464  CONTKIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.         [bull.  260. 
to  the  escape  of  this  compound  of  sulphur  are  the  so-called  "  sour 
Waters."  These  are  best  known  at  Sour  Lake,  but  are  by  no  means 
uncommon.  Their  salts  are  largely  sulphates  and  their  significance 
may  be  similar  to  that  of  sulphureted  l^drogen,  all  being  members 
of  a  group  of  substances  which  are  related  in  origin.  It  is  by  no> 
means  to  be  inferred  that  the  escape  of  a  large  amount  of  gas  is  a 
better  indication  than  that  of  a  small  amount.  Indeed,  the  best  oil 
fields  of  the  Coastal  Plain  have  been  marked  by  very  modest  surface 
indications. 
A  phenomenon  which  has  carried  far  more  weight  than  justly 
belongs  to  it  is  found  in  the  so-called  "  gas  mounds."  These  are  low,, 
rounded  mounds,  averaging  perhaps  2  feet  in  height  and  several  rods 
in  diameter.  Considering  their  vast  numbers  on  the  flat  Coastal 
Plain,  they  show  remarkable  uniformity  in  size  and  shape.  They  are 
popularly  supposed  to  be  connected  with  the  escape  of  gases  from  the 
soil.  Whatever  be  their  origin,  there  is  as  yet  no  evidence  that  they 
are  in  any  way  related  to  oil  bodies.  Even  were  such  a  relation 
assumed,  these  mounds  are  so  Avidely  distributed  over  the  flat  Coastal  1 
Plain  that  as  guides  to  drilling  they  are  of  no  value  whatever. 
Approximately  one-half  of  the  developed  and  prospective  fieldf 
belong  to  that  part  of  the  Coastal  Plain  Avhose  topography  is  as  yet 
unaffected  by  erosion.  On  this  part  of  the  Coastal  Plain  there  is  onei 
class  of  topographic  features  which  rises  prominent  over  all  others. 
This  class  comprises  the  so-called  "  mounds  "  (not  the  "  gas  mounds  '; 
spoken  of  above),  which  in  size  range  from  several  hundred  to  j 
several  thousand  acres  and  in  altitude  to  a  maximum  of  75  or  100 
feet  above  the  surrounding  plain.  This  class  of  mounds,  of  which  the 
Spindletop  oil  field  and  the  Salt  Islands  of  Louisiana  are  the  best 
known  examples,  has  doubtless  been  represented  on  the  older  and  now 
eroded  parts  of  the  Coastal  Plain,  but  their  former  presence  there  is 
now  difficult  to  determine.  Corresponding  to  the  topographic  mound 
there  is,  beneath  each  of  these  limited  areas,  an  assemblage  of  rocks 
and  a  geologic  structure  which  do  not  exist  outside  the  area  covered 
by  the  elevation.  It  is  in  mounds  of  this  class  that  drilling  for  oil  has 
been  persistent,  on  account  of  the  phenomenal  success  of  the  search 
at  Spindletop.  Of  those  oil  fields,  both  developed  and  prospective, 
which  lie  upon  the  eroded  part  of  the  Coastal  Plain,  some  show  the1 
peculiar  internal  structure  of  the  mounds.  The  same  structure  might 
perhaps  be  found  in  all  of  the  productive  fields  by  sufficiently  deep 
and  thorough  exploration.  Where  the  surficial  mound  is  absent  or 
obscure,  it  may  be  on  account  of  subsequent  erosion,  but  it  must  also 
be  noted  that  the  elevation  is  not  a  necessary  accompaniment  of  the 
peculiar  geologic  structure.  In  the  most  typical  case  of  all  (Spindle- 
top) the  elevation  above  the  surrounding  flat  is  but  7  feet,     It  is 
