﻿Tivenhofel 
  — 
  Wreford 
  and 
  Foraker 
  Limestones. 
  427 
  

  

  no 
  good 
  reason 
  for 
  the 
  preservation 
  of 
  structures 
  which 
  

   were 
  not 
  yet 
  in 
  solid 
  form, 
  and 
  these 
  might, 
  or 
  might 
  not, 
  

   appear 
  in 
  the 
  cherts. 
  Fossils 
  would 
  quite 
  certainly 
  be 
  

   preserved 
  and 
  the 
  preservation 
  could 
  be 
  better 
  than, 
  as 
  

   good 
  as, 
  or 
  worse 
  than, 
  that 
  of 
  those 
  in 
  the 
  associated 
  

   materials. 
  

  

  The 
  process 
  of 
  the 
  growth 
  of 
  a 
  nodule 
  is 
  thought 
  to 
  

   have 
  been 
  something 
  as 
  follows. 
  As 
  silica 
  in 
  solution 
  

   was 
  carried 
  through 
  the 
  sediments 
  by 
  diffusion, 
  it 
  is 
  

   believed 
  to 
  have 
  been 
  precipitated 
  about 
  centers, 
  such 
  as 
  

   small 
  pieces 
  of 
  silica, 
  fragments 
  or 
  organic 
  matter, 
  and, 
  

   slightly 
  extending 
  previous 
  conceptions, 
  perhaps 
  a 
  very 
  

   small 
  "gel-mass" 
  of 
  the 
  type 
  postulated 
  by 
  Tarr. 
  The 
  

   dominating 
  factor 
  in 
  the 
  process 
  of 
  nodule 
  growth 
  was 
  

   the 
  attraction 
  exercised 
  on 
  the 
  dissolved 
  silica 
  by 
  the 
  

   growing 
  nodule. 
  As 
  silica 
  was 
  taken 
  from 
  solution, 
  it 
  

   destroyed 
  the 
  equilibrium 
  of 
  the 
  solute, 
  resulting 
  in 
  more 
  

   silica 
  being 
  diffused 
  toward 
  the 
  nodule. 
  A 
  relatively 
  

   sharp 
  boundary 
  was 
  maintained 
  between 
  the 
  limestone 
  

   and 
  the 
  chert 
  by 
  the 
  pressure 
  which 
  the 
  growing 
  nodule 
  

   exercised 
  on 
  the 
  associated 
  lime 
  sediments 
  and 
  which 
  

   favored 
  their 
  going 
  into 
  solution. 
  13 
  That 
  this 
  boundary 
  

   is 
  graclative 
  so 
  far 
  as 
  the 
  two 
  substances 
  are 
  concerned 
  

   is 
  shown 
  by 
  the 
  cherts 
  which 
  have 
  been 
  described 
  in 
  this 
  

   article. 
  In 
  the 
  development 
  of 
  the 
  nodule 
  any 
  organic 
  

   substances 
  could 
  become 
  incorporated 
  and 
  these 
  could 
  be 
  

   oriented 
  in 
  any 
  direction. 
  Masses 
  of 
  lime 
  sediments, 
  

   crystals 
  of 
  calcite, 
  and 
  all 
  sorts 
  of 
  material 
  could 
  and 
  

   should 
  be 
  present. 
  As 
  the 
  movement 
  of 
  silica 
  through 
  

   the 
  sediments 
  would 
  be 
  greatest 
  laterally 
  because 
  of 
  bed- 
  

   ding 
  and 
  lamination 
  planes, 
  and 
  least 
  from 
  below 
  because 
  

   of 
  greater 
  pressure 
  and 
  hence 
  less 
  permeability 
  in 
  that 
  

   direction, 
  the 
  greater 
  width 
  of 
  the 
  bands 
  laterally 
  and 
  

   their 
  least 
  width 
  below 
  is 
  readily 
  explained. 
  The 
  differ- 
  

   ent 
  widths 
  of 
  bands 
  on 
  opposite 
  sides 
  would 
  arise 
  from 
  

   differences 
  in 
  the 
  quantity 
  of 
  silica 
  supplied 
  from 
  differ- 
  

   ent 
  directions, 
  while 
  the 
  different 
  widths 
  of 
  the 
  bands 
  in 
  

   radial 
  section 
  could 
  be 
  referred 
  to 
  differences 
  in 
  the 
  quan- 
  

   tity 
  of 
  silica 
  supplied 
  from 
  period 
  to 
  period. 
  

  

  The 
  chert 
  of 
  the 
  Foraker 
  limestone 
  is 
  believed 
  to 
  have 
  

   developed 
  as 
  outlined. 
  The 
  theory 
  harmonizes 
  with 
  the 
  

   facts 
  as 
  they 
  relate 
  to 
  this 
  chert 
  and 
  fully 
  explains 
  the 
  

   distribution 
  and 
  preservation 
  of 
  Fusulina. 
  

  

  13 
  Faber, 
  S., 
  this 
  Journal, 
  41, 
  555, 
  1916. 
  

  

  