﻿412 
  MESSRS. 
  A. 
  J. 
  JUKES-BROWNE 
  AND 
  W. 
  HILL 
  ON 
  COLLOID 
  

  

  gations 
  of 
  them. 
  These 
  globules 
  are 
  smaller 
  and 
  less 
  even 
  in 
  their 
  

   outline 
  than 
  those 
  of 
  the 
  Malmstone, 
  but 
  are 
  identical 
  with 
  those 
  

   of 
  the 
  upper 
  bed 
  in 
  the 
  Chilton 
  cutting. 
  The 
  marl 
  immediately 
  

   surrounding 
  the 
  chert-like 
  nodules 
  shows 
  no 
  difference 
  in 
  its 
  cha- 
  

   racter 
  ; 
  but 
  the 
  globules 
  of 
  colloid 
  silica 
  are 
  distinctly 
  larger 
  and 
  

   more 
  closely 
  resemble 
  those 
  of 
  the 
  Malmstone. 
  

  

  The 
  material 
  of 
  the 
  nodules 
  themselves 
  is 
  the 
  same 
  as 
  the 
  sur- 
  

   rounding 
  marl. 
  Grains 
  of 
  glauconite 
  and 
  quartz 
  together 
  with 
  

   Foraminifera 
  and 
  fragments 
  of 
  shells 
  are 
  scattered 
  through 
  the 
  mass 
  

   just 
  as 
  they 
  are 
  through 
  the 
  surrounding 
  chalk, 
  from 
  which 
  it 
  only 
  

   differs 
  in 
  the 
  presence 
  of 
  the 
  chalcedonic 
  silica, 
  which 
  appears 
  to 
  

   permeate 
  and 
  partially 
  replace 
  the 
  calcareous 
  material. 
  That 
  some 
  

   calcareous 
  matter 
  remains 
  is 
  proved 
  by 
  effervescence 
  when 
  a 
  piece 
  

   is 
  placed 
  in 
  acid 
  ; 
  but 
  the 
  action 
  is 
  much 
  less 
  brisk 
  than 
  in 
  the 
  

   case 
  of 
  the 
  surrounding 
  material, 
  and 
  shows 
  that 
  the 
  amount 
  of 
  

   calcareous 
  matter 
  in 
  the 
  chert 
  is 
  small. 
  Spongc-spicules 
  are 
  

   abundant, 
  many 
  are 
  of 
  the 
  type 
  found 
  throughout 
  the 
  marl, 
  fine 
  

   and 
  thread-like 
  in 
  form 
  ; 
  but 
  others 
  occur 
  much 
  larger 
  in 
  size. 
  In 
  

   the 
  greater 
  number 
  of 
  them 
  the 
  silica 
  is 
  either 
  wholly 
  or 
  in 
  part 
  

   in 
  the 
  condition 
  of 
  transparent 
  chalcedony, 
  and 
  chalcedonic 
  silica 
  

   also 
  fills 
  cavities 
  which 
  appear 
  to 
  have 
  been 
  produced 
  by 
  the 
  decay 
  

   of 
  organic 
  fragments 
  other 
  than 
  spicules, 
  and 
  also 
  the 
  interior 
  of 
  

   the 
  cells 
  of 
  Eoraminifera, 
  which 
  yet 
  retain 
  their 
  calcareous 
  tests. 
  

  

  There 
  are 
  spicules, 
  however, 
  with 
  their 
  silica 
  still 
  in 
  the 
  amor- 
  

   phous" 
  or 
  colloid 
  state, 
  and 
  the 
  sections 
  of 
  the 
  nodules 
  exhibit 
  

   remarkably 
  well 
  the 
  various 
  changes 
  in 
  their 
  condition. 
  

  

  Colloid 
  silica 
  in 
  globular 
  form 
  is 
  very 
  abundant, 
  the 
  globules 
  

   being 
  in 
  all 
  respects 
  similar 
  to 
  those 
  of 
  the 
  Malmstone 
  and 
  Green- 
  

   sands 
  described 
  by 
  Dr. 
  Hinde. 
  Globules 
  are 
  especially 
  abundant 
  in 
  

   the 
  vicinity 
  of 
  sponge-spicules 
  and 
  may 
  be 
  seen 
  filling 
  the 
  spicular 
  

   canals 
  even 
  where 
  the 
  walls 
  are 
  of 
  transparent 
  chalcedony. 
  In 
  

   fact 
  they 
  frequently 
  appear 
  surrounded 
  by 
  chalcedonic 
  silica, 
  and 
  

   it 
  is 
  difficult 
  to 
  say 
  whether 
  they 
  are 
  or 
  are 
  not 
  " 
  chalcedonified 
  ; 
  " 
  

   but 
  after 
  the 
  action 
  of 
  acid 
  and 
  heated 
  caustic 
  potash 
  we 
  have 
  

   been 
  unable 
  to 
  detect 
  a 
  single 
  free 
  globule 
  of 
  chalcedonic 
  silica, 
  

   and 
  only 
  those 
  remain 
  which 
  appear 
  to 
  have 
  been 
  protected 
  by 
  

   the 
  surrounding 
  chalcedonic 
  material. 
  Spicules 
  treated 
  for 
  30 
  

   minutes 
  in 
  heated 
  caustic 
  potash 
  present 
  a 
  rugged 
  and 
  uneven 
  

   outline, 
  showing 
  that 
  the 
  silica 
  is 
  not 
  completely 
  crystalline. 
  

  

  It 
  is 
  interesting 
  also 
  to 
  find 
  that 
  the 
  constitution 
  of 
  the 
  hard 
  

   siliceous 
  nodules 
  in 
  this 
  chalk 
  is 
  similar 
  to 
  that 
  of 
  the 
  nodules 
  

   which 
  occur 
  in 
  the 
  Malmstone. 
  Dr. 
  Hinde, 
  in 
  a 
  letter 
  to 
  one 
  of 
  

   us, 
  says 
  " 
  the 
  portion 
  of 
  the 
  chert-like 
  nodule 
  closely 
  resembles 
  the 
  

   bluish 
  nodules 
  which 
  occur 
  in 
  the 
  Malm 
  or 
  Upper 
  Greensand 
  of 
  

   Merstham 
  and 
  Godstonc, 
  which 
  are 
  locally 
  known 
  as 
  flints 
  ; 
  and 
  

   the 
  sections 
  from 
  them 
  likewise 
  correspond 
  to 
  a 
  great 
  extent 
  with 
  

   those 
  I 
  have 
  from 
  the 
  Merstham 
  nodules, 
  save 
  that 
  in 
  your 
  speci- 
  

   mens 
  the 
  minute 
  siliceous 
  discs 
  or 
  globules 
  of 
  colloid 
  silica 
  are 
  less 
  

   abundant 
  and 
  seem 
  to 
  have 
  been 
  converted 
  into 
  transparent 
  chalce- 
  

   dony 
  The 
  silica 
  is 
  partly 
  amorphous 
  and 
  partly 
  chalcedonic; 
  

  

  