﻿416 
  MESSRS. 
  A. 
  J. 
  JTJKES-BRO 
  WiXE 
  AXD 
  W. 
  HILL 
  ON 
  COLLOID 
  

  

  Let 
  us 
  now 
  recapitulate 
  the 
  several 
  conditions 
  in 
  which 
  the 
  silica 
  

   occurs 
  in 
  the 
  Lower 
  Chalk, 
  stating 
  them 
  in 
  an 
  order 
  which 
  proceeds 
  

   from 
  the 
  least 
  to 
  the 
  most 
  siliceous 
  kind 
  of 
  rock 
  : 
  — 
  

  

  1. 
  There 
  is 
  a 
  chalk 
  which 
  differs 
  from 
  ordinary 
  Chalk 
  "Marl 
  only 
  

   in 
  containing 
  numerous 
  sponge-spicules, 
  and 
  these 
  are 
  all, 
  or 
  nearly 
  

   all, 
  of 
  silica, 
  which 
  is 
  but 
  slightly 
  changed 
  from 
  the 
  clear 
  colloid 
  

   condition 
  in 
  which 
  it 
  originally 
  existed. 
  

  

  2. 
  There 
  is 
  chalk 
  in 
  which 
  a 
  certain 
  amount 
  of 
  free 
  colloid 
  silica 
  

   occurs, 
  hut 
  it 
  is 
  disseminated 
  through 
  the 
  mass 
  in 
  the 
  form 
  of 
  very 
  

   minute 
  globules, 
  so 
  small 
  that 
  they 
  can 
  hardly 
  be 
  recognized 
  with 
  a 
  

   power 
  of 
  less 
  than 
  an 
  eighth 
  of 
  an 
  inch 
  ( 
  = 
  640 
  diams.). 
  This 
  

   silica 
  is 
  generally 
  associated 
  with 
  very 
  small 
  sponge-spicules. 
  

  

  3. 
  There 
  is 
  chalk 
  which 
  contains 
  aggregations 
  of 
  larger 
  globules 
  

   of 
  silica, 
  easily 
  distinguishable 
  with 
  a 
  quarter-inch 
  objective 
  (i. 
  e. 
  

   200 
  diams.), 
  and 
  this 
  chalk 
  always 
  contains 
  a 
  larger 
  number 
  of 
  

   spicules, 
  many 
  of 
  them 
  being 
  of 
  large 
  size. 
  Such 
  chalk 
  does 
  not 
  

   fall 
  to 
  powder 
  when 
  placed 
  in 
  acid, 
  but 
  the 
  insoluble 
  residue 
  stands 
  

   as 
  a 
  firm 
  siliceous 
  mass. 
  

  

  4. 
  There 
  is 
  a 
  chert-like 
  nodule 
  which 
  differs 
  from 
  the 
  last 
  only 
  

   in 
  being 
  permeated 
  and 
  indurated 
  by 
  an 
  infiltration 
  of 
  chalcedonic 
  

   silica, 
  this 
  having 
  replaced 
  some, 
  but 
  not 
  all, 
  of 
  the 
  calcareous 
  

   matter. 
  

  

  Leaving 
  the 
  chalcedonic 
  silica 
  out 
  of 
  consideration 
  for 
  the 
  moment, 
  

   we 
  have 
  in 
  the 
  most 
  siliceous 
  chalk 
  an 
  amount 
  of 
  colloid 
  silica 
  

   estimated 
  at 
  from 
  15 
  to 
  19 
  per 
  cent, 
  by 
  weight, 
  and 
  possibly 
  more 
  in 
  

   volume. 
  Again 
  the 
  matrix 
  of 
  some 
  portions 
  of 
  the 
  Surrey 
  Malmstone 
  

   is 
  almost 
  entirely 
  made 
  up 
  of 
  such 
  silica, 
  and 
  the 
  total 
  amount 
  o'f 
  so- 
  

   called 
  soluble 
  (or 
  colloid) 
  silica 
  present 
  in 
  such 
  samples 
  was 
  esti- 
  

   mated 
  by 
  Messrs. 
  Paine 
  and 
  "Way 
  at 
  from 
  50 
  to 
  75 
  per 
  cent. 
  Now, 
  

   as 
  the 
  mass 
  of 
  siliceous 
  chalk 
  at 
  Eastcott 
  is 
  at 
  least 
  10 
  feet 
  thick, 
  

   as 
  the 
  Surrey 
  Malmstone 
  is 
  estimated 
  at 
  25 
  feet, 
  and 
  that 
  of 
  

   Oxfordshire 
  and 
  Berkshire 
  attains 
  a 
  maximum 
  thickness 
  of 
  70 
  or 
  

   80 
  feet, 
  the 
  amount 
  of 
  disseminated 
  colloid 
  silica 
  in 
  these 
  rocks 
  is 
  

   obviously 
  very 
  large. 
  

  

  If, 
  therefore, 
  we 
  are 
  to 
  assume 
  that 
  all 
  this 
  silica 
  has 
  in 
  some 
  

   way 
  or 
  other 
  been 
  furnished 
  by 
  the 
  spicules 
  of 
  siliceous 
  sponges, 
  we 
  

   may 
  feel 
  a 
  difficulty 
  in 
  understanding 
  how 
  there 
  came 
  to 
  be 
  such 
  

   an 
  accumulation 
  of 
  spicules 
  as 
  would 
  be 
  required 
  for 
  the 
  supply 
  of 
  

   this 
  silica. 
  Without 
  the 
  knowledge 
  of 
  some 
  recently 
  published 
  

   facts, 
  it 
  would 
  undoubtedly 
  have 
  been 
  difficult 
  to 
  account 
  satis- 
  

   factorily 
  for 
  the 
  existence 
  of 
  so 
  many 
  separate 
  spicules. 
  We 
  might 
  

   certainly 
  have 
  supposed 
  that 
  many 
  hundred 
  thousand 
  siliceous 
  

   sponges 
  have 
  lived 
  and 
  died 
  on 
  the 
  sea-floors 
  represented 
  by 
  these 
  

   beds, 
  and 
  that 
  they 
  were 
  of 
  species 
  whose 
  skeletons 
  were 
  entirely 
  

   composed 
  of 
  spicules 
  ; 
  this, 
  indeed, 
  Dr. 
  Hinde 
  informs 
  us, 
  was 
  the 
  

   case 
  with 
  the 
  sponges 
  to 
  which 
  the 
  spicules 
  occurring 
  in 
  the 
  Malm- 
  

   stones 
  belong. 
  We 
  might, 
  too, 
  have 
  supplemented 
  this 
  source 
  

   of 
  supply 
  by 
  supposing 
  that 
  many 
  were 
  drifted 
  from 
  distant 
  colonies 
  

   of 
  sponges, 
  borne 
  by 
  the 
  same 
  currents 
  that 
  carried 
  the 
  fine 
  inor- 
  

   ganic 
  particles 
  which 
  occur 
  in 
  the 
  matrix. 
  These, 
  however, 
  were 
  

  

  