﻿8. 
  Powers 
  — 
  The 
  Butler 
  Salt 
  Dome. 
  141 
  

  

  Texas," 
  13 
  and 
  for 
  volcanic 
  ash 
  from 
  west 
  Texas 
  erup- 
  

   tions 
  interbedded 
  in 
  the 
  Jackson 
  formation 
  of 
  Eocene 
  

   age. 
  In 
  contrast 
  to 
  the 
  single 
  established 
  period 
  of 
  vul- 
  

   canism 
  there 
  are 
  at 
  least 
  two 
  generations 
  of 
  salt 
  domes, 
  

   and 
  salt 
  domes 
  grew 
  periodically 
  from 
  Upper 
  Creta- 
  

   ceous 
  to 
  Pliocene 
  and 
  even 
  post-Pliocene 
  time. 
  Second, 
  

   the 
  fact 
  that 
  salt 
  domes 
  never 
  formed 
  over 
  such 
  igneous 
  

   masses 
  as 
  that 
  at 
  Thrall, 
  which 
  is 
  still 
  buried, 
  or 
  as 
  the 
  

   Uvalde 
  buttes, 
  most 
  of 
  which 
  are 
  exposed 
  by 
  erosion, 
  al- 
  

   though 
  these 
  igneous 
  masses 
  were 
  covered 
  by 
  the 
  same 
  

   sediments 
  as 
  the 
  interior 
  salt 
  dome. 
  

  

  The 
  tectonic 
  theory, 
  based 
  on 
  a 
  comparison 
  of 
  these 
  

   domes 
  with 
  those 
  of 
  Europe, 
  requires 
  a 
  thick 
  body 
  of 
  salt 
  

   at 
  depth. 
  Granting 
  the 
  existence 
  of 
  the 
  salt, 
  the 
  theory 
  

   appears 
  to 
  be 
  acceptable. 
  But 
  a 
  Permian 
  age 
  is 
  as- 
  

   signed 
  to 
  this 
  hypothetical 
  salt 
  body 
  although 
  it 
  appears 
  

   that 
  central 
  Texas 
  was 
  undergoing 
  erosion 
  during 
  the 
  

   Permian. 
  It 
  is 
  well 
  established 
  that 
  this 
  area 
  was 
  

   undergoing 
  erosion 
  during 
  the 
  deposition 
  of 
  the 
  Penn- 
  

   sylvanian 
  sediments 
  to 
  the 
  north 
  and 
  west 
  and 
  this 
  land 
  

   area 
  is 
  supposed 
  to 
  have 
  persisted 
  until 
  Cretaceous 
  de- 
  

   position. 
  The 
  Central 
  Mineral 
  region 
  is 
  not 
  an 
  uplifted 
  

   mountain-built 
  area. 
  It 
  is 
  a 
  horst 
  — 
  a 
  positive 
  element 
  — 
  

   which 
  first 
  affected 
  sedimentation 
  in 
  post-Bend 
  (Pennsyl- 
  

   vanian) 
  time. 
  It 
  probably 
  had 
  direct 
  connection 
  with 
  or 
  

   was 
  an 
  outlier 
  of 
  the 
  main 
  land 
  area. 
  Eecent 
  borings 
  east 
  

   and 
  southeast 
  of 
  this 
  region 
  on 
  both 
  sides 
  of 
  the 
  Balcones 
  

   fault, 
  starting 
  in 
  one 
  instance 
  in 
  the 
  Tertiary 
  and 
  in 
  the 
  

   other 
  instances 
  in 
  the 
  Cretaceous, 
  at 
  "Waco, 
  Georgetown, 
  

   Maxwell, 
  San 
  Antonio, 
  and 
  Leon 
  Springs, 
  find 
  pre-Cam- 
  

   brian 
  schist 
  beneath 
  the 
  Cretaceous 
  at 
  depths 
  of 
  3700, 
  

   1100, 
  3000, 
  1800, 
  and 
  1100 
  feet, 
  respectively. 
  14 
  This 
  pre- 
  

  

  13 
  G. 
  S. 
  Sogers, 
  Intrusive 
  origin 
  of 
  the 
  Gulf 
  Coast 
  salt 
  domes, 
  Economic 
  

   Geol., 
  vol. 
  13, 
  p. 
  458, 
  1918. 
  This 
  plug 
  may 
  possibly 
  be 
  the 
  " 
  porcellanite 
  ' 
  ' 
  

   in 
  Grimes 
  County, 
  Texas, 
  along 
  the 
  Brazos 
  Eiver, 
  in 
  which 
  fossil 
  leaves 
  

   are 
  reported. 
  One 
  suggestion 
  is 
  that 
  the 
  rock 
  is 
  rhyolite. 
  Another 
  is 
  

   that 
  it 
  is 
  an 
  alteration 
  product 
  caused 
  by 
  the 
  heat 
  of 
  burning 
  lignite. 
  

  

  14 
  Two 
  wells 
  have 
  been 
  drilled 
  near 
  Waco, 
  one 
  four 
  miles 
  north 
  and 
  the 
  

   other 
  in 
  the 
  city. 
  Cuttings 
  from 
  the 
  former 
  well, 
  which 
  had 
  just 
  been 
  com- 
  

   pleted 
  (Sept., 
  1919), 
  have 
  been 
  examined 
  by 
  Dr. 
  J. 
  A. 
  Udden 
  and 
  those 
  

   below 
  2,700 
  feet 
  are 
  pre-Cambrian 
  graphitic 
  schist. 
  Cuttings 
  from 
  the 
  

   latter 
  well, 
  drilled 
  in 
  1914, 
  were 
  examined 
  by 
  Mr. 
  E. 
  G. 
  Woodruff, 
  of 
  Tulsa, 
  

   Okla., 
  and 
  Ordovician 
  fossils 
  were 
  found 
  at 
  a 
  depth 
  of 
  2,300 
  feet, 
  which 
  

   is 
  the 
  base 
  of 
  the 
  Cretaceous 
  in 
  both 
  wells. 
  (Personal 
  communication 
  

   from 
  Dr. 
  Udden, 
  Mr. 
  Woodruff, 
  and 
  Mr. 
  E. 
  T. 
  Hill, 
  of 
  Dallas, 
  Texas.) 
  

  

  The 
  data 
  concerning 
  the 
  schist 
  were 
  taken 
  from 
  J. 
  A. 
  Udden, 
  Amer. 
  

   Inst. 
  Min. 
  Engrs., 
  Bull. 
  133, 
  1918, 
  p. 
  94; 
  and 
  from 
  Eeview 
  of 
  the 
  geology 
  

   of 
  Texas, 
  Univ. 
  of 
  Texas, 
  Bull. 
  44, 
  3d 
  ed., 
  1919, 
  p. 
  42. 
  

  

  