﻿COLLOID 
  SILICA 
  IN 
  LOWER 
  CHALK 
  OF 
  BERKSHIRE 
  AND 
  WILTSHIRE. 
  403 
  

  

  27. 
  The 
  Occurrence 
  of 
  Colloid 
  Silica 
  in 
  the 
  Low 
  t 
  er 
  Chalk 
  of 
  Berk- 
  

   shire 
  and 
  Wiltshire. 
  By 
  A. 
  J. 
  Jukes-Browne, 
  Esq., 
  F.G.S. 
  

   and 
  W. 
  Hill, 
  Esq., 
  F.G.S. 
  (Read 
  March 
  20, 
  1889.) 
  

  

  The 
  existence 
  in 
  the 
  Lower 
  Chalk 
  of 
  beds 
  containing 
  a 
  certain 
  

   proportion 
  of 
  pure 
  disseminated 
  silica 
  is 
  not 
  a 
  new 
  discovery, 
  the 
  

   occurrence 
  of 
  such 
  material 
  having 
  been 
  proved 
  by 
  Messrs. 
  Way 
  and 
  

   Paine 
  in 
  1851 
  * 
  ; 
  but 
  we 
  are 
  not 
  aware 
  that 
  it 
  has 
  ever 
  before 
  been 
  

   found 
  in 
  such 
  quantity, 
  or 
  that 
  any 
  detailed 
  investigation 
  of 
  its 
  mode 
  

   of 
  occurrence 
  has 
  previously 
  been 
  made. 
  

  

  The 
  beds 
  which 
  are 
  the 
  subject 
  of 
  the 
  present 
  communication 
  

   were 
  discovered 
  during 
  the 
  examination 
  of 
  the 
  Lower 
  Chalk 
  of 
  

   Berkshire 
  and 
  Wiltshire 
  in 
  1887-88, 
  for 
  the 
  purposes 
  of 
  the 
  Geo- 
  

   logical 
  Survey, 
  and 
  we 
  are 
  indebted 
  to 
  the 
  Director-General 
  of 
  the 
  

   Survey 
  for 
  permission 
  to 
  make 
  use 
  of 
  the 
  stratigraphical 
  information 
  

   then 
  obtained. 
  Beds 
  of 
  siliceous 
  chalk 
  were 
  first 
  observed 
  near 
  

   Wantage 
  and 
  Letcombe 
  Bassett, 
  and 
  attracted 
  attention 
  from 
  their 
  

   hard 
  and 
  compact 
  character 
  ; 
  they 
  occurred 
  near 
  the 
  horizon 
  at 
  

   which 
  the 
  Totternhoe 
  Stone 
  is 
  found 
  further 
  north, 
  and 
  it 
  was 
  at 
  

   first 
  thought 
  possible 
  that 
  one 
  of 
  them 
  might 
  be 
  the 
  representative 
  

   of 
  that 
  stone. 
  Specimens 
  were 
  therefore 
  submitted 
  to 
  microscopical 
  

   examination, 
  and 
  were 
  found 
  to 
  be 
  different 
  from 
  Totternhoe 
  Stone 
  

   and 
  from 
  any 
  other 
  samples 
  of 
  Lower 
  Chalk 
  previously 
  examined 
  

   by 
  us. 
  The 
  exact 
  stratigraphical 
  position 
  of 
  these 
  beds 
  was 
  subse- 
  

   quently 
  ascertained 
  by 
  means 
  of 
  the 
  excellent 
  section 
  exposed 
  by 
  

   the 
  cutting 
  on 
  the 
  Didcot, 
  Newbury, 
  and 
  Winchester 
  line, 
  near 
  

   Chilton. 
  This 
  section 
  we 
  visited 
  together, 
  and 
  the 
  hard 
  siliceous 
  

   beds 
  were 
  found 
  to 
  occur 
  in 
  the 
  upper 
  part 
  of 
  the 
  Chalk 
  Marl 
  or 
  

   zone 
  of 
  Ammonites 
  varians, 
  a 
  little 
  way 
  below 
  the 
  horizon 
  of 
  the 
  

   Totternhoe 
  Stone, 
  of 
  which 
  a 
  thin 
  representative 
  is 
  seen 
  in 
  the 
  

  

  The 
  following 
  is 
  the 
  succession 
  seen 
  in 
  the 
  cutting 
  about 
  half 
  a 
  

   mile 
  east 
  of 
  Chilton 
  : 
  — 
  

  

  ft. 
  

  

  in. 
  

  

  ( 
  Grey 
  bedded 
  chalk, 
  soft 
  below, 
  harder 
  above 
  12 
  

  

  p 
  I 
  Hard 
  grey 
  rocky 
  chalk 
  1 
  

  

  p-i 
  lQ 
  -n 
  i 
  Hard 
  grey 
  bedded 
  chalk 
  with 
  partings 
  of 
  marly 
  chalk 
  5 
  

  

  a 
  | 
  Dark 
  grey 
  gritty 
  stone 
  with 
  many 
  small 
  phosphatic 
  nodules 
  

  

  1, 
  (Totternhoe 
  Stone) 
  ,. 
  2 
  

  

  (Soft 
  grey 
  marl, 
  breaking 
  conchoidally 
  5 
  

  

  Hard 
  compact 
  siliceous 
  limestone 
  1 
  3 
  

  

  Soft 
  shaly 
  chalk 
  6 
  

  

  Hard 
  grey 
  siliceous 
  chalk 
  9 
  

  

  Marl. 
  "] 
  Loose 
  grey 
  marly 
  chalk 
  3 
  

  

  Hard 
  rough 
  grey 
  chalk 
  6 
  

  

  j 
  Loose 
  grey 
  marly 
  chalk 
  2 
  

  

  ^ 
  Passing 
  down 
  into 
  rather 
  hard 
  blocky 
  chalk 
  3 
  

  

  Chalk 
  

  

  36 
  

  

  * 
  See 
  Joum. 
  Roy. 
  Agric. 
  Soc. 
  xii. 
  p. 
  544. 
  

  

  2*2 
  

  

  