﻿Tivenhofel 
  — 
  Wreford 
  and 
  Foraker 
  Limestones. 
  421 
  

  

  ments 
  from 
  which 
  the 
  limestone 
  was 
  formed 
  remains 
  to 
  

   be 
  decided. 
  It 
  is 
  difficult 
  to 
  understand 
  how 
  a 
  replace- 
  

   ment 
  of 
  shells 
  could 
  have 
  been 
  so 
  general 
  in 
  horizontal 
  

   distribution, 
  and 
  so 
  slight 
  in 
  vertical 
  distribution, 
  as 
  the 
  

   bed 
  immediately 
  below 
  contains 
  its 
  fossils 
  still 
  almost 
  

   wholly 
  in 
  carbonate 
  form. 
  The 
  fact 
  that 
  the 
  shells 
  

   were 
  replaced 
  while 
  the 
  surrounding 
  limestone 
  was 
  not 
  

   affected 
  might 
  be 
  explained 
  on 
  the 
  ground 
  that 
  the 
  shells 
  

   were 
  composed 
  of 
  a 
  less 
  stable 
  form 
  of 
  calcium 
  carbonate 
  

   than 
  that 
  of 
  the 
  surrounding 
  limestone. 
  It 
  would 
  seem, 
  

   however, 
  that 
  in 
  the 
  recrystallization 
  of 
  the 
  sediments 
  to 
  

   form 
  the 
  limestone 
  the 
  shells 
  would 
  also 
  have 
  assumed 
  the 
  

   stable 
  form 
  of 
  lime 
  carbonate. 
  An 
  additional 
  source 
  of 
  

   difficulty 
  is 
  how 
  to 
  account 
  for 
  the 
  entrance 
  of 
  water 
  to 
  

   the 
  shells, 
  as 
  the 
  enclosing 
  limestone 
  is 
  quite 
  fine-grained, 
  

   without 
  being 
  affected 
  in 
  any 
  apparent 
  degree 
  and 
  with- 
  

   out 
  any 
  nodules 
  forming. 
  These 
  difficulties 
  have 
  led 
  to 
  

   the 
  rejection 
  of 
  the 
  theory 
  that 
  the 
  chert 
  fossils 
  are 
  

   replacements 
  subsequent 
  to 
  solidification 
  of 
  the 
  limestone. 
  

  

  The 
  cherts 
  of 
  the 
  second 
  zone 
  of 
  the 
  Wreford 
  and 
  those 
  

   of 
  the 
  third 
  are 
  totally 
  different 
  in 
  appearance 
  and 
  struc- 
  

   ture. 
  In 
  fact, 
  they 
  are 
  about 
  as 
  different 
  as 
  any 
  two 
  

   cherts 
  could 
  very 
  well 
  be. 
  The 
  distribution 
  of 
  each 
  of 
  

   these 
  cherts 
  is 
  horizontally 
  uniform 
  over 
  an 
  area 
  of 
  at 
  

   least 
  one 
  hundred 
  square 
  miles, 
  with 
  the 
  probability 
  that 
  

   the 
  horizontal 
  distribution 
  far 
  exceeds 
  this 
  figure. 
  In 
  the 
  

   upper 
  zone 
  there 
  is 
  no 
  trace 
  of 
  the 
  bedding 
  of 
  the 
  lime- 
  

   stone 
  in 
  the 
  chert, 
  while 
  fossils 
  are 
  preserved 
  therein. 
  

   The 
  second 
  zone 
  shows 
  faint 
  traces 
  of 
  bedding, 
  but 
  fossils 
  

   are 
  as 
  a 
  rule 
  better 
  preserved 
  in 
  the 
  chert 
  than 
  in 
  the 
  

   limestone. 
  No 
  chert 
  of 
  the 
  character 
  of 
  that 
  of 
  the 
  third 
  

   zone 
  is 
  found 
  in 
  the 
  second 
  zone 
  and 
  the 
  reciprocal 
  is 
  also 
  

   true. 
  

  

  These 
  facts 
  strongly 
  militate 
  against 
  the 
  view 
  that 
  

   these 
  cherts 
  developed 
  through 
  replacement 
  of 
  limestone 
  ; 
  

   in 
  fact, 
  they 
  prove 
  the 
  contrary. 
  It 
  is 
  therefore 
  con- 
  

   cluded 
  that 
  the 
  cherts 
  being 
  considered 
  did 
  not 
  develop 
  

   subseouent 
  to 
  the 
  solidification 
  of 
  the 
  containing 
  strata. 
  

   The 
  alternative 
  is 
  that 
  they 
  are 
  antecedent 
  in 
  origin 
  to 
  

   solidification 
  or 
  contemporaneous 
  therewith. 
  

  

  Theory 
  that 
  the 
  chert 
  nodules 
  originated 
  from 
  direct 
  

   precipitation 
  of 
  silica 
  from 
  sea 
  water. 
  — 
  This 
  theory, 
  first 
  

   enunciated 
  by 
  Prestwich, 
  9 
  has 
  lately 
  been 
  independently 
  

  

  9 
  Prestwich. 
  J.. 
  Geology, 
  Chemical, 
  Physical, 
  and 
  Stratigraphical, 
  vol. 
  2, 
  

   320-324. 
  Oxford, 
  1887. 
  

  

  