﻿420 
  MESSES. 
  A. 
  J. 
  JTTKES-BEOWNE 
  AND 
  W. 
  HILL 
  ON 
  COLLOID 
  

  

  of 
  siliceous 
  chalk 
  mentioned 
  on 
  pp. 
  415-416 
  is 
  suggestive 
  of 
  the 
  infer- 
  

   ence 
  that 
  the 
  first 
  three 
  are 
  stages 
  in 
  the 
  production 
  of 
  highly 
  siliceous 
  

   chalk, 
  and 
  that 
  the 
  fourth 
  process 
  has 
  resulted 
  in 
  the 
  formation 
  of 
  

   a 
  nodule 
  which 
  bears 
  a 
  very 
  strong 
  resemblance 
  to 
  a 
  flint. 
  In 
  this 
  

   connexion 
  we 
  may 
  refer 
  to 
  a 
  paper 
  by 
  Prof. 
  W. 
  J. 
  Sollas, 
  written 
  

   in 
  1880, 
  wherein 
  he 
  states 
  his 
  belief 
  that 
  flints 
  have 
  been 
  formed 
  

   by 
  the 
  gradual 
  elimination 
  of 
  the 
  calcareous 
  matter 
  from 
  a 
  mass 
  of 
  

   siliceous 
  chalk, 
  and 
  its 
  replacement 
  by 
  silica. 
  " 
  Briefly 
  to 
  sum 
  up," 
  

   he 
  says 
  *, 
  " 
  a 
  deposit 
  of 
  sponge-spicules 
  accumulated 
  in 
  the 
  chalk 
  

   ooze, 
  and 
  in 
  the 
  presence 
  of 
  sea-water 
  under 
  pressure, 
  entered 
  into 
  

   solution. 
  Eeplacement 
  of 
  the 
  calcareous 
  material 
  of 
  the 
  ooze 
  then 
  

   ensued, 
  small 
  shells, 
  and 
  many 
  large 
  ones 
  too, 
  being 
  converted 
  into 
  

   silex 
  ; 
  and 
  siliceous 
  chalk, 
  not 
  flint, 
  was 
  the 
  result. 
  The 
  chambers 
  

   of 
  the 
  Eoraminifera 
  and 
  the 
  interstices 
  of 
  the 
  chalk 
  were 
  now 
  filled 
  

   up 
  by 
  a 
  simple 
  deposition 
  of 
  silica, 
  and 
  the 
  siliceous 
  chalk 
  became 
  

   converted 
  into 
  black 
  flint, 
  an 
  incompletely 
  silicified 
  layer 
  of 
  chalk 
  

   remaining 
  as 
  the 
  white 
  layer 
  of 
  its 
  surface." 
  

  

  This 
  theory 
  was 
  suggested 
  by 
  the 
  presence 
  of 
  siliceous 
  chalk, 
  a 
  

   mixture 
  of 
  chalky 
  powder 
  and 
  siliceous 
  spicules, 
  inside 
  certain 
  flint 
  

   nodules, 
  and 
  he 
  was 
  therefore 
  imagining 
  a 
  different 
  kind 
  of 
  siliceous 
  

   chalk 
  from 
  that 
  described 
  in 
  the 
  preceding 
  pages. 
  The 
  chert 
  

   nodules, 
  however, 
  may 
  be 
  regarded 
  as 
  siliceous 
  chalk 
  (i. 
  e. 
  a 
  mixture 
  

   of 
  chalk 
  and 
  colloid 
  silica) 
  still 
  further 
  silicified 
  in 
  the 
  manner 
  

   suggested 
  by 
  Prof. 
  Sollas, 
  but 
  not 
  so 
  completely 
  silicified 
  as 
  to 
  be 
  

   converted 
  into 
  flint. 
  It 
  is 
  not 
  difficult 
  to 
  imagine 
  the 
  effect 
  of 
  a 
  

   further 
  influx 
  of 
  silica 
  in 
  sufficient 
  quantity 
  to 
  replace 
  all 
  the 
  

   remaining 
  calcareous 
  matter 
  ; 
  all 
  the 
  separate 
  particles 
  which 
  are 
  

   now 
  distinguishable 
  would 
  then 
  be 
  obscured 
  and 
  merged 
  into 
  the 
  

   solid 
  translucent 
  chalcedonic 
  matrix 
  or 
  medium, 
  just 
  as 
  the 
  spicules 
  

   which 
  can 
  sometimes 
  be 
  seen 
  in 
  hollow 
  flints, 
  half 
  freely 
  projecting, 
  

   and 
  half 
  completely 
  merged 
  in 
  the 
  solid 
  flint. 
  Probably 
  if 
  these 
  

   nodules 
  had 
  been 
  converted 
  into 
  flints 
  their 
  colour 
  would 
  have 
  been 
  

   grey 
  like 
  those 
  in 
  the 
  Middle 
  Chalk 
  of 
  Lincolnshire. 
  

  

  Discussion. 
  

  

  Dr. 
  Evans 
  was 
  struck 
  with 
  the 
  difficulty 
  which 
  had 
  been 
  expe- 
  

   rienced 
  in 
  determining 
  what 
  was 
  colloid 
  silica. 
  The 
  term 
  was 
  liable 
  

   to 
  mislead. 
  Plint 
  had 
  been 
  defined 
  as 
  being 
  of 
  two 
  kinds, 
  a 
  crys- 
  

   talline 
  and 
  an 
  amorphous 
  silica. 
  The 
  solubility 
  materially 
  varied 
  

   with 
  the 
  solvent 
  used. 
  The 
  ordinary 
  chalk-flints 
  contain 
  much 
  

   silica 
  soluble 
  in 
  ordinary 
  water 
  ; 
  this 
  is 
  strictly 
  colloid 
  silica, 
  and 
  it 
  

   has 
  been 
  removed 
  from 
  flint 
  implements 
  within 
  human 
  times. 
  The 
  

   question 
  before 
  the 
  Society 
  would 
  be 
  simplified 
  if 
  the 
  real 
  nature 
  of 
  

   colloid 
  silica 
  were 
  better 
  defined. 
  

  

  Dr. 
  Hinde 
  stated 
  that 
  the 
  specimens 
  and 
  sections 
  from 
  the 
  Chalk 
  

  

  Marl 
  exhibited 
  by 
  the 
  Authors 
  were 
  of 
  a 
  precisely 
  similar 
  character 
  

  

  to 
  those 
  which 
  he 
  had 
  described 
  from 
  the 
  Malmstones 
  of 
  the 
  Upper 
  

  

  Greensand. 
  His 
  own 
  work 
  was 
  directed 
  not 
  so 
  much 
  to 
  ascertain 
  the 
  

  

  * 
  Ann. 
  & 
  Mag. 
  Nat. 
  Hist. 
  ser. 
  5. 
  vol. 
  vi. 
  p. 
  449. 
  

  

  