﻿SILICA 
  IN 
  THE 
  LOWER 
  CHALK 
  OE 
  BERKSHIRE 
  AND 
  WILTSHIRE. 
  411 
  

  

  Allington. 
  — 
  A 
  specimen 
  of 
  marl 
  140 
  feet 
  below 
  the 
  Melbourn 
  

   Rock 
  contains 
  but 
  few 
  spicules, 
  and 
  siliceous 
  globules 
  occur 
  only 
  

   sparingly 
  in 
  the 
  residue. 
  About 
  80 
  feet 
  below 
  the 
  Melbourn 
  Rock 
  

   at 
  Allington 
  is 
  a 
  bed 
  of 
  hard 
  Chalk 
  Marl 
  in 
  which 
  spicules 
  occur. 
  

   Treatment 
  with 
  acid 
  easily 
  reduced 
  the 
  material 
  to 
  powder, 
  the 
  

   residue 
  consisting 
  of 
  quartz-sand, 
  a 
  few 
  siliceous 
  spicules, 
  &c, 
  and 
  

   a 
  large 
  proportion 
  of 
  nocculent 
  matter. 
  This, 
  when 
  dried, 
  mounted 
  

   in 
  Canada 
  Balsam, 
  and 
  viewed 
  with 
  a 
  power 
  magnifying 
  200 
  dia- 
  

   meters, 
  appeared 
  full 
  of 
  specks, 
  but 
  otherwise 
  structureless 
  ; 
  entangled 
  

   in 
  it 
  were 
  minute 
  particles 
  of 
  quartz. 
  When 
  magnified 
  640 
  

   diameters 
  many 
  of 
  the 
  specks 
  could 
  be 
  resolved 
  into 
  minute 
  discs 
  

   rarely 
  more 
  than 
  -0002 
  inch 
  in 
  diameter, 
  bearing 
  a 
  strong 
  resem- 
  

   blance 
  to 
  the 
  globules 
  of 
  colloid 
  silica. 
  As 
  a 
  rule 
  these 
  minute 
  

   discs 
  are 
  single, 
  but 
  groups 
  of 
  three 
  and 
  four 
  can 
  be 
  seen. 
  Their 
  

   outline 
  appears 
  generally 
  circular, 
  but 
  not 
  always 
  so; 
  like 
  the 
  larger 
  

   globules 
  they 
  are 
  neutral 
  to 
  polarized 
  light 
  and 
  easily 
  destroyed 
  in 
  

   caustic 
  potash. 
  

  

  In 
  a 
  hard 
  bed 
  40 
  feet 
  below 
  the 
  Melbourn 
  Rock 
  at 
  Allington, 
  

   the 
  highest 
  well-defined 
  bed 
  in 
  the 
  Lower 
  Chalk 
  of 
  this 
  district, 
  

   sponge-spicules 
  are 
  seen 
  sparingly. 
  Their 
  condition 
  appears 
  to 
  be 
  

   as 
  usual, 
  some 
  being 
  siliceous, 
  while 
  in 
  others 
  the 
  silica 
  is 
  replaced 
  by 
  

   crystalline 
  calcite. 
  This 
  bed 
  contains 
  a 
  large 
  proportion 
  of 
  coarse 
  

   shelly 
  fragments, 
  chiefly 
  Inoceramus-^risms, 
  reminding 
  one 
  of 
  the 
  

   Totternhoe 
  Stone. 
  Scarcely 
  any 
  globules 
  of 
  colloid 
  silica 
  occur 
  in 
  

   the 
  residue, 
  but 
  a 
  large 
  proportion 
  of 
  the 
  Inoceramus-yiisms 
  are 
  

   completely 
  silicifled 
  and 
  are 
  unaffected 
  by 
  acid 
  ; 
  immersion 
  for 
  30 
  

   minutes 
  in 
  a 
  heated 
  solution 
  of 
  caustic 
  potash 
  likewise 
  appeared 
  to 
  

   have 
  no 
  effect 
  on 
  them. 
  

  

  Etchilhampton 
  Hill. 
  — 
  Two 
  specimens 
  of 
  Chalk 
  Marl 
  were 
  taken 
  

   from 
  a 
  pit 
  on 
  this 
  hill 
  about 
  100 
  feet 
  above 
  the 
  Chloritic 
  Marl. 
  

   The 
  chalk 
  here 
  contains 
  a 
  large 
  proportion 
  of 
  smallish 
  shell-frag- 
  

   ments, 
  none 
  of 
  which 
  are 
  silicifled. 
  The 
  specimen 
  from 
  the 
  base 
  

   of 
  the 
  pit 
  contains 
  but 
  few 
  spicules 
  and 
  little 
  colloid 
  silica 
  ; 
  but 
  in 
  

   that 
  from 
  the 
  upper 
  part 
  colloid 
  silica 
  in 
  well-marked 
  globules 
  or 
  

   aggregations 
  of 
  them 
  occurs 
  plentifully. 
  Sponge-spicules 
  can 
  be 
  

   seen 
  in 
  thin 
  sections 
  of 
  this 
  specimen, 
  but 
  they 
  are 
  not 
  well 
  defined 
  ; 
  

   calcite 
  appears 
  to 
  replace 
  the 
  silica 
  in 
  most 
  of 
  them 
  ; 
  a 
  few 
  spicules 
  

   with 
  their 
  silica 
  in 
  the 
  amorphous 
  condition 
  were 
  found 
  in 
  the 
  

   residue 
  after 
  treatment 
  with 
  acid. 
  

  

  Eastcott. 
  — 
  Sections 
  of 
  the 
  Chalk 
  Marl 
  from 
  various 
  parts 
  of 
  the 
  

   whitish 
  friable 
  chalk 
  in 
  which 
  the 
  chert-like 
  nodules 
  occur 
  at 
  

   Eastcott 
  show 
  that 
  it 
  does 
  not 
  differ 
  materially 
  from 
  the 
  marl 
  of 
  

   the 
  district. 
  Sponge-spicules 
  are 
  common, 
  though 
  not 
  abundant, 
  and 
  

   are 
  seen 
  perhaps 
  more 
  distinctly 
  than 
  usual. 
  The 
  greater 
  part 
  of 
  

   them 
  are 
  fine 
  and 
  thread-like, 
  and 
  in 
  these 
  the 
  silica 
  seems 
  to 
  be 
  

   replaced 
  by 
  calcite 
  ; 
  but 
  in 
  the 
  larger 
  ones 
  it 
  is 
  chiefly 
  in 
  the 
  amor- 
  

   phous 
  or 
  colloid 
  condition. 
  

  

  In 
  none 
  does 
  the 
  silica 
  appear 
  chalcedonic. 
  The 
  material 
  will 
  

   not 
  break 
  up 
  on 
  treatment 
  with 
  acid, 
  and 
  the 
  finer 
  part 
  of 
  the 
  

   residue 
  consists 
  almost 
  entirely 
  of 
  colloid 
  silica 
  in 
  globules 
  or 
  aggre- 
  

  

  