﻿SILICA 
  IN 
  THE 
  LOWER 
  CHALK 
  OF 
  BERKSHIRE 
  AND 
  WILTSHIRE. 
  421 
  

  

  method 
  by 
  which 
  the 
  silica 
  had 
  reached 
  its 
  present 
  condition 
  as 
  to 
  show 
  

   that 
  it 
  had 
  been 
  derived 
  from 
  sponge-remains, 
  and 
  this 
  view 
  was 
  

   fully 
  borne 
  out 
  by 
  the 
  Authors. 
  He 
  had 
  depended 
  to 
  a 
  great 
  extent 
  

   on 
  optical 
  evidence 
  as 
  to 
  the 
  presence 
  of 
  amorphous 
  silica. 
  The 
  

   sections 
  in 
  which 
  the 
  silica 
  was 
  negative 
  in 
  polarized 
  light 
  between 
  

   crossed 
  nicols 
  he 
  had 
  regarded 
  as 
  amorphous. 
  The 
  view 
  that 
  the 
  accu- 
  

   mulation 
  of 
  spicules 
  in 
  these 
  fossil 
  sponge-beds 
  was 
  mainly 
  owing 
  to 
  

   the 
  shedding 
  of 
  the 
  spicules 
  by 
  the 
  living 
  sponges 
  required 
  confirma- 
  

   tion 
  ; 
  he 
  believed 
  it 
  was 
  rather 
  due 
  to 
  the 
  disintegrated 
  skeletons 
  of 
  

   successive 
  generations 
  of 
  these 
  organisms. 
  

  

  Prof. 
  Seelet 
  had 
  doubts 
  about 
  the 
  formation 
  of 
  colloid 
  silica 
  at 
  

   the 
  period 
  of 
  deposition 
  of 
  the 
  roek. 
  He 
  believed 
  the 
  sponges 
  

   furnished 
  soluble 
  matter 
  to 
  water 
  percolating 
  through 
  the 
  rock, 
  and 
  

   that 
  the 
  process 
  had 
  been 
  going 
  on 
  for 
  long 
  ages. 
  

  

  Dr. 
  Hicks 
  differed 
  from 
  the 
  last 
  speaker 
  as 
  to 
  his 
  way 
  of 
  account- 
  

   ing 
  for 
  the 
  silica 
  found 
  among 
  the 
  older 
  rocks. 
  The 
  siliceous 
  bands 
  

   in 
  the 
  Carboniferous 
  Limestone, 
  for 
  instance, 
  must 
  be 
  considered 
  

   contemporaneous 
  deposits. 
  

  

  Mr. 
  Hill 
  could 
  not 
  define 
  at 
  present 
  what 
  was 
  or 
  was 
  not 
  colloid 
  

   silica. 
  In 
  answer 
  to 
  Prof. 
  Seeley, 
  he 
  observed 
  that 
  there 
  were 
  in 
  

   the 
  Chalk-Marl 
  of 
  Wiltshire 
  beds 
  containing 
  quantities 
  of 
  shell-frag- 
  

   ments 
  the 
  material 
  of 
  which 
  was 
  replaced 
  by 
  silica 
  ; 
  he 
  thought 
  if 
  

   silica 
  was 
  deposited 
  by 
  percolating 
  water, 
  as 
  Prof. 
  Seeley 
  suggested, 
  

   it 
  would 
  hardly 
  be 
  in 
  the 
  globular 
  form. 
  

  

  