﻿SILICA 
  IN 
  THE 
  LOWER 
  CHALK 
  OE 
  BERKSHIRE 
  AND 
  WILTSHIRE. 
  415 
  

  

  " 
  Soluble 
  in 
  alkaline 
  carbonate 
  after 
  

  

  boiling 
  15 
  minutes 
  12'61 
  

  

  Insoluble 
  in 
  tbe 
  same 
  22*56 
  

  

  35-17 
  

  

  " 
  The 
  silica 
  formed 
  such 
  a 
  network 
  that 
  it 
  prevented 
  the 
  acid 
  

   getting 
  at 
  the 
  carbonate 
  of 
  lime 
  to 
  dissolve 
  it, 
  the 
  effervescence 
  

   ceasing 
  till 
  a 
  portion 
  was 
  broken 
  off 
  and 
  a 
  fresh 
  surface 
  exposed. 
  

   The 
  mass 
  had 
  to 
  be 
  rubbed 
  down 
  with 
  a 
  pestle 
  to 
  make 
  sure 
  of 
  the 
  

   acid 
  dissolving 
  out 
  all 
  soluble 
  matter 
  and 
  thoroughly 
  decomposing 
  

   all 
  soluble 
  silicates 
  [if 
  any 
  existed]. 
  I 
  preferred 
  to 
  do 
  this 
  rather 
  

   than 
  powder 
  the 
  chalk 
  previously, 
  as 
  it 
  afforded 
  a 
  better 
  idea 
  of 
  

   the 
  distribution 
  of 
  the 
  silica 
  and 
  the 
  constitution 
  of 
  the 
  sample." 
  

  

  The 
  only 
  question 
  which 
  remains 
  is 
  whether 
  any 
  silicates 
  exist 
  

   in 
  the 
  specimen 
  which 
  would 
  be 
  decomposed 
  by 
  the 
  action 
  of 
  the 
  

   hydrochloric 
  acid 
  and 
  leave 
  their 
  silica 
  free 
  to 
  swell 
  the 
  amount 
  

   subsequently 
  dissolved 
  in 
  the 
  alkaline 
  carbonate. 
  The 
  only 
  silicate 
  

   present 
  in 
  appreciable 
  quantity 
  is 
  glauconite, 
  and, 
  so 
  far 
  as 
  we 
  can 
  

   ascertain, 
  this 
  is 
  not 
  acted 
  on 
  by 
  the 
  acid 
  at 
  ordinary 
  temperatures 
  ; 
  

   for 
  the 
  grains 
  and 
  rod-like 
  fragments 
  of 
  this 
  mineral 
  appeared 
  to 
  be 
  

   unaltered 
  when 
  examined 
  under 
  the 
  microscope 
  after 
  the 
  chalk 
  was 
  

   treated 
  with 
  cold 
  acid 
  and 
  the 
  residue 
  boiled 
  for 
  20 
  minutes 
  in 
  

   caustic 
  potash. 
  Hot 
  acid, 
  however, 
  decomposes 
  glauconite 
  with 
  

   separation 
  of 
  silica, 
  and 
  the 
  amounts 
  of 
  soluble 
  silica 
  in 
  Mr. 
  Knights' 
  

   and 
  Mr. 
  Brierley's 
  analyses 
  may 
  include 
  such 
  separated 
  silica. 
  Prof. 
  

   Harrison 
  used 
  a 
  special 
  method 
  of 
  estimation, 
  which 
  avoids 
  this 
  diffi- 
  

   culty 
  and 
  gives 
  a 
  more 
  accurate 
  result 
  (see 
  p. 
  414). 
  

  

  Resume 
  and 
  Conclusions. 
  

  

  We 
  have 
  shown 
  that 
  in 
  the 
  Lower 
  Chalk 
  of 
  Berkshire 
  and 
  Wiltshire 
  

   there 
  are 
  beds 
  which 
  contain 
  a 
  large 
  amount 
  of 
  disseminated 
  colloid 
  

   silica, 
  and 
  are 
  comparable 
  in 
  general 
  structure 
  to 
  the 
  Malm 
  stones 
  

   of 
  the 
  Upper 
  Greensand. 
  Dr. 
  Hinde's 
  study 
  of 
  the 
  latter 
  led 
  him 
  to 
  

   believe 
  that 
  the 
  globular 
  colloid 
  silica 
  which 
  they 
  contain 
  has 
  been 
  

   directly 
  derived 
  from 
  the 
  remains 
  of 
  siliceous 
  sponges, 
  and 
  the 
  study 
  

   of 
  our 
  Chalk 
  specimens 
  has 
  confirmed 
  his 
  conclusion 
  by 
  adding 
  several 
  

   important 
  pieces 
  of 
  evidence. 
  Thus 
  the 
  contrast 
  between 
  the 
  speci- 
  

   mens 
  which 
  contain 
  disseminated 
  colloid 
  silica, 
  and 
  that 
  which 
  con- 
  

   tains 
  none 
  but 
  in 
  which 
  the 
  sponge-spicules 
  are 
  mostly, 
  if 
  not 
  all, 
  

   siliceous, 
  is 
  highly 
  suggestive 
  of 
  the 
  inference 
  that 
  the 
  disseminated 
  

   silica 
  was 
  largely, 
  if 
  not 
  wholly, 
  derived 
  from 
  the 
  spicules. 
  Again, 
  

   we 
  find 
  that 
  the 
  amount 
  of 
  free 
  disseminated 
  silica 
  increases 
  in 
  pro- 
  

   portion 
  to 
  the 
  number 
  of 
  spicules, 
  and 
  calcite 
  casts 
  of 
  spicules, 
  which 
  

   occur 
  in 
  the 
  rock. 
  The 
  great 
  similarity 
  between 
  the 
  siliceous 
  chalk 
  

   and 
  the 
  Malmstone 
  is 
  also 
  heightened 
  by 
  the 
  occurrence 
  of 
  similar 
  

   siliceous 
  concretions 
  in 
  both 
  rocks, 
  the 
  material 
  of 
  which 
  may 
  be 
  

   described 
  as 
  siliceous 
  chalk 
  indurated 
  by 
  a 
  cement 
  of 
  chalcedonic 
  

   silica. 
  

  

  