﻿SILICA 
  IN 
  THE 
  LOWEK 
  CHALK 
  OF 
  BERKSHIRE 
  AND 
  WILTSHIRE. 
  407 
  

  

  tributed 
  through 
  the 
  matrix 
  of 
  the 
  malmstone, 
  and 
  in 
  some 
  cases 
  

   almost 
  entirely 
  composing 
  that 
  matrix. 
  This 
  silica 
  is 
  also 
  in 
  a 
  

   colloid 
  condition, 
  being 
  quite 
  neutral 
  between 
  crossed 
  nicols, 
  and 
  

   it 
  occurs 
  either 
  in 
  the 
  form 
  of 
  minute 
  granules 
  similar 
  to 
  those 
  seen 
  

   in 
  the 
  spicules 
  or 
  else 
  in 
  a 
  globular 
  form, 
  " 
  that 
  is 
  to 
  say, 
  in 
  very 
  

   minute 
  bodies 
  with 
  circular 
  outlines 
  though 
  not 
  strictly 
  of 
  spherical 
  

   form." 
  

  

  Describing 
  this 
  globular 
  silica, 
  Dr. 
  Hinde 
  says, 
  " 
  the 
  globular 
  

   bodies 
  are 
  either 
  single 
  and 
  quite 
  free, 
  or 
  in 
  groups 
  of 
  two, 
  three, 
  

   or 
  several 
  individuals 
  united 
  together. 
  The 
  single 
  globules 
  present 
  

   well-defined 
  circular 
  outlines 
  ; 
  the 
  individuals 
  forming 
  the 
  groups 
  

   are 
  also 
  circular, 
  except 
  where, 
  in 
  contact, 
  they 
  partially 
  coalesce, 
  

   and 
  their 
  margins 
  become 
  abruptly 
  truncated. 
  . 
  . 
  . 
  "When 
  mag- 
  

   nified 
  about 
  5u0 
  diameters 
  they 
  present 
  well-marked 
  variations. 
  

   In 
  the 
  commonest 
  forms 
  there 
  is 
  a 
  marginal 
  ring, 
  about 
  one 
  sixth 
  

   the 
  diameter 
  of 
  the 
  globule, 
  which 
  seems 
  to 
  be 
  faintly 
  striate, 
  while 
  

   the 
  central 
  portion 
  has 
  a 
  granular 
  appearance. 
  In 
  another 
  form 
  

   the 
  surface 
  is 
  covered 
  with 
  faint 
  striae 
  which 
  radiate 
  from 
  an 
  in- 
  

   definite 
  central 
  granule 
  The 
  very 
  smallest 
  and 
  simplest 
  

  

  globules 
  consist 
  of 
  perfectly 
  clear 
  silica 
  without 
  strise 
  or 
  granules." 
  

   In 
  size 
  they 
  vary 
  from 
  -0014 
  millim. 
  to 
  *045 
  millim. 
  They 
  dissolve 
  

   readily 
  in 
  heated 
  caustic 
  potash. 
  

  

  Finally, 
  Dr. 
  Hinde 
  remarks 
  that 
  " 
  the 
  presence 
  in 
  the 
  same 
  beds 
  

   of 
  silica 
  in 
  an 
  amorphous 
  condition, 
  with 
  the 
  siliceous 
  spicules 
  in 
  a 
  

   similar 
  state, 
  taken 
  in 
  connection 
  with 
  the 
  fact 
  that 
  the 
  beds 
  are 
  

   filled 
  with 
  empty 
  spicular 
  casts 
  from 
  which 
  the 
  spicules 
  have 
  been 
  

   dissolved 
  .... 
  points 
  to 
  the 
  conclusion 
  that 
  the 
  colloidal 
  silica 
  

   in 
  the 
  beds 
  has 
  been 
  directly 
  derived 
  from 
  the 
  breaking-up 
  and 
  

   dissolution 
  of 
  the 
  sponge-remains/'" 
  He 
  also 
  observes 
  that 
  sedi- 
  

   mentary 
  deposits 
  consisting 
  in 
  part 
  of 
  colloidal 
  silica 
  in 
  a 
  globular 
  

   form 
  seem 
  to 
  be 
  of 
  rare 
  occurrence, 
  and 
  that 
  he 
  had 
  not 
  been 
  able 
  

   to 
  find 
  any 
  previous 
  notice 
  of 
  such. 
  He 
  is 
  unable 
  to 
  explain 
  the 
  

   causes 
  which 
  have 
  produced 
  this 
  singular 
  form 
  of 
  colloid 
  silica 
  or 
  to 
  

   say 
  why 
  the 
  silica 
  of 
  the 
  sponge-spicules 
  should 
  not 
  have 
  passed 
  

   into 
  the 
  more 
  stable 
  condition 
  of 
  chalcedony 
  or 
  crystalline 
  quartz, 
  

   as 
  is 
  the 
  case 
  with 
  those 
  of 
  most 
  other 
  fossil 
  sponge-beds. 
  

  

  With 
  regard 
  to 
  the 
  bluish 
  nodular 
  concretions 
  found 
  in 
  the 
  

   malmstone, 
  he 
  represents 
  a 
  section 
  of 
  one 
  (pi. 
  40. 
  fig. 
  2) 
  as 
  exhi- 
  

   biting 
  numerous 
  spicules 
  in 
  a 
  siliceo-calcareous 
  matrix, 
  and 
  he 
  

   states 
  that 
  the 
  minute 
  globules 
  of 
  silica 
  which 
  are 
  free 
  or 
  but 
  

   lightly 
  aggregated 
  together 
  in 
  the 
  friable 
  portions 
  of 
  the 
  rock, 
  are 
  

   in 
  the 
  nodules 
  enclosed 
  by 
  transparent 
  chalcedonic 
  silica 
  and 
  seem 
  

   to 
  be 
  gradually 
  passing 
  into 
  the 
  crypto 
  -crystalline 
  condition. 
  

  

  Coming 
  now 
  to 
  the 
  beds 
  which 
  form 
  the 
  subject 
  of 
  the 
  present 
  

   paper, 
  it 
  will 
  be 
  convenient 
  to 
  begin 
  with 
  the 
  lowest 
  horizon, 
  and 
  

   describe 
  a 
  specimen 
  from 
  the 
  cutting 
  some 
  250 
  yards 
  south-west 
  of 
  

   Upton 
  Station 
  from 
  what 
  seemed 
  a 
  rather 
  harder 
  course 
  than 
  usual, 
  

   about 
  70 
  feet 
  above 
  the 
  base 
  of 
  the 
  Chalk 
  Marl. 
  

  

  UiAon.-^-K 
  thin 
  section 
  of 
  this, 
  viewed 
  under 
  the 
  microscope, 
  

  

  