﻿SILICA 
  IN 
  THE 
  LOWER 
  CHALK 
  OP 
  BERKSHIRE 
  AND 
  WILTSHIRE. 
  419 
  

  

  Again, 
  colloid 
  silica 
  of 
  the 
  same 
  character 
  often 
  fills 
  the 
  chambers 
  

   of 
  Foraminifera, 
  and 
  in 
  other 
  cases 
  half-globules 
  Or 
  discs 
  of 
  such 
  

   silica 
  are 
  seen 
  clinging 
  to 
  both 
  the 
  inner 
  and 
  outer 
  surfaces 
  of 
  a 
  

   Foraminiferal 
  chamber 
  — 
  modes 
  of 
  occurrence 
  which 
  it 
  is 
  difficult, 
  if 
  

   not 
  impossible, 
  to 
  account 
  for 
  on 
  any 
  theory 
  except 
  that 
  of 
  pre- 
  

   cipitation 
  from 
  solution. 
  

  

  We 
  are 
  therefore 
  compelled 
  to 
  suppose 
  that 
  the 
  globular 
  silica 
  

   was 
  precipitated 
  from 
  solution, 
  and 
  that 
  this 
  precipitation 
  took 
  

   place 
  before 
  the 
  consolidation 
  of 
  the 
  beds, 
  and 
  while 
  they 
  were 
  

   still 
  permeated 
  by 
  sea-water. 
  That 
  oceanic 
  waters 
  contain 
  silica 
  in 
  

   solution 
  is 
  certain, 
  because 
  it 
  must 
  be 
  from 
  such 
  a 
  solution 
  of 
  silica 
  

   that 
  the 
  sponges 
  obtain 
  the 
  material 
  secreted 
  in 
  their 
  spicular 
  

   skeletons, 
  and 
  in 
  deep 
  water, 
  where 
  the 
  pressure 
  is 
  great, 
  the 
  

   amount 
  of 
  silica 
  in 
  solution 
  may 
  be 
  considerable 
  ; 
  but 
  it 
  is 
  very 
  diffi- 
  

   cult 
  to 
  understand 
  the 
  precipitation 
  of 
  any 
  of 
  this 
  dissolved 
  silica 
  with- 
  

   out 
  the 
  intervention 
  of 
  organic 
  agencies. 
  It 
  is 
  hardly 
  possible 
  to 
  

   conceive 
  conditions 
  under 
  which 
  the 
  water 
  could 
  contain 
  a 
  saturated 
  

   solution 
  of 
  silica 
  ; 
  but 
  we 
  may 
  perhaps 
  imagine 
  that 
  the 
  water 
  

   which 
  lies 
  over 
  large 
  fields 
  of 
  sponge-spicules 
  holds 
  more 
  silica 
  in 
  

   solution 
  than 
  that 
  overlying 
  more 
  calcareous 
  bottoms, 
  which 
  may 
  

   perhaps 
  be 
  saturated 
  with 
  carbonate 
  of 
  lime. 
  If 
  this 
  were 
  so, 
  and 
  

   if 
  a 
  gentle 
  current 
  then 
  brought 
  the 
  lime-bearing 
  water 
  into 
  the 
  

   siliceous 
  tract, 
  it 
  is 
  perhaps 
  conceivable 
  that 
  some 
  of 
  the 
  silica 
  

   might 
  be 
  displaced 
  by 
  the 
  lime 
  ; 
  but 
  we 
  admit 
  that 
  we 
  are 
  here 
  losing 
  

   ourselves 
  in 
  the 
  realms 
  of 
  speculation. 
  

  

  In 
  the 
  above 
  considerations 
  we 
  have 
  already 
  dealt 
  only 
  with 
  the 
  

   colloid 
  silica, 
  because 
  it 
  seemed 
  clear 
  to 
  us 
  that 
  the 
  precipitation 
  of 
  

   the 
  chalcedonic 
  silica 
  of 
  the 
  cherty 
  nodules 
  was 
  a 
  secondary 
  and 
  

   subsequent 
  operation, 
  and 
  that 
  the 
  conditions 
  under 
  which 
  it 
  took 
  

   place 
  were 
  different 
  from 
  those 
  which 
  favoured 
  the 
  precipitation 
  of 
  

   the 
  colloid 
  globular 
  silica. 
  We 
  are 
  disposed 
  to 
  regard 
  all 
  nodular 
  

   concretions 
  resembling 
  flints 
  and 
  phosphatic 
  nodules 
  as 
  growths 
  which 
  

   were 
  more 
  or 
  less 
  contemporaneous 
  with 
  the 
  deposition 
  of 
  the 
  

   materials 
  of 
  the 
  enclosing 
  rock 
  ; 
  and 
  we 
  should 
  not 
  accept 
  the 
  view 
  

   that 
  the 
  cherty 
  nodules 
  in 
  the 
  Malmstone 
  and 
  Lower 
  Chalk 
  were 
  

   formed 
  after 
  the 
  consolidation 
  of 
  those 
  rocks 
  without 
  very 
  strong 
  

   evidence 
  to 
  that 
  effect. 
  We 
  are, 
  however, 
  inclined 
  to 
  connect 
  the 
  

   disappearance 
  of 
  the 
  silica 
  which 
  once 
  occupied 
  the 
  empty 
  spicule- 
  

   casts, 
  and 
  that 
  of 
  the 
  spicules 
  which 
  are 
  now 
  replaced 
  by 
  calcite, 
  with 
  

   the 
  formation 
  of 
  the 
  chalcedonic 
  nodules 
  ; 
  and 
  if 
  this 
  is 
  correct, 
  the 
  

   rocks 
  must 
  have 
  been 
  so 
  far 
  consolidated 
  that 
  little 
  mechanical 
  

   movement 
  of 
  their 
  component 
  particles 
  could 
  be 
  effected. 
  If, 
  how- 
  

   ever, 
  masses 
  of 
  colloid 
  protoplasm 
  like 
  those 
  described 
  by 
  Dr. 
  

   Wallich 
  * 
  were 
  enclosed 
  in 
  the 
  deposit, 
  their 
  slow 
  and 
  gradual 
  

   decomposition 
  might 
  conceivably 
  set 
  up 
  chemical 
  reactions, 
  whereby 
  

   the 
  silica 
  of 
  the 
  spicules 
  in 
  the 
  surrounding 
  mass 
  might 
  be 
  dissolved 
  

   and 
  re-deposited 
  in 
  the 
  place 
  of 
  the 
  organic 
  matter. 
  

  

  We 
  are 
  here 
  approaching 
  the 
  problem 
  of 
  the 
  formation 
  of 
  flints, 
  and 
  

   we 
  think 
  it 
  will 
  hardly 
  be 
  denied 
  that 
  the 
  existence 
  of 
  the 
  four 
  kinds 
  

  

  * 
  Quart, 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xxxvi. 
  p. 
  79. 
  

  

  2h2 
  

  

  