﻿604 
  DE. 
  W. 
  EEASEE 
  HUME 
  ON 
  [Dec. 
  T897, 
  

  

  with, 
  in 
  stances 
  of 
  fossil 
  sponges 
  presenting 
  a 
  well-preserved 
  form 
  

   externally 
  and 
  a 
  curious 
  mixture 
  of 
  spicules 
  within. 
  

  

  Considering 
  the 
  question 
  of 
  silicification 
  as 
  a 
  whole, 
  we 
  are 
  now 
  

   able 
  to 
  trace 
  out 
  some 
  interesting 
  facts 
  with 
  regard 
  to 
  the 
  character 
  

   of 
  the 
  preservation 
  of 
  sponge-spicules 
  and 
  other 
  organisms 
  as 
  

   illustrated 
  by 
  the 
  Irish 
  strata 
  : 
  — 
  

  

  (a) 
  In 
  the 
  Yellow 
  Sandstones 
  where 
  detrital 
  quartz 
  is 
  plentiful, 
  

   but 
  fossils 
  capable 
  of 
  silicification 
  are 
  not 
  numerous, 
  the 
  sponge- 
  

   spicules 
  which 
  were 
  present 
  have 
  been 
  partly 
  or 
  completely 
  dissolved, 
  

   giving 
  rise 
  to 
  definite 
  chert-beds 
  or 
  cherty 
  nodules. 
  

  

  (6) 
  In 
  the 
  Inoceramus-zone 
  and 
  upward, 
  the 
  abundant 
  glauconitic 
  

   casts 
  of 
  spicules 
  prove 
  the 
  former 
  presence 
  of 
  a 
  sponge-fauna 
  which 
  

   in 
  other 
  respects 
  has 
  left 
  no 
  trace 
  of 
  its 
  former 
  existence. 
  The 
  

   silica, 
  thus 
  passed 
  into 
  solution, 
  has 
  been 
  redeposited 
  in 
  the 
  interior, 
  

   or 
  as 
  the 
  replacement, 
  of 
  shells 
  of 
  Inoceramus 
  and 
  brachiopoda. 
  

  

  (c) 
  Where 
  large 
  mineral 
  fragments 
  are 
  no 
  longer 
  carried 
  by 
  the 
  

   currents, 
  solution 
  appears 
  to 
  have 
  been 
  at 
  its 
  minimum 
  and 
  glauco- 
  

   nitic 
  replacement 
  absent, 
  the 
  result 
  being 
  the 
  preservation 
  of 
  the 
  

   sponges 
  and 
  the 
  formation 
  of 
  a 
  definite 
  sponge-bed. 
  

  

  (d) 
  In 
  the 
  pure 
  limestones 
  the 
  solution 
  of 
  the 
  spicules 
  is 
  again 
  

   considerable 
  or 
  complete, 
  and 
  organisms 
  capable 
  of 
  silicification 
  are 
  

   either 
  small 
  or 
  rare, 
  so 
  that 
  a 
  great 
  excess 
  of 
  colloid 
  silica 
  is 
  formed, 
  

   which 
  is 
  subsequently 
  redeposited 
  as 
  flint. 
  

  

  In 
  addition 
  to 
  the 
  formation 
  of 
  flint, 
  silicification 
  has 
  proceeded 
  to 
  

   an 
  extraordinary 
  extent 
  in 
  the 
  chalk, 
  as 
  is 
  seen 
  from 
  the 
  statement 
  

   of 
  Mr. 
  J. 
  "Wright, 
  1 
  after 
  examining 
  the 
  powder 
  in 
  the 
  interior 
  of 
  a 
  

   number 
  of 
  flints 
  : 
  — 
  ' 
  Among 
  the 
  many 
  specimens 
  collected 
  from 
  these 
  

   various 
  places, 
  I 
  have 
  recognized, 
  besides 
  corals 
  and 
  polyzoa, 
  17 
  

   species 
  of 
  ostracoda, 
  106 
  species 
  and 
  well-marked 
  varieties 
  of 
  

   foraminifera, 
  and 
  27 
  forms 
  of 
  sponge-spiculae 
  ; 
  many 
  of 
  these 
  

   attain 
  fine 
  proportions, 
  being 
  much 
  larger 
  than 
  those 
  usually 
  obtained 
  

   from 
  the 
  washings 
  of 
  English 
  Chalk. 
  The 
  microzoa, 
  on 
  being 
  placed 
  

   in 
  hydrochloric 
  acid, 
  were 
  found 
  to 
  be 
  either 
  not 
  at 
  all 
  or 
  but 
  slightly 
  

   affected 
  by 
  it, 
  showing 
  that 
  their 
  original 
  composition 
  has, 
  to 
  a 
  

   great 
  extent, 
  been 
  replaced 
  by 
  silica.' 
  

  

  The 
  same 
  statement 
  holds 
  good 
  for 
  the 
  spicules 
  described 
  by 
  Dr. 
  

   G. 
  J. 
  Hinde 
  from 
  the 
  Upper 
  Chalk 
  of 
  Horstead, 
  and 
  Prof. 
  Sollas 
  has 
  

   also 
  discussed 
  and 
  emphasized 
  the 
  same 
  point 
  in 
  the 
  paper 
  mentioned 
  

   above, 
  laying 
  stress 
  on 
  the 
  fact 
  that 
  when 
  siliceous 
  solutions 
  are 
  

   present 
  they 
  replace 
  the 
  molecule 
  of 
  carbonate 
  of 
  lime 
  as 
  a 
  whole. 
  

   In 
  consequence 
  of 
  this 
  latter 
  property, 
  not 
  only 
  internal 
  casts, 
  but 
  

   absolute 
  replacements 
  of 
  the 
  original 
  tests 
  are 
  met 
  with. 
  

  

  Glauconite 
  appears 
  to 
  behave 
  in 
  a 
  precisely 
  similar 
  manner, 
  the 
  

   internal 
  casts 
  being 
  frequently 
  of 
  a 
  very 
  perfect 
  character. 
  Prof. 
  

   Sollas 
  has 
  suggested 
  that 
  these 
  may 
  be 
  the 
  result 
  of 
  the 
  combination 
  

   of 
  silica, 
  set 
  free 
  from 
  decaying 
  sponges, 
  with 
  alumina, 
  iron, 
  and 
  

   alkalies, 
  to 
  the 
  entire 
  exclusion 
  of 
  lime. 
  At 
  present 
  I 
  am 
  unable 
  to 
  

   obtain 
  any 
  evidence 
  of 
  the 
  existence 
  of 
  sponges 
  during 
  the 
  formation 
  

   of 
  an 
  extremely 
  rich 
  deposit 
  of 
  glauconitic 
  grains, 
  as, 
  for 
  example, 
  

  

  1 
  ' 
  Irish 
  Cret. 
  Microzoa,' 
  Proc. 
  Belfast 
  Nat. 
  Field 
  Club, 
  ser. 
  2, 
  vol. 
  i. 
  p. 
  74. 
  

  

  