﻿418 
  Twenhofel 
  — 
  Wreford 
  and 
  Foraher 
  Limestones. 
  

  

  certain. 
  Respecting 
  the 
  time 
  and 
  method 
  of 
  segregation 
  

   there 
  is 
  considerable 
  diversity 
  of 
  opinion. 
  

  

  Time 
  and 
  Method 
  of 
  Origin 
  of 
  the 
  Chert 
  Nodules. 
  

  

  Views 
  relating 
  to 
  the 
  time 
  and 
  method 
  of 
  origin 
  of 
  chert 
  

   nodules 
  fall 
  into 
  two 
  general 
  classes, 
  namely: 
  (1) 
  the 
  

   cherts 
  develop 
  subsequent 
  to 
  the 
  solidification 
  of 
  the 
  

   strata 
  in 
  which 
  they 
  are 
  imbedded 
  through 
  replacement 
  

   of 
  certain 
  parts 
  or 
  through 
  the 
  filling 
  of 
  cavities 
  ; 
  and 
  (2) 
  

   the 
  cherts 
  develop 
  prior 
  to, 
  or 
  contemporaneous 
  with, 
  the 
  

   solidification 
  of 
  the 
  containing 
  strata, 
  either 
  as 
  original 
  

   precipitates 
  or 
  as 
  segregations 
  in 
  the 
  unconsolidated 
  sedi- 
  

   ments. 
  

  

  Of 
  those 
  who 
  hold 
  that 
  the 
  chert 
  develops 
  subsequent 
  

   to 
  the 
  solidification 
  of 
  the 
  containing 
  strata, 
  one 
  group 
  

   maintains 
  that 
  the 
  chert 
  forms 
  in 
  the 
  zone 
  of 
  cementation 
  

   through 
  replacement 
  of 
  the 
  carbonates 
  or 
  through 
  filling 
  

   of 
  cavities, 
  the 
  silica 
  being 
  derived 
  from 
  that 
  held 
  in 
  solu- 
  

   tion 
  in 
  ground 
  water. 
  Another 
  group 
  maintains 
  that 
  

   chert 
  develops 
  as 
  a 
  consequence 
  of 
  weathering 
  and 
  that 
  

   it 
  is 
  far 
  more 
  abundant 
  in 
  exposed 
  strata, 
  or 
  strata 
  which 
  

   at 
  one 
  time 
  have 
  been 
  exposed, 
  than 
  in 
  strata 
  which 
  have 
  

   not 
  been 
  subjected 
  to 
  weathering 
  agencies 
  ; 
  it 
  being 
  rare 
  

   or 
  wanting 
  in 
  the 
  latter. 
  

  

  Likewise 
  there 
  are 
  two 
  groups 
  of 
  those 
  who 
  consider 
  

   that 
  chert 
  is 
  formed 
  prior 
  to, 
  or 
  contemporaneous 
  with, 
  

   the 
  solidification 
  of 
  the 
  containing 
  strata. 
  According 
  to 
  

   one 
  group 
  the 
  cherts 
  are 
  deposited 
  contemporaneously 
  

   with 
  the 
  associated 
  materials 
  ; 
  according 
  to 
  the 
  other 
  the 
  

   nodules 
  develop 
  by 
  segregation 
  of 
  silica 
  in 
  the 
  sediments 
  

   before 
  the 
  latter 
  become 
  consolidated 
  and 
  cemented, 
  or 
  

   while 
  this 
  is 
  being 
  accomplished. 
  

  

  It 
  is 
  quite 
  possible, 
  and 
  even 
  probable, 
  that 
  chert 
  has 
  

   been 
  formed 
  in 
  each 
  of 
  the 
  ways 
  postulated. 
  Further- 
  

   more, 
  it 
  is 
  but 
  fair 
  to 
  state 
  that 
  it 
  is 
  certain 
  that 
  the 
  stu- 
  

   dents 
  who 
  hold 
  to 
  the 
  several 
  theories 
  which 
  have 
  been 
  

   outlined 
  do 
  not 
  insist 
  that 
  chert 
  originates 
  in 
  any 
  one 
  par- 
  

   ticular 
  way. 
  Each 
  of 
  these 
  theories 
  will 
  now 
  be 
  examined 
  

   with 
  particular 
  reference 
  to 
  the 
  cherts 
  which 
  are 
  con- 
  

   sidered 
  in 
  this 
  paper. 
  

  

  Theory 
  that 
  the 
  chert 
  is 
  a 
  consequence 
  of 
  weathering. 
  — 
  

   This 
  view 
  appears 
  to 
  have 
  been 
  developed 
  by 
  Dr. 
  E. 
  0. 
  

   Ulrich," 
  who 
  considered 
  that 
  the 
  chert 
  in 
  the 
  Potosi 
  group 
  

  

  Bain, 
  H. 
  F. 
  and 
  Ulrich, 
  E. 
  O., 
  U. 
  S. 
  Geol. 
  Survey, 
  Bull. 
  267, 
  27, 
  1905. 
  

  

  