﻿SILICA 
  IN 
  THE 
  LOWER 
  CHALK 
  OF 
  BERKSHIRE 
  AND 
  WILTSHIRE. 
  413 
  

  

  and 
  the 
  gradual 
  change 
  from 
  the 
  amorphous 
  to 
  the 
  chalcedonic 
  

   condition 
  is 
  very 
  distinctly 
  visible." 
  

  

  Dr. 
  Hinde 
  thinks 
  that 
  the 
  nodules 
  can 
  hardly 
  be 
  correctly 
  termed 
  

   chert, 
  because 
  the 
  silica 
  of 
  ordinary 
  chert 
  is 
  entirely 
  chalcedonic, 
  

   whereas 
  our 
  specimens 
  have 
  an 
  admixture 
  of 
  amorphous 
  silica 
  and 
  

   of 
  amorphous 
  calcareous 
  matter, 
  so 
  as 
  to 
  be 
  really 
  calcareo-siliceous 
  

   concretions. 
  But 
  we 
  think 
  the 
  preponderance 
  of 
  chalcedony 
  and 
  

   the 
  general 
  similarity 
  of 
  the 
  nodules 
  to 
  those 
  of 
  chert 
  in 
  other 
  lime- 
  

   stones 
  entitle 
  them 
  to 
  come 
  under 
  that 
  designation. 
  

  

  We 
  have 
  also 
  examined 
  the 
  two 
  conspicuously 
  hard 
  beds 
  which 
  

   occur 
  in 
  the 
  chalk 
  above 
  the 
  horizon 
  of 
  the 
  chert-like 
  nodules. 
  

   Neither 
  of 
  them 
  contains 
  any 
  colloid 
  silica, 
  but 
  many 
  of 
  the 
  coarser 
  

   shelly 
  fragments 
  they 
  contain 
  appear 
  to 
  have 
  been 
  replaced 
  by 
  chal- 
  

   cedonic 
  or 
  crystalline 
  silica. 
  The 
  silicined 
  prisms 
  of 
  Inoceramus- 
  

   shell 
  are 
  common 
  in 
  the 
  uppermost 
  bed, 
  which 
  is 
  probably 
  a 
  con- 
  

   tinuation 
  of 
  that 
  about 
  the 
  same 
  horizon 
  at 
  Allington. 
  

  

  Chemical 
  Composition 
  of 
  the 
  Siliceous 
  Beds. 
  

  

  Microscopical 
  examination 
  having 
  revealed 
  the 
  presence 
  of 
  a 
  

   large 
  amount 
  of 
  siliceous 
  matter 
  in 
  some 
  of 
  our 
  specimens, 
  we 
  

   were 
  desirous 
  of 
  ascertaining 
  the 
  precise 
  amount 
  of 
  this, 
  and 
  at 
  the 
  

   same 
  time 
  of 
  determining 
  the 
  relative 
  proportions 
  of 
  the 
  amorphous 
  

   or 
  colloid 
  silica 
  in 
  the 
  beds 
  where 
  that 
  seemed 
  to 
  be 
  most 
  abundant. 
  

  

  We 
  had 
  made 
  a 
  rough 
  estimate 
  of 
  the 
  proportionate 
  amounts 
  of 
  

   the 
  principal 
  ingredients 
  of 
  the 
  highest 
  siliceous 
  beds 
  near 
  Chilton 
  

   by 
  estimating 
  the 
  bulk 
  of 
  each 
  visible 
  in 
  thin 
  slices 
  under 
  the 
  

   microscope, 
  with 
  the 
  following 
  results 
  : 
  — 
  

  

  Amorphous 
  organic 
  silica 
  .... 
  about 
  20 
  per 
  cent. 
  

   Inorganic 
  silica 
  and 
  glauconite 
  „ 
  15 
  „ 
  

   Calcareous 
  matter 
  „ 
  65 
  

  

  »? 
  

  

  This, 
  however, 
  could 
  only 
  give 
  a 
  general 
  idea 
  of 
  the 
  proportions, 
  

   and 
  we 
  therefore 
  had 
  some 
  analyses 
  made 
  for 
  the 
  purpose 
  of 
  obtaining 
  

   more 
  accurate 
  results. 
  The 
  following 
  analysis 
  of 
  the 
  highest 
  bed 
  

   in 
  the 
  Chilton 
  cutting 
  was 
  made 
  for 
  us 
  by 
  Mr. 
  J. 
  West 
  Knights, 
  

   the 
  borough 
  analyst 
  for 
  Cambridgeshire 
  : 
  — 
  

  

  Siliceous 
  matter 
  insoluble 
  in 
  hydrochloric 
  acid. 
  33*6 
  

  

  Silica 
  soluble 
  in 
  hydrochloric 
  acid 
  *4 
  

  

  Oxide 
  of 
  iron 
  and 
  alumina 
  *8 
  

  

  Carbonate 
  of 
  lime 
  63-4 
  

  

  Carbonate 
  of 
  magnesia 
  1;5 
  

  

  Trace 
  of 
  alkalies 
  and 
  loss 
  -3 
  

  

  100-0 
  

  

  Of 
  the 
  33*6 
  per 
  cent, 
  of 
  silica 
  insoluble 
  in 
  hydrochloric 
  acid, 
  15 
  

   per 
  cent, 
  was 
  dissolved 
  by 
  a 
  boiling 
  solution 
  of 
  sodium 
  carbonate. 
  

   This 
  analysis 
  serves 
  to 
  give 
  an 
  idea 
  of 
  the 
  general 
  composition 
  of 
  

   the 
  rock. 
  The 
  material 
  was 
  dried 
  at 
  a 
  temperature 
  high 
  enough 
  to 
  

   dissipate 
  all 
  water; 
  hence 
  the 
  absence 
  of 
  water 
  in 
  the 
  analysis, 
  

  

  