﻿418 
  MESSES. 
  A. 
  J. 
  jrXES-BROTVJTE 
  AND 
  W. 
  HILL 
  ON 
  COLLOID 
  

  

  bed 
  of 
  chalk, 
  the 
  re-arrangement 
  of 
  particles 
  throughout 
  the 
  whole 
  

   mass 
  of 
  the 
  rock 
  can 
  hardly 
  be 
  imagined 
  as 
  having 
  taken 
  place 
  after 
  

   even 
  partial 
  consolidation. 
  In 
  the 
  case 
  of 
  a 
  calcareous 
  ooze 
  con- 
  

   taining 
  numerous 
  spicules, 
  and 
  partially 
  consolidated, 
  i. 
  e., 
  com- 
  

   pacted 
  and 
  compressed 
  by 
  the 
  weight 
  of 
  overlying 
  deposits, 
  the 
  

   spicules 
  might 
  be 
  dissolved, 
  but 
  the 
  silica 
  could 
  hardly 
  be 
  re-deposited 
  

   within 
  the 
  same 
  bed 
  without 
  a 
  replacement 
  of 
  some 
  part 
  of 
  the 
  

   material 
  forming 
  that 
  bed. 
  

  

  It 
  seems 
  to 
  us 
  that 
  the 
  most 
  natural 
  conclusion 
  to 
  be 
  drawn 
  from 
  

   the 
  facts 
  is 
  that 
  the 
  intimate 
  mixture 
  of 
  calcareous 
  and 
  siliceous 
  

   material 
  was 
  formed 
  before 
  the 
  consolidation 
  of 
  the 
  rock, 
  and 
  while 
  

   it 
  was 
  still 
  in 
  a 
  sufficiently 
  oozy 
  condition 
  to 
  admit 
  of 
  easy 
  molecular 
  

   redistribution 
  ; 
  and 
  we 
  think 
  that 
  this 
  was 
  the 
  case 
  with 
  the 
  Malm- 
  

   stone 
  as 
  well. 
  

  

  We 
  are, 
  of 
  course, 
  prepared 
  to 
  admit 
  that 
  the 
  empty 
  casts 
  of 
  

   spicules 
  which 
  occur 
  in 
  the 
  Malm 
  stones, 
  and 
  occasionally 
  in 
  the 
  

   Chalk, 
  prove 
  that 
  some 
  spicules 
  were 
  dissolved 
  after 
  the 
  consolida- 
  

   tion 
  of 
  the 
  rocks 
  ; 
  but 
  we 
  see 
  no 
  reason 
  to 
  think 
  that 
  the 
  silica 
  from 
  

   these 
  particular 
  spicules 
  has 
  gone 
  to 
  form 
  any 
  part 
  of 
  the 
  globular 
  

   silica 
  which 
  is 
  disseminated 
  through 
  the 
  rock, 
  and 
  makes 
  up 
  so 
  

   large 
  a 
  part 
  of 
  their 
  mass. 
  

  

  The 
  final 
  problem 
  presented 
  for 
  our 
  consideration 
  is 
  to 
  ascertain 
  

   the 
  manner 
  in 
  which 
  the 
  globular 
  silica 
  was 
  formed 
  and 
  dissemi- 
  

   nated 
  through 
  the 
  beds 
  of 
  chalk. 
  This 
  is 
  a 
  question 
  in 
  chemical 
  

   geology, 
  for 
  the 
  proper 
  discussion 
  of 
  which 
  we 
  do 
  not 
  feel 
  ourselves 
  

   very 
  well 
  fitted, 
  and 
  the 
  remarks 
  we 
  venture 
  to 
  offer 
  must 
  be 
  taken 
  

   as 
  suggestive, 
  rather 
  than 
  as 
  the 
  expressions 
  of 
  qualified 
  opinion. 
  

  

  "We 
  were 
  at 
  first 
  inclined 
  to 
  think 
  that 
  the 
  disseminated 
  colloid 
  

   6ilica 
  had 
  been 
  derived 
  directly 
  from 
  the 
  disintegration 
  of 
  spicules 
  

   in 
  which 
  the 
  globular 
  structure 
  had 
  been 
  previously 
  developed 
  ; 
  

   this 
  molecular 
  change 
  in 
  the 
  substance 
  of 
  the 
  spicules 
  may, 
  in 
  fact, 
  

   be 
  seen 
  in 
  every 
  stage 
  of 
  progress, 
  and 
  we 
  thought 
  it 
  possible 
  that 
  

   when 
  the 
  globules 
  had 
  been 
  so 
  formed 
  they 
  might 
  resist 
  the 
  action 
  

   of 
  solvents 
  better 
  than 
  the 
  unaltered 
  outer 
  wall 
  of 
  the 
  spicule, 
  so 
  

   that 
  when 
  set 
  free 
  by 
  the 
  solution 
  of 
  the 
  latter 
  they 
  would 
  become 
  

   mixed 
  with 
  and 
  distributed 
  through 
  the 
  surrounding 
  calcareous 
  

   ooze. 
  

  

  This 
  theory 
  would 
  satisfactorily 
  explain 
  the 
  existence 
  of 
  the 
  

   disseminated 
  silica 
  in 
  a 
  globular 
  form 
  without 
  the 
  necessity 
  of 
  sup- 
  

   posing 
  that 
  it 
  had 
  gone 
  through 
  a 
  process 
  of 
  solution 
  and 
  precipi- 
  

   tation 
  as 
  colloid 
  silica, 
  but 
  it 
  did 
  not 
  prove 
  to 
  be 
  in 
  accordance 
  with 
  

   all 
  the 
  observed 
  facts. 
  If 
  the 
  silica 
  only 
  occurred 
  in 
  the 
  shape 
  of 
  

   free 
  globules 
  or 
  in 
  linear 
  series 
  of 
  globules, 
  there 
  would 
  be 
  less 
  

   difficulty; 
  but 
  it 
  often 
  occurs 
  in 
  irregular 
  masses 
  or 
  lumps 
  far 
  larger 
  

   than 
  the 
  diameter 
  of 
  any 
  spicule, 
  and 
  in 
  such 
  lumps 
  it 
  is 
  only 
  on 
  

   the 
  outside 
  that 
  well-formed 
  globules 
  can 
  be 
  seen, 
  the 
  inside 
  show- 
  

   ing 
  only 
  an 
  indefinite 
  kind 
  of 
  segregation, 
  as 
  if 
  the 
  mass 
  had 
  grown 
  

   too 
  rapidly 
  for 
  the 
  development 
  of 
  free 
  circular 
  surfaces. 
  The 
  

   existence 
  of 
  such 
  bunches 
  or 
  aggregations 
  of 
  malformed 
  globules 
  

   seems, 
  indeed, 
  to 
  indicate 
  growth 
  or 
  precipitation 
  from 
  solution. 
  

  

  