﻿420 
  Twenhofel 
  — 
  Wreford 
  and 
  Foraker 
  Limestones. 
  

  

  in 
  solution 
  in 
  water 
  has 
  been 
  demonstrated, 
  8 
  and 
  it 
  is 
  

   quite 
  probable 
  that 
  a 
  great 
  deal 
  of 
  chert 
  develops 
  in 
  this 
  

   way. 
  When 
  solid 
  limestone 
  or 
  dolomite 
  is 
  thus 
  replaced 
  

   by 
  silica, 
  the 
  original 
  structures 
  of 
  the 
  replaced 
  rock 
  

   should 
  be 
  duplicated, 
  or 
  be 
  less 
  perfect, 
  in 
  the 
  resulting 
  

   chert. 
  Original 
  structures 
  would 
  include 
  bedding, 
  lami- 
  

   nation, 
  fossils, 
  etc. 
  It 
  is 
  impossible 
  that 
  they 
  could 
  be 
  

   better 
  and 
  it 
  is 
  more 
  than 
  likely 
  that 
  they 
  should 
  have 
  

   lost 
  something 
  in 
  the 
  process 
  of 
  replacement. 
  As 
  a 
  sub- 
  

   stance 
  of 
  which 
  fossils 
  may 
  be 
  composed, 
  silica 
  is 
  about 
  

   but 
  hardly 
  quite 
  so 
  good 
  as 
  calcite. 
  If 
  the 
  fossils 
  held 
  

   in 
  the 
  cherts 
  are 
  of 
  better 
  preservation 
  than 
  those 
  in 
  the 
  

   limestone, 
  it 
  should 
  be 
  considered 
  positive 
  evidence 
  that 
  

   the 
  chert 
  is 
  not 
  a 
  replacement 
  of 
  solidified 
  limestone. 
  

   Cherts 
  which 
  are 
  replacements 
  of 
  solid 
  limestone 
  should 
  

   have 
  bedding 
  and 
  lamination 
  planes 
  continued 
  from 
  the 
  

   limestone 
  into 
  the 
  chert 
  and, 
  if 
  these 
  structures 
  end 
  

   abruptly 
  at 
  the 
  chert, 
  the 
  latter 
  could 
  hardly 
  be 
  considered 
  

   of 
  this 
  origin. 
  The 
  conclusion 
  would 
  be 
  stronger 
  if 
  the 
  

   cherts 
  were 
  fossiliferous, 
  as 
  it 
  does 
  not 
  appear 
  possible 
  

   that 
  in 
  the 
  same 
  chert 
  fossils 
  could 
  be 
  retained 
  and 
  bed- 
  

   ding 
  planes 
  eliminated. 
  Replacements 
  of 
  this 
  kind 
  

   should 
  be 
  related 
  to 
  fractures 
  and 
  should 
  also 
  give 
  the 
  

   same 
  character 
  of 
  chert 
  in 
  vertical 
  distribution 
  in 
  beds 
  

   which 
  are 
  essentially 
  similar 
  — 
  quite 
  similar 
  limestones 
  

   within 
  less 
  than 
  live 
  feet 
  of 
  each 
  other 
  vertically 
  should 
  

   have 
  similar 
  cherts 
  — 
  , 
  while 
  in 
  horizontal 
  distribution 
  the 
  

   chert 
  should 
  be 
  considerably 
  localized. 
  

  

  The 
  cherts 
  of 
  the 
  Foraker 
  limestone 
  appear 
  to 
  be 
  con- 
  

   fined 
  to 
  certain 
  beds 
  of 
  this 
  member 
  and 
  in 
  these 
  there 
  

   seems 
  to 
  be 
  general 
  horizontal 
  distribution. 
  The 
  Fusu- 
  

   linas 
  of 
  these 
  cherts 
  are 
  exquisitely 
  preserved 
  and, 
  in 
  

   general, 
  they 
  are 
  of 
  better 
  preservation 
  than 
  are 
  those 
  of 
  

   the 
  enclosing 
  limestone 
  ; 
  in 
  collecting, 
  few 
  of 
  the 
  latter 
  

   were 
  considered 
  of 
  much 
  value. 
  Not 
  only 
  is 
  the 
  detail 
  of 
  

   the 
  interior 
  much 
  better 
  shown 
  in 
  the 
  silicified 
  fossils, 
  but 
  

   also 
  the 
  character 
  of 
  the 
  surface. 
  Few 
  traces 
  of 
  bedding 
  

   are 
  shown 
  in 
  the 
  specimens 
  which 
  have 
  been 
  studied. 
  On 
  

   a 
  basis 
  of 
  these 
  characteristics 
  it 
  is 
  considered 
  that 
  these 
  

   cherts 
  are 
  not 
  replacements 
  of 
  solid 
  limestone. 
  

  

  The 
  siliceous 
  fossils 
  in 
  the 
  lower 
  chert 
  zone 
  of 
  the 
  Wre- 
  

   ford 
  are 
  without 
  doubt 
  replacements 
  of 
  solid 
  material. 
  

   That 
  point 
  is 
  settled, 
  but 
  whether 
  this 
  replacement 
  

   occurred 
  before 
  or 
  after 
  the 
  solidification 
  of 
  the 
  sedi- 
  

  

  8 
  Church, 
  A. 
  II., 
  Journ. 
  Chem. 
  Soc, 
  15, 
  107, 
  1862. 
  

  

  

  