  200  feet  below  the  oil  horizon  were  as  well  explored  as  are  the 
beds  above  that  horizon  the  sulphur  would  be  found  in  larger  quan- 
Bull.  260—05  m 30 
