﻿SILICA 
  IN 
  THE 
  LOWER 
  CHALK 
  OF 
  BERKSHIRE 
  AND 
  WILTSHIRE. 
  405 
  

  

  overlying 
  a 
  bed 
  of 
  rather 
  hard 
  greyish 
  chalk, 
  which 
  has 
  somewhat 
  

   the 
  appearance 
  of 
  Totternhoe 
  Stono. 
  Examination 
  by 
  hand-lens 
  

   shows 
  that 
  both 
  are 
  full 
  of 
  minute 
  glauconite 
  grains, 
  and 
  also 
  

   present 
  many 
  minute 
  sparkling 
  specks 
  which 
  seem 
  to 
  have 
  too 
  great 
  

   a 
  brilliancy 
  for 
  mica. 
  No 
  siliceous 
  nodules, 
  however, 
  occur 
  here. 
  

  

  Near 
  Allington, 
  north-east 
  of 
  Devizes, 
  the 
  Lower 
  Chalk 
  appears 
  

   to 
  be 
  about 
  240 
  feet 
  thick, 
  and 
  an 
  almost 
  continuous 
  exposure 
  of 
  

   the 
  upper 
  part 
  may 
  be 
  found 
  along 
  the 
  cart-road 
  that 
  leads 
  north- 
  

   ward 
  up 
  to 
  the 
  Downs. 
  Firm 
  white 
  chalk 
  is 
  seen 
  just 
  above 
  the 
  

   500 
  feet 
  contour, 
  and 
  about 
  50 
  or 
  60 
  feet 
  higher 
  a 
  set 
  of 
  hard 
  beds 
  

   makes 
  a 
  succession 
  of 
  ridges 
  across 
  the 
  roadway 
  : 
  there 
  are 
  five 
  or 
  

   six 
  of 
  them, 
  and 
  the 
  highest 
  is 
  not 
  more 
  than 
  40 
  feet 
  below 
  the 
  

   Melbourn 
  Eock, 
  which 
  crosses 
  the 
  road 
  at 
  an 
  elevation 
  of 
  about 
  

   650 
  feet. 
  This 
  topmost 
  hard 
  bed 
  is 
  a 
  compact 
  heavy 
  grey 
  chalk 
  

   with 
  numerous 
  black 
  or 
  dark 
  green 
  grains, 
  characters 
  which 
  are 
  

   very 
  unusual 
  at 
  so 
  high 
  a 
  horizon 
  in 
  the 
  Chalk. 
  

  

  A 
  still 
  more 
  interesting 
  exposure 
  occurs 
  on 
  the 
  road 
  leading 
  up 
  

   the 
  hill, 
  east 
  of 
  Eastcott, 
  near 
  Market 
  Lavington. 
  This 
  we 
  judge 
  to 
  

   be 
  about 
  halfway 
  between 
  the 
  base-line 
  and 
  the 
  horizon 
  of 
  the 
  

   Melbourn 
  Hock, 
  and 
  therefore 
  about 
  120 
  feet 
  above 
  the 
  former. 
  

   About 
  10 
  feet 
  of 
  flaggy 
  whitish 
  chalk 
  is 
  here 
  seen, 
  and 
  one 
  of 
  the 
  

   beds 
  contains 
  many 
  definite 
  siliceous 
  concretions 
  of 
  irregular 
  shapes 
  

   comparable 
  to 
  nodules 
  of 
  flint 
  or 
  chert, 
  but 
  unlike 
  such 
  nodules 
  in 
  

   being 
  so 
  closely 
  united 
  with 
  the 
  surrounding 
  matrix 
  that 
  they 
  are 
  

   not 
  readily 
  separated 
  from 
  it. 
  They 
  are 
  very 
  hard. 
  When 
  broken 
  

   they 
  are 
  seen 
  to 
  be 
  of 
  a 
  yellowish 
  or 
  bluish-grey 
  colour, 
  with 
  a 
  dull 
  

   earthy 
  fracture, 
  which 
  is 
  quite 
  different 
  from 
  that 
  of 
  flint. 
  Examined 
  

   with 
  a 
  lens 
  the 
  fractured 
  surface 
  is 
  seen 
  to 
  be 
  rough 
  and 
  granular, 
  

   and 
  has 
  somewhat 
  the 
  appearance 
  of 
  a 
  piece 
  of 
  grey 
  chalk 
  which 
  

   has 
  been 
  hardened 
  by 
  an 
  infiltration 
  of 
  gum 
  or 
  cement. 
  The 
  nodules 
  

   occur 
  only 
  through 
  a 
  depth 
  of 
  12 
  or 
  18 
  inches 
  of 
  the 
  chalk; 
  and 
  

   about 
  4 
  feet 
  above 
  this 
  bed, 
  the 
  chalk 
  becomes 
  greyish 
  and 
  harder, 
  

   but 
  is 
  still 
  siliceous. 
  

  

  Higher 
  up 
  the 
  road 
  and 
  at 
  distances 
  of 
  about 
  80 
  and 
  40 
  feet 
  

   respectively, 
  below 
  the 
  Melbourn 
  Eock, 
  there 
  are 
  two 
  very 
  hard 
  

   courses 
  of 
  grey 
  chalk, 
  the 
  lower 
  about 
  2 
  feet 
  and 
  the 
  higher 
  about 
  

   3 
  feet 
  thick. 
  The 
  lowest 
  of 
  these 
  occupies 
  the 
  position 
  of 
  the 
  

   Totternhoe 
  Stone, 
  but 
  is 
  much 
  more 
  compact 
  and 
  does 
  not 
  contain 
  

   any 
  of 
  the 
  small 
  phosphatic 
  nodules 
  which 
  always 
  occur 
  in 
  that 
  

   stone 
  ; 
  the 
  upper 
  bed 
  has 
  a 
  rather 
  coarser 
  grain 
  and 
  more 
  gritty 
  

   appearance. 
  

  

  It 
  is 
  interesting 
  also 
  to 
  note 
  that 
  these 
  hard 
  beds 
  containing 
  silica 
  

   and 
  sponge-remains 
  set 
  in 
  concurrently 
  with 
  a 
  decided 
  increase 
  in 
  

   the 
  thickness 
  of 
  the 
  Lower 
  Chalk. 
  Throughout 
  the 
  counties 
  of 
  

   Cambridge, 
  Bedford, 
  Hertford, 
  Buckingham, 
  and 
  Oxford 
  the 
  thick- 
  

   ness 
  of 
  the 
  Lower 
  Chalk, 
  wherever 
  it 
  can 
  be 
  measured, 
  ranges 
  

   between 
  150 
  and 
  170 
  feet; 
  the 
  thickness 
  of 
  the 
  beds 
  below 
  the 
  

   Totternhoe 
  Stone, 
  usually 
  known 
  as 
  Chalk 
  Marl, 
  being 
  generally 
  

   about 
  70 
  feet. 
  In 
  Berkshire 
  this 
  lower 
  division 
  suddenly 
  increases 
  to 
  

   about 
  140 
  feet 
  ; 
  the 
  Totternhoe 
  Stone 
  is 
  very 
  thin, 
  and 
  the 
  overlying 
  

  

  