﻿Twenhofel 
  — 
  Wreford 
  and 
  Foraker 
  Limestones. 
  425 
  

  

  which 
  they 
  may 
  have 
  floated. 
  If 
  they 
  were 
  dissolved 
  and 
  

   precipitated 
  about 
  other 
  sponge 
  spicules, 
  those 
  in 
  the 
  

   chert, 
  for 
  instance, 
  a 
  method 
  of 
  origin 
  considered 
  in 
  the 
  

   succeeding 
  topic 
  is 
  approximated. 
  

  

  The 
  third 
  chert 
  zone 
  of 
  the 
  Wreford, 
  that 
  in 
  which 
  the 
  

   large 
  banded 
  nodules 
  occur, 
  is 
  also 
  difficult 
  to 
  explain 
  on 
  

   the 
  theory 
  of 
  original 
  deposition. 
  According 
  to 
  this 
  

   theory, 
  the 
  nodules 
  should 
  be 
  flattened 
  parallel 
  to 
  the 
  bed- 
  

   ding, 
  and 
  this 
  is 
  true 
  for 
  many 
  and 
  perhaps 
  most 
  of 
  them 
  ; 
  

   but 
  there 
  are 
  many 
  others 
  which 
  are 
  globular 
  and 
  not 
  flat- 
  

   tened 
  at 
  all. 
  Others 
  are 
  of 
  irregular 
  shapes 
  and 
  still 
  

   others 
  have 
  their 
  longest 
  axes 
  perpendicular 
  to 
  the 
  bed- 
  

   ding. 
  In 
  places 
  the 
  chert 
  composes 
  the 
  entire 
  zone. 
  

   Many 
  of 
  the 
  nodules 
  have 
  diameters 
  of 
  three 
  feet 
  or 
  more, 
  

   and 
  these, 
  like 
  the 
  small 
  ones, 
  have 
  concentric 
  structure. 
  

   If 
  these 
  were 
  once 
  " 
  gel-masses' 
  ' 
  of 
  silica 
  which 
  were 
  

   rolled 
  around 
  so 
  as 
  to 
  give 
  them 
  concentric 
  structure, 
  it 
  

   implies 
  rather 
  strong 
  currents 
  for 
  which 
  the 
  associated 
  

   material 
  gives 
  no 
  evidence 
  whatever. 
  If 
  they 
  were 
  not 
  

   rolled 
  around, 
  the 
  greater 
  additions 
  of 
  silica 
  should 
  have 
  

   been 
  from 
  above 
  and 
  laterally 
  and, 
  if 
  this 
  could 
  have 
  been 
  

   deposited 
  in 
  banded 
  form, 
  the 
  banding 
  which 
  is 
  present 
  

   in 
  these 
  nodules 
  would 
  have 
  been 
  realized. 
  At 
  the 
  lat- 
  

   eral 
  peripheries, 
  the 
  bands 
  are 
  two 
  to 
  three 
  times 
  as 
  wide 
  

   as 
  on 
  top. 
  This 
  might 
  have 
  been 
  brought 
  about 
  by 
  the 
  

   flattening 
  through 
  their 
  own 
  weight 
  or 
  the 
  overburden; 
  

   but, 
  as 
  will 
  be 
  shown 
  subsequently, 
  the 
  banding 
  may 
  be 
  

   explained 
  equally 
  as 
  well 
  in 
  another 
  way. 
  The 
  some- 
  

   what 
  botryoidal 
  aspect 
  of 
  the 
  exterior 
  shows 
  the 
  develop- 
  

   ment 
  of 
  many 
  centers 
  of 
  accretion; 
  but 
  these, 
  too, 
  have 
  

   a 
  concentric 
  structure 
  which 
  appears 
  to 
  be 
  no 
  more 
  than 
  

   an 
  outpushing 
  of 
  the 
  interior 
  bands. 
  These 
  could 
  not 
  

   have 
  been 
  developed 
  through 
  any 
  process 
  of 
  rolling 
  or 
  by 
  

   the 
  cohering 
  of 
  one 
  "gel-mass" 
  to 
  another. 
  

   ^ 
  The 
  observed 
  facts 
  relating 
  to 
  the 
  cherts 
  under 
  con- 
  

   sideration 
  do 
  not 
  favor 
  an 
  origin 
  through 
  direct 
  pre- 
  

   cipitation 
  from 
  silica 
  in 
  solution 
  in 
  the 
  sea 
  water, 
  the 
  

   precipitation 
  and 
  formation 
  of 
  the 
  nodules 
  taking 
  place 
  

   in 
  accordance 
  with 
  the 
  theory 
  outlined 
  in 
  this 
  topic. 
  

   That 
  processes 
  of 
  the 
  nature 
  considered 
  may 
  have 
  

   assisted 
  in 
  the 
  formation 
  is 
  quite 
  possible. 
  Further- 
  

   more, 
  the 
  writer 
  considers 
  it 
  extremely 
  improbable 
  that 
  

   large 
  "gel-masses" 
  could 
  have 
  developed 
  in 
  the 
  manner 
  

   postulated, 
  and 
  more 
  probable 
  that 
  the 
  silica 
  is 
  precipi- 
  

  

  