﻿Twenhofel 
  — 
  Wreford 
  and 
  Foraker 
  Limestones. 
  419 
  

  

  of 
  southeastern 
  Missouri 
  developed 
  "largely 
  as 
  the 
  result 
  

   of 
  segregation 
  of 
  siliceous 
  matter 
  under 
  conditions 
  of 
  

   slow 
  subaerial 
  decomposition 
  of 
  the 
  limestone.'' 
  The 
  

   present 
  writer 
  believes 
  that 
  some 
  chert 
  has 
  been 
  devel- 
  

   oped 
  in 
  this 
  way, 
  observations 
  made 
  at 
  fossil 
  localities 
  

   suggesting 
  that 
  in 
  some 
  cases 
  the 
  replacement 
  of 
  fossils 
  

   by 
  silica 
  is 
  consequent 
  to 
  weathering. 
  

  

  It 
  is 
  quite 
  certain, 
  however, 
  that 
  none 
  of 
  the 
  chert 
  which 
  

   is 
  considered 
  in 
  this 
  article 
  developed 
  in 
  this 
  fashion. 
  

   As 
  has 
  already 
  been 
  stated; 
  the 
  chert 
  has 
  been 
  observed 
  

   under 
  essentially 
  every 
  condition 
  of 
  exposure, 
  and 
  in 
  fresh 
  

   rock 
  it 
  is 
  just 
  as 
  abundant 
  and, 
  except 
  for 
  oxidation, 
  of 
  

   the 
  same 
  character 
  as 
  in 
  that 
  in 
  which 
  it 
  has 
  been 
  exposed 
  

   to 
  weathering 
  agents. 
  The 
  test 
  would 
  come 
  more 
  par- 
  

   ticularly 
  in 
  the 
  case 
  of 
  the 
  first 
  chert 
  zone 
  of 
  the 
  Wreford 
  

   limestone, 
  that 
  in 
  which 
  the 
  chert 
  occurs 
  in 
  fossils 
  (chiefly 
  

   Composita 
  subtilita). 
  This 
  zone 
  was 
  seen 
  in 
  hundreds 
  of 
  

   places 
  under 
  all 
  conditions, 
  and 
  invariably 
  the 
  shells 
  are 
  

   composed 
  of 
  silica, 
  while 
  the 
  material 
  filling 
  the 
  hollow 
  

   interiors 
  is 
  composed 
  of 
  calcite 
  and 
  quartz. 
  Had 
  the 
  sili- 
  

   cification 
  been 
  a 
  consequence 
  of 
  weathering, 
  it 
  would 
  seem 
  

   that 
  somewhere 
  the 
  specimens 
  of 
  Composita 
  should 
  have 
  

   been 
  composed 
  of 
  calcium 
  carbonate, 
  especially 
  as 
  places 
  

   were 
  observed 
  where 
  the 
  immediately 
  underlying 
  bed 
  has 
  

   its 
  fossils 
  in 
  that 
  form. 
  

  

  It 
  may 
  be 
  argued 
  that 
  the 
  exposure 
  antedated 
  the 
  pres- 
  

   ent 
  and. 
  that 
  the 
  cherts 
  lie 
  along 
  disconformities 
  or 
  old 
  

   erosion 
  surfaces. 
  This 
  view 
  can 
  not 
  be 
  sustained. 
  

  

  Theory' 
  that 
  the 
  chert 
  developed 
  through 
  filling 
  of 
  cav- 
  

   ities, 
  or 
  replacement 
  of 
  solid 
  rock. 
  — 
  That 
  some 
  chert 
  

   occurs 
  as 
  fillings 
  of 
  cavities 
  and 
  fissures 
  rests 
  on 
  many 
  

   observations, 
  so 
  there 
  is 
  no 
  doubt 
  that 
  it 
  is 
  of 
  quite 
  com- 
  

   mon 
  occurrence, 
  7 
  but 
  that 
  the 
  chert 
  under 
  consideration 
  

   did 
  not 
  develop 
  through 
  filling 
  of 
  cavities 
  is 
  quite 
  evident 
  

   from 
  the 
  distribution 
  of 
  fossils 
  therein. 
  In 
  no 
  instance 
  

   were 
  these 
  found 
  to 
  be 
  confined 
  to 
  the 
  lower 
  portions 
  of 
  

   the 
  nodules 
  ; 
  but, 
  so 
  far 
  as 
  observed, 
  and 
  fully 
  a 
  thousand 
  

   nodules 
  were 
  examined 
  in 
  the 
  field, 
  they 
  are 
  uniformly 
  

   distributed 
  throughout. 
  Furthermore, 
  none 
  of 
  the 
  cherts 
  

   shows 
  any 
  relations 
  to 
  fractures 
  and 
  not 
  a 
  single 
  fracture 
  

   was 
  observed 
  which 
  had 
  been 
  filled 
  by 
  chert. 
  

  

  That 
  chert 
  may 
  develop 
  as 
  a 
  replacement 
  of 
  solid 
  lime- 
  

   stone 
  through 
  replacement 
  of 
  the 
  carbonate 
  by 
  silica 
  held 
  

  

  7 
  Van 
  Tuyl, 
  F. 
  M., 
  this 
  Journal, 
  44, 
  450, 
  1918. 
  

  

  