﻿426 
  Twenhofel 
  — 
  Wreford 
  and 
  Foraher 
  Limestones. 
  

  

  tated 
  in 
  extremely 
  small 
  particles 
  and 
  that 
  these 
  are 
  

   buried 
  with 
  other 
  sediments. 
  No 
  such 
  "gel-masses" 
  

   appear 
  to 
  have 
  been 
  reported 
  from 
  ocean 
  waters 
  or 
  ocean 
  

   sediments. 
  

  

  Theory 
  of 
  partial 
  replacement 
  of 
  unconsolidated 
  lime 
  sediments 
  

   either 
  before 
  or 
  during 
  the 
  process 
  of 
  consolidation 
  

  

  This 
  theory 
  in 
  its 
  broadest 
  statement 
  maintains 
  that 
  

   the 
  silica 
  replaced 
  unconsolidated 
  sediments 
  either 
  before 
  

   or 
  during 
  the 
  process 
  of 
  consolidation, 
  the 
  silica 
  being 
  

   derived 
  from 
  organic 
  and 
  other 
  silica 
  mingled 
  with 
  the 
  

   sediments, 
  either 
  in 
  solid 
  form 
  or 
  in 
  solution. 
  The 
  silica 
  

   in 
  solution 
  in 
  the 
  sea 
  water 
  is 
  not 
  excluded 
  from 
  aiding 
  

   in 
  the 
  formation 
  of 
  the 
  chert. 
  The 
  term 
  replacement 
  is 
  

   used 
  not 
  only 
  to 
  include 
  actual 
  substitution 
  of 
  the 
  lime 
  

   sediments, 
  but 
  also 
  replacement 
  of 
  the 
  space 
  held 
  by 
  them 
  

   through 
  their 
  being 
  pushed 
  out 
  of 
  the 
  way. 
  Organic 
  sub- 
  

   stances 
  and 
  organic 
  silica 
  are 
  believed 
  to 
  have 
  quite 
  com- 
  

   monly 
  served 
  as 
  nuclei 
  for 
  the 
  nodules. 
  

  

  As 
  conditions 
  on 
  the 
  sea 
  bottom 
  change 
  from 
  time 
  to 
  

   time, 
  and 
  as 
  each 
  bed 
  represents 
  a 
  somewhat 
  different 
  

   condition 
  from 
  that 
  obtaining 
  during 
  the 
  formation 
  of 
  a 
  

   previous 
  or 
  subsequent 
  bed, 
  it 
  readily 
  follows 
  that 
  the 
  

   chert 
  of 
  each 
  bed 
  may 
  be 
  quite 
  different 
  from 
  that 
  of 
  any 
  

   other 
  bed. 
  Each 
  separate 
  band 
  probably 
  represents 
  a 
  

   separate 
  period 
  of 
  accretion 
  under 
  slightly 
  different 
  con- 
  

   ditions, 
  and 
  if 
  the 
  bands 
  show 
  a 
  rhythmic 
  character, 
  such 
  

   as 
  exists 
  in 
  the 
  third 
  chert 
  zone 
  of 
  the 
  Wreford, 
  it 
  points 
  

   to 
  a 
  recurrence 
  of 
  similar 
  conditions 
  which 
  may 
  well 
  have 
  

   been 
  seasonal. 
  If 
  the 
  recent 
  suggestion 
  of 
  Barrell, 
  12 
  that 
  

   geologic 
  time 
  should 
  be 
  expanded 
  in 
  large 
  multiples 
  and 
  

   that 
  each 
  bed 
  represents 
  a 
  much 
  longer 
  time 
  than 
  has 
  

   previously 
  been 
  supposed, 
  approximate 
  a 
  correct 
  esti- 
  

   mate 
  of 
  geologic 
  time 
  and 
  the 
  duration 
  of 
  sedimentary 
  

   processes, 
  there 
  would 
  be 
  given 
  time 
  for 
  as 
  many 
  periods 
  

   of 
  accretion 
  as 
  one 
  could 
  well 
  wish. 
  At 
  any 
  rate, 
  not 
  

   more 
  than 
  one 
  hundred 
  years 
  would 
  be 
  required 
  for 
  the 
  

   development 
  of 
  a 
  Wreford 
  nodule 
  of 
  the 
  largest 
  size 
  on 
  

   the 
  assumption 
  that 
  each 
  band 
  is 
  an 
  annual 
  accretion, 
  and 
  

   a 
  much 
  shorter 
  time 
  would 
  be 
  required 
  if 
  the 
  bands 
  rep- 
  

   resent 
  seasonal 
  accretions. 
  Since 
  the 
  sediments 
  were 
  

   still 
  soft 
  while 
  the 
  nodules 
  were 
  growing, 
  there 
  would 
  be 
  

  

  12 
  Barrell, 
  J., 
  Bull. 
  Geol. 
  Soc. 
  America, 
  28, 
  884, 
  1917. 
  

  

  