﻿410 
  MESSRS. 
  A. 
  J. 
  JUKES-BROWNE 
  AND 
  W. 
  HILL 
  ON 
  COLLOID 
  

  

  rately 
  the 
  quantity 
  of 
  silica 
  in 
  the 
  globular 
  form 
  which 
  is 
  dissemi- 
  

   nated 
  through 
  the 
  marl 
  ; 
  but 
  we 
  believe 
  it 
  forms 
  a 
  much 
  larger 
  pro- 
  

   portion 
  of 
  the 
  bulk 
  of 
  the 
  material 
  than 
  chemical 
  analysis 
  would 
  lead 
  

   one 
  to 
  suppose. 
  

  

  As 
  already 
  stated, 
  we 
  have 
  also 
  found 
  disseminated 
  colloid 
  silica 
  

   in 
  considerable 
  quantities 
  in 
  the 
  Chalk 
  Marl 
  of 
  Wiltshire. 
  The 
  

   amount 
  varies 
  in 
  different 
  specimens, 
  and 
  in 
  some 
  cases 
  the 
  

   globules 
  are 
  exceedingly 
  minute. 
  

  

  In 
  all 
  specimens 
  examined 
  spicules 
  of 
  sponges 
  were 
  present, 
  in 
  

   some 
  only 
  sparingly, 
  in 
  others 
  they 
  were 
  more 
  abundant. 
  Usually 
  

   they 
  are 
  rather 
  small 
  and 
  thread-like, 
  and 
  appear 
  merely 
  as 
  shadowy 
  

   outlines 
  in 
  the 
  matrix 
  of 
  the 
  marl, 
  but 
  here 
  and 
  there 
  in 
  thin 
  

   sections 
  spicules 
  can 
  be 
  clearly 
  seen 
  ; 
  in 
  some 
  the 
  silica 
  remains 
  in 
  

   the 
  amorphous 
  or 
  colloid 
  condition, 
  in 
  others 
  it 
  is 
  clearly 
  replaced 
  by 
  

   calcite. 
  Siliceous 
  spicules 
  and 
  rods 
  of 
  glauconite, 
  the 
  casts 
  of 
  the 
  

   spicular 
  canals, 
  may 
  also 
  be 
  found 
  in 
  the 
  residue 
  of 
  the 
  marl 
  after 
  

   treatment 
  with 
  acid. 
  

  

  Compton 
  Basset. 
  — 
  Sponge, 
  spicules 
  are 
  numerous 
  in 
  a 
  hard 
  bed 
  

   40 
  feet 
  above 
  the 
  Chloritic 
  Marl. 
  In 
  this 
  the 
  silica 
  of 
  many 
  

   spicules 
  can 
  be 
  seen 
  to 
  be 
  replaced 
  by 
  calcite. 
  Much 
  colloid 
  silica, 
  

   in 
  globules 
  rarely 
  exceeding 
  *0(J05 
  of 
  an 
  inch 
  in 
  diameter, 
  occurred 
  

   in 
  the 
  residue 
  after 
  treatment 
  with 
  acid. 
  As 
  in 
  the 
  Chilton 
  

   specimens 
  their 
  outline 
  is 
  not 
  so 
  even 
  as 
  in 
  those 
  of 
  the 
  Malmstone 
  ; 
  

   but 
  in 
  most 
  other 
  respects 
  they 
  are 
  precisely 
  similar, 
  occurring 
  in 
  

   linear 
  series, 
  in 
  groups 
  and 
  larger 
  aggregations, 
  or 
  singly. 
  Some 
  

   still 
  remain 
  attached 
  to 
  the 
  rods 
  of 
  glauconite 
  which 
  once 
  filled 
  

   the 
  spicular 
  canals. 
  

  

  Colloid 
  silica 
  also 
  appears 
  to 
  be 
  present 
  in 
  a 
  specimen 
  of 
  the 
  

   marl 
  taken 
  60 
  feet 
  above 
  the 
  Chloritic 
  Marl 
  at 
  Compton. 
  In 
  this 
  

   case 
  its 
  form 
  can 
  hardly 
  be 
  termed 
  globular. 
  A 
  large 
  part 
  of 
  the 
  

   finer 
  residue, 
  after 
  treatment 
  with 
  acid, 
  consists 
  of 
  aggregations 
  of 
  

   minute 
  but 
  defined 
  particles 
  of 
  irregular 
  shape, 
  amongst 
  which 
  a 
  

   few 
  well-formed 
  globules 
  occur. 
  The 
  material 
  seems 
  to 
  be 
  of 
  the 
  

   same 
  nature 
  as 
  the 
  globules, 
  it 
  is 
  neutral 
  with 
  polarized 
  light 
  and 
  

   dissolves 
  readily 
  in 
  caustic 
  potash. 
  

  

  Stocldey. 
  — 
  This 
  is 
  a 
  small 
  place 
  about 
  4 
  miles 
  north 
  of 
  Devizes, 
  

   and 
  the 
  pit 
  where 
  our 
  specimen 
  was 
  taken 
  is 
  on 
  the 
  slope 
  of 
  the 
  

   hill 
  a 
  very 
  little 
  way, 
  probably 
  not 
  more 
  than 
  20 
  feet, 
  above 
  the 
  

   base 
  of 
  the 
  Chalk 
  Marl. 
  The 
  specimen 
  is 
  more 
  marly 
  and 
  less 
  

   hard 
  than 
  is 
  usual 
  in 
  beds 
  which 
  we 
  have 
  found 
  to 
  contain 
  much 
  

   colloid 
  silica. 
  Well-defined 
  sponge-spicules 
  are 
  seen 
  to 
  be 
  fairly 
  

   common 
  in 
  thin 
  sections. 
  Calcite 
  has 
  replaced 
  the 
  silica 
  in 
  some 
  

   of 
  them, 
  in 
  others 
  it 
  is 
  in 
  the 
  amorphous 
  or 
  colloid 
  condition. 
  

   Rods 
  of 
  glauconite, 
  which 
  appear 
  to 
  have 
  been 
  the 
  casts 
  of 
  the 
  spi- 
  

   cular 
  canals, 
  may 
  also 
  be 
  seen 
  in 
  the 
  sections, 
  the 
  walls 
  of 
  the 
  

   spicules 
  having 
  disappeared 
  entirely. 
  The 
  material 
  does 
  not 
  break 
  

   up 
  on 
  treatment 
  with 
  acid, 
  and 
  colloid 
  silica 
  in 
  well-marked 
  

   globules 
  occurs 
  abundantly 
  in 
  the 
  residue. 
  This 
  is 
  the 
  lowest 
  

   horizon 
  at 
  which 
  we 
  have 
  found 
  disseminated 
  silica. 
  

  

  