﻿SILICA 
  IN 
  THE 
  LOWER 
  CHALK 
  OF 
  BERKSHIRE 
  AND 
  WILTSHIRE. 
  409 
  

  

  Marl 
  the 
  silica 
  is 
  for 
  the 
  most 
  part 
  retained 
  in 
  the 
  spicules. 
  In 
  

   examining 
  the 
  sections 
  of 
  Marl 
  it 
  is 
  not 
  always 
  easy 
  to 
  determine 
  

   whether 
  the 
  material 
  of 
  which 
  the 
  spicules 
  now 
  consist 
  is 
  silica 
  or 
  

   calcite 
  ; 
  but 
  the 
  greater 
  abundance 
  of 
  free 
  spicules 
  found 
  in 
  the 
  

   residue 
  of 
  this 
  specimen 
  after 
  treatment 
  with 
  acid 
  and 
  the 
  absence 
  

   of 
  colloid 
  silica 
  in 
  globular 
  form 
  leads 
  us 
  to 
  think 
  a 
  large 
  proportion 
  

   of 
  them 
  remain 
  siliceous 
  and 
  have 
  not 
  calcareous 
  casts. 
  

  

  Chilton. 
  — 
  Specimens 
  of 
  the 
  Chalk 
  Marl 
  taken 
  from 
  the 
  hard 
  beds 
  

   shown 
  in 
  the 
  railway-cutting 
  near 
  Chilton, 
  each 
  at 
  a 
  slightly 
  higher 
  

   horizon, 
  give 
  somewhat 
  different 
  results. 
  

  

  Viewed 
  in 
  thin 
  sections 
  all 
  show 
  the 
  presence 
  of 
  sponge-spicules, 
  

   these 
  being 
  more 
  clearly 
  denned 
  and 
  more 
  abundant 
  in 
  the 
  upper- 
  

   most 
  of 
  five 
  beds 
  examined. 
  

  

  The 
  silica 
  of 
  the 
  spicules 
  can 
  here 
  be 
  seen 
  to 
  be 
  replaced 
  by 
  

   calcite, 
  though 
  in 
  some 
  cases 
  it 
  still 
  remains 
  in 
  the 
  colloid 
  state, 
  

   negative 
  to 
  polarized 
  light. 
  Incomplete 
  elliptical 
  rings 
  can 
  be 
  some- 
  

   times 
  perceived 
  traversing 
  the 
  material 
  of 
  the 
  large 
  spicules, 
  and 
  

   glauconite 
  often 
  fills 
  the 
  spicnlar 
  canal. 
  

  

  Treating 
  these 
  specimens 
  with 
  acid, 
  a 
  moderate 
  reaction 
  ensued 
  ; 
  

   but 
  the 
  material 
  did 
  not 
  break 
  up 
  to 
  powder, 
  and 
  even 
  after 
  the 
  

   acid 
  had 
  removed 
  the 
  calcareous 
  matter 
  pressure 
  was 
  necessary 
  to 
  

   break 
  even 
  small 
  lumps. 
  

  

  Examination 
  of 
  the 
  residue 
  showed 
  that 
  comparatively 
  few 
  of 
  

   the 
  spicules 
  retained 
  their 
  silica, 
  the 
  greater 
  part 
  of 
  them 
  having 
  

   been 
  destroyed 
  by 
  the 
  acid, 
  and 
  only 
  a 
  few 
  of 
  Monactinellid 
  and 
  

   Tetractinellid 
  types 
  remaining. 
  In 
  these 
  the 
  silica 
  preserved 
  the 
  

   characters 
  already 
  described. 
  

  

  In 
  the 
  finer 
  part 
  of 
  the 
  residue 
  colloid 
  silica 
  in 
  the 
  globular 
  form 
  

   existed 
  in 
  considerable 
  quantities. 
  The 
  globules 
  correspond 
  very 
  

   closely 
  with 
  those 
  described 
  by 
  Dr. 
  Binde, 
  have 
  circular 
  outlines, 
  

   and 
  occur 
  singly, 
  or 
  in 
  groups, 
  or 
  in 
  linear 
  series, 
  their 
  outline 
  

   being 
  broken 
  at 
  the 
  point 
  of 
  contact. 
  They 
  are, 
  however, 
  smaller 
  

   than 
  those 
  of 
  the 
  greensands 
  and 
  malm, 
  their 
  diameter 
  being 
  about 
  

   •0006 
  of 
  an 
  inch, 
  and 
  their 
  outline 
  is 
  less 
  even 
  and 
  regular. 
  Some 
  

   appear 
  to 
  be 
  discs 
  with 
  greater 
  or 
  less 
  convexity, 
  others 
  are 
  cer- 
  

   tainly 
  spherical, 
  the 
  majority 
  have 
  a 
  nuclear 
  spot; 
  but 
  in 
  these 
  we 
  

   have 
  been 
  unable 
  to 
  detect 
  any 
  radiate 
  striation. 
  Small 
  masses 
  of 
  

   the 
  globules 
  have 
  a 
  brownish 
  tint. 
  All 
  globules 
  and 
  spicules 
  dis- 
  

   solved 
  readily 
  in 
  heated 
  caustic 
  potash 
  and 
  are 
  neutral 
  with 
  pola- 
  

   rized 
  light. 
  

  

  Sponge-remains 
  were 
  most 
  abundant 
  in 
  the 
  higher 
  bed 
  about 
  

   6 
  feet 
  below 
  the 
  Totternhoe 
  Stone, 
  and 
  it 
  is 
  here 
  that 
  the 
  condi- 
  

   tion 
  of 
  the 
  spicules 
  can 
  be 
  most 
  conveniently 
  examined. 
  Crystal- 
  

   line 
  calcite 
  can 
  be 
  seen 
  to 
  have 
  replaced 
  the 
  silica 
  of 
  many 
  spicules, 
  

   but 
  in 
  others 
  it 
  still 
  remains 
  in 
  the 
  amorphous 
  or 
  colloid 
  state. 
  

  

  With 
  the 
  increase 
  in 
  the 
  number 
  of 
  spicules 
  in 
  this 
  bed 
  the 
  

   amount 
  of 
  colloid 
  silica 
  in 
  globular 
  form 
  increases 
  also. 
  It 
  can 
  be 
  

   seen 
  in 
  thin 
  sections 
  dispersed 
  through 
  the 
  matrix 
  and 
  also 
  filling 
  

   the 
  cells 
  of 
  Foraminifera, 
  the 
  calcareous 
  tests 
  of 
  which 
  still 
  remain. 
  

  

  It 
  is 
  impossible, 
  by 
  microscopic 
  examination, 
  to 
  estimate 
  accu- 
  

  

  