466  CONTKIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.         [bull.  260. 
titles  at  the  greater  depths.  Thus  far  it  can  only  be  said  that  the 
sulphur  found  has  always  been  in  positions  in  which  a  body  of  oil 
might  have  been  for  a  time  retained.  It  is  not  an  infrequent  occur- 
rence to  find  the  cavities  of  the  oil-filled  rock  lined  with  sulphur 
crystals.  It  is  not  improbable  that  the  occurrence  of  these  sulphur 
deposits,  the  large  amount  of  sulphur  dissolved  in  the  crude  oil,  and 
the  escape  of  sulphurous  gases  at  the  surface  are  genetically  related. 
Gypsum  has  been  found  in  considerable  quantities  beneath  the  oil 
rock,  both  at  Spindletop  and  elsewhere.  It  is  frequently  separated 
from  the  oil  rock  by  many  feet  of  clay,  marl,  or  even  sand.  In  some 
of  the  mounds  this  gypsum  has  a  thickness  of  hundreds  of  feetv 
Beneath  the  gypsum  horizon  rock  salt  has  been  encountered  in  so 
large  a  number  of  the  mounds  that  it  is  impossible  to  draw  a  line 
distinguishing  the  Salt  Islands  on  the  one  hand  from  the  oil  fields  on 
the  other.  The  same  materials  arranged  in  the  same  order  character- 
ize both  types.  These  materials,  named  in  descending  order,  are  (1) 
the  unconsolidated  clays,  etc.,  (2)  porous  limestone,  with  oil  and  sul- 
phur, (3)  gypsum,  (4)  salt.  Not  all  the  members  of  this  series  are 
present  in  each  case,  but  the  number  of  repetitions  of  this  series,  in 
whole  or  in  part,  is  strikingly  large.  There  are  no  significant  depart- 
ures from  the  order  here  given  for  the  solid  materials.  The  oil,  as 
pointed  out  above,  is  frequently  found  in  the  overlying  sandy  beds, 
and  in  the  two  cases  named  small  amounts  of  oil  Avere  found  after 
passing  through  streaks  of  salt  branching  out  from  the  main  body. 
ORIGIN  OF  THE  MOUNDS. 
The  origin  of  these  several  materials  characteristically  associated 
with  the  oil-producing  mounds  is  not  fully  determined.  That  the 
porous  limestone  is  a  deposit  from  solution  in  water  is  evident  on 
account  of  its  thoroughly  crystalline  texture.  The  sulphur  may  per- 
haps have  been  deposited  from  solution  in  the  oil.  That  it  has  been 
known  to  precipitate  in  tanks  lends  plausibility  to  this  supposition, 
and  the  fact  that  its  position  wherever  found  has  been  essentially  the 
same  as  that  in  which  oil  bodies  are  found  further  strengthens  the 
assumption.  Of  the  origin  of  the  gypsum  and  salt  nothing  definite 
has  been  ascertained. 
The  mounds  at  the  surface  plainly  owe  their  existence  to  some  force 
exerted  from  beneath  upward.  It  is  inconceivable  that  any  lateral 
pressure  in  these  uncompacted  Coastal  Plain  deposits  should  have 
resulted  in  approximately  circular  hills  rising  above  the  dead  flat. 
It  is  too  early  to  frame  an  exact  hypothesis  as  to  the  nature  of  that 
force  exerted  from  beneath,  but  the  indications  in  the  field  strongly 
suggest  some  association  with  an  expansive  force  accompanying  the 
crystallization  of  the  minerals  characteristic  of  such  mounds. 
