﻿S. 
  Powers— 
  The 
  Butler 
  Salt 
  Dome. 
  135 
  

  

  tions 
  are 
  about 
  1 
  foot 
  wide 
  and 
  4 
  feet 
  long. 
  They 
  consist 
  

   of 
  an 
  outer 
  layer 
  of 
  cone-in-cone 
  structure 
  6 
  from 
  1 
  inch 
  

   to 
  6 
  inches 
  in 
  thickness 
  with 
  a 
  center 
  3 
  to 
  8 
  inches 
  thick 
  

   composed 
  of 
  calcareous, 
  sandy 
  siderite. 
  Small 
  balls 
  % 
  

   inch 
  to. 
  1 
  inch 
  in 
  diameter 
  also 
  occur 
  in 
  the 
  center 
  and 
  

   represent 
  small 
  siderite 
  concretions. 
  Fossils, 
  espe- 
  

   cially 
  small 
  corals, 
  are 
  found 
  throughout 
  the 
  larger 
  

   concretions, 
  but 
  they 
  are 
  practically 
  destroyed 
  in 
  the 
  

   cone-in-cone 
  rims. 
  They 
  'are 
  not 
  found 
  in 
  the 
  smaller 
  

   concretions, 
  which 
  are 
  almost 
  wholly 
  limonite 
  replacing 
  

   siderite 
  without 
  cone-in-cone 
  structure. 
  The 
  shales 
  ex- 
  

   posed 
  at 
  the 
  side 
  of 
  the 
  road 
  contain 
  a 
  few 
  flat 
  limonite 
  

   concretions 
  less 
  than 
  a 
  foot 
  in 
  diameter 
  and 
  the 
  shales 
  

   are 
  occasionally 
  streaked 
  with 
  yellow. 
  Selenite 
  crystals 
  

   of 
  small 
  size 
  are 
  also 
  present. 
  The 
  shale 
  breaks 
  up 
  into 
  

   tiny 
  angular 
  fragments. 
  It 
  is 
  a 
  very 
  pure 
  clay, 
  is 
  very 
  

   different 
  from 
  the 
  typical 
  sandy 
  shales 
  of 
  the 
  "Wilcox 
  and 
  

   is 
  believed 
  to 
  be 
  of 
  Midway 
  age. 
  

  

  Large 
  unfossiliferous 
  concretions 
  similar 
  to 
  some 
  of 
  

   those 
  associated 
  with 
  the 
  fossils 
  are 
  exposed 
  north 
  of 
  the 
  

   Hollow 
  near 
  the 
  bridge 
  and 
  also 
  south 
  of 
  the 
  fossil 
  local- 
  

   ity 
  in 
  three 
  places. 
  The 
  latter 
  outcrops 
  show 
  argilla- 
  

   ceous 
  sands 
  of 
  white 
  to 
  yellow 
  color. 
  

  

  At 
  the 
  south 
  road 
  to 
  Butler 
  a 
  few 
  fossiliferous 
  concre- 
  

   tions 
  and 
  cone-in-cone 
  concretions 
  traversed 
  by 
  selenite 
  

   veins 
  are 
  exposed 
  above 
  a 
  thick 
  series 
  of 
  blue 
  shales 
  

   exactly 
  like 
  those 
  at 
  the 
  north 
  road. 
  To 
  the 
  south 
  there 
  

   are 
  outcrops 
  of 
  heavy 
  limonite 
  beds 
  and 
  there 
  is 
  one 
  out- 
  

   crop 
  of 
  impure 
  limestone 
  of 
  yellow 
  color. 
  

  

  Another 
  locality 
  where 
  fossils 
  were 
  collected 
  is 
  on 
  the 
  

   Oakwood 
  road 
  at 
  the 
  top 
  of 
  the 
  hill 
  north 
  of 
  the 
  saline. 
  

   Other 
  similar 
  concretions, 
  but 
  without 
  fossils, 
  were 
  found 
  

   between 
  the 
  Bonner's 
  Ferry 
  road 
  and 
  the 
  saline 
  as 
  indi- 
  

   cated 
  on 
  the 
  map. 
  

  

  Limestone 
  is 
  practically 
  unknown 
  in 
  the 
  "Wilcox 
  forma- 
  

   tion 
  although 
  concretionary 
  masses 
  of 
  very 
  calcareous 
  

   shale 
  are 
  occasionally 
  found, 
  as 
  near 
  Farrar 
  and 
  near 
  

   Freestone. 
  At 
  Mexia 
  and 
  in 
  the 
  Mexia 
  gas 
  field 
  beds 
  of 
  

   Midway 
  limestone 
  are 
  widespread. 
  It 
  is 
  believed 
  that 
  

   the 
  siderite 
  concretions 
  were 
  originally 
  thin 
  lenses 
  of 
  

   very 
  calcareous 
  shale 
  or 
  of 
  limestone 
  and 
  that 
  they 
  are 
  

  

  8 
  The 
  origin 
  of 
  cone-in-cone 
  structure 
  is 
  well 
  described 
  by 
  W. 
  S. 
  Gresley, 
  

   Cone-in-Cone, 
  Quart. 
  Jour. 
  Geol. 
  Soc, 
  London, 
  vol. 
  50, 
  pp. 
  731-739, 
  1894. 
  

  

  Am. 
  Jour. 
  Sci.— 
  Fourth 
  Series, 
  Vol. 
  XLIX, 
  No. 
  290.— 
  February, 
  1920. 
  

   10 
  

  

  