﻿422 
  Twenhofel 
  — 
  Wreford 
  and 
  Foraker 
  Limestones. 
  

  

  elaborated 
  by 
  Tarr, 
  10 
  who 
  came 
  to 
  the 
  conclusion 
  that 
  the 
  

   Burlington 
  chert 
  nodules 
  resulted 
  from 
  direct 
  precipi- 
  

   tation 
  of 
  silica 
  and 
  that 
  these 
  nodules 
  were 
  subsequently 
  

   buried 
  in 
  the 
  sediments. 
  "The 
  silica," 
  he 
  says, 
  "is 
  

   believed 
  to 
  have 
  been 
  derived 
  from 
  the 
  land 
  by 
  chemical 
  

   weathering 
  and 
  transported 
  to 
  the 
  sea 
  by 
  the 
  streams 
  as 
  

   colloidal 
  silica," 
  where 
  "it 
  is 
  believed 
  to 
  have 
  been 
  pre- 
  

   cipitated 
  by 
  the 
  alkali 
  salts 
  in 
  the 
  sea 
  water 
  after 
  it 
  had 
  

   undergone 
  considerable 
  dispersion 
  and 
  a 
  certain 
  amount 
  

   of 
  concentration 
  — 
  ; 
  the 
  silica 
  thus 
  precipitated 
  on 
  the 
  

   sea 
  bottom 
  would 
  tend 
  to 
  assume 
  more 
  or 
  less 
  globular 
  or 
  

   elliptical 
  forms. 
  Upon 
  burial 
  these 
  forms 
  would 
  become 
  

   compressed 
  into 
  their 
  elliptical 
  or 
  lenticular 
  shapes 
  by 
  

   weight 
  of 
  the 
  accumulating 
  sediments. 
  Fossils 
  falling 
  

   into 
  this 
  soft 
  colloidal 
  mass 
  would 
  be 
  perfectly 
  pre- 
  

   served" 
  (p. 
  428). 
  It 
  is 
  argued 
  that 
  the 
  "gel-mass" 
  was 
  

   for 
  a 
  while 
  carried 
  about 
  by 
  currents, 
  until 
  its 
  mass 
  had 
  

   become 
  sufficiently 
  great 
  to 
  cause 
  it 
  to 
  sink 
  to 
  the 
  bottom. 
  

   "If 
  the 
  waters 
  were 
  shallow, 
  currents 
  and 
  waves 
  might 
  

   have 
  shifted 
  and 
  rolled 
  the 
  masses 
  of 
  gel 
  about, 
  thus 
  tend- 
  

   ing 
  to 
  form 
  larger 
  aggregates. 
  This 
  rolling 
  aided 
  in 
  pro- 
  

   ducing 
  the 
  banding 
  of 
  the 
  chert, 
  but 
  only 
  the 
  relatively 
  

   small 
  masses 
  were 
  rolled 
  about. 
  Circular 
  bands 
  are 
  

   found 
  only 
  in 
  the 
  smaller 
  aggregates, 
  those 
  not 
  exceeding 
  

   eight 
  to 
  ten 
  inches 
  in 
  diameter. 
  " 
  "If 
  the 
  currents 
  were 
  

   unable 
  to 
  move 
  the 
  masses, 
  the 
  latter 
  would 
  have 
  grown 
  

   through 
  mass 
  action, 
  the 
  larger 
  aggregates 
  attracting 
  the 
  

   smaller 
  ones. 
  As 
  growth 
  proceeded, 
  the 
  banding 
  and 
  

   mottling 
  of 
  the 
  chert 
  was 
  developed, 
  due 
  to 
  carbonaceous 
  

   materials 
  that 
  are 
  believed 
  to 
  have 
  been 
  included 
  in 
  the 
  

   chert 
  as 
  it 
  grew" 
  (p. 
  437). 
  "When 
  several 
  nodules 
  were 
  

   formed 
  close 
  together 
  they 
  united 
  to 
  form 
  a 
  larger 
  mass 
  

   which 
  shows 
  a 
  lobed 
  circular 
  outline 
  from 
  the 
  flattening 
  of 
  

   the 
  several 
  spherical 
  aggregates" 
  (p. 
  439). 
  

  

  Certain 
  other 
  characteristics 
  of 
  cherts 
  which 
  might 
  

   have 
  developed 
  in 
  this 
  manner 
  may 
  be 
  suggested. 
  If 
  the 
  

   postulated 
  masses 
  of 
  "gel" 
  were 
  rolled 
  around 
  by 
  waves 
  

   and 
  currents 
  so 
  as 
  to 
  develop 
  the 
  banding 
  through 
  the 
  

   inclusion 
  of 
  carbonaceous 
  matter, 
  there 
  would 
  also 
  be 
  

   included 
  the 
  shells 
  of 
  small 
  organisms 
  which 
  lay 
  upon 
  the 
  

   sen 
  bottom, 
  and 
  as 
  a 
  consequence 
  there 
  ought 
  to 
  be 
  some 
  

   soil 
  of 
  banded 
  arrangement 
  in 
  the 
  distribution 
  of 
  the 
  

   fossils, 
  and 
  their 
  longer 
  axes 
  should 
  be 
  parallel 
  to 
  the 
  tan- 
  

  

  10 
  Tarr, 
  op. 
  cit. 
  

  

  