﻿1864.] 
  DUNCAN 
  — 
  WEST 
  INDIAN 
  COEALS. 
  367 
  

  

  large 
  Astraean 
  the 
  superficial 
  parts 
  of 
  the 
  corallum 
  are 
  well 
  pre- 
  

   served, 
  but 
  the 
  centre 
  and 
  base 
  of 
  the 
  mass 
  are 
  turned 
  into 
  dark, 
  

   semitransparent, 
  homogeneous 
  flint, 
  without 
  a 
  trace 
  of 
  structure. 
  

   There 
  are 
  many 
  grades 
  of 
  the 
  destructive 
  form 
  of 
  silicification 
  

   between 
  those 
  just 
  described 
  and 
  that 
  which 
  is 
  so 
  slight 
  as 
  to 
  be 
  

   distinguished 
  with 
  difficulty, 
  or 
  not 
  at 
  all, 
  from 
  the 
  results 
  of 
  pre- 
  

   fossil 
  wear 
  and 
  tear. 
  Simple 
  loss 
  of 
  the 
  septal 
  granules, 
  thinning 
  

   of 
  the 
  septa, 
  costse, 
  and 
  of 
  the 
  wall, 
  are 
  very 
  commonly 
  observed 
  ; 
  

   these 
  conditions 
  may 
  have 
  resulted 
  from 
  prefossil 
  accidents 
  ; 
  but 
  

   when 
  the 
  wall 
  is 
  hardly 
  distinguishable, 
  the 
  septa 
  and 
  costse 
  re- 
  

   maining 
  as 
  well 
  proportioned 
  as 
  ever*, 
  or 
  when 
  the 
  dissepiments 
  

   are 
  wanting, 
  the 
  rest 
  of 
  the 
  sclerenchyma 
  being 
  intact, 
  something 
  

   more 
  than 
  prefossil 
  wear 
  and 
  tear 
  must 
  be 
  considered 
  in 
  explana- 
  

   tion. 
  In 
  some 
  long 
  corallites 
  of 
  a 
  llliodarcea 
  or 
  a 
  Stephanoccenia 
  

   the 
  walls 
  and 
  septa 
  do 
  not 
  exist; 
  they 
  have 
  been 
  absorbed 
  and 
  

   replaced 
  by 
  silica 
  ; 
  but 
  the 
  columella 
  and 
  its 
  attached 
  pari, 
  greatly 
  

   thinned, 
  are 
  found 
  in 
  the 
  centre 
  of 
  each 
  corallite. 
  This 
  state 
  of 
  

   things 
  must 
  have 
  been 
  brought 
  about 
  after 
  the 
  infiltration 
  of 
  the 
  

   interspaces 
  ; 
  otherwise 
  the 
  corallites 
  would 
  have 
  fallen 
  to 
  pieces. 
  

   Again, 
  the 
  presence 
  of 
  imperfect 
  corallites 
  separated 
  from 
  the 
  mass 
  

   of 
  perfectly 
  silicified 
  coral, 
  and 
  joined 
  to 
  it 
  by 
  opalescent 
  or 
  even 
  

   nearly 
  transparent 
  homogeneous 
  silica, 
  is 
  now 
  and 
  then 
  seen 
  in 
  

   specimens 
  where 
  there 
  is 
  a 
  greater 
  amount 
  of 
  destruction 
  of 
  the 
  

   walls 
  and 
  septa 
  of 
  the 
  corallites 
  than 
  is 
  consistent 
  with 
  the 
  integrity 
  

   of 
  the 
  mass 
  when 
  formed 
  of 
  its 
  usual 
  coral-salts. 
  It 
  is 
  in 
  these 
  

   minor 
  grades, 
  especially 
  where 
  the 
  siliceous 
  replacement 
  is 
  in 
  the 
  

   transparent 
  or 
  slightly 
  opalescent 
  form, 
  that 
  the 
  wavy 
  lines 
  of 
  more 
  

   opaque 
  silica 
  curve 
  around 
  and 
  bound 
  the 
  parts 
  where 
  destructive 
  

   silicification 
  has 
  been 
  very 
  decided. 
  The 
  parts 
  beyond 
  are 
  often 
  

   quite 
  semitransparent, 
  and 
  the 
  wavy 
  lines 
  or 
  broad 
  belts 
  are 
  gene- 
  

   rally 
  singularly 
  milky 
  in 
  colour. 
  The 
  occasional 
  and 
  general 
  connexion 
  

   of 
  homogeneous 
  black 
  flint, 
  and 
  the 
  wavy 
  lines 
  of 
  agates 
  and 
  

   of 
  more 
  or 
  less 
  coloured 
  siliceous 
  stones, 
  with 
  the 
  former 
  existence 
  

   of 
  organized 
  structures 
  is 
  very 
  evident 
  after 
  the 
  study 
  of 
  the 
  An- 
  

   tiguan 
  fossil 
  corals. 
  

  

  The 
  first 
  variety 
  of 
  this 
  kind 
  of 
  fossilization 
  complicates 
  the 
  

   appearance 
  of 
  the 
  corals 
  greatly, 
  on 
  account 
  of 
  the 
  interspaces 
  being 
  

   filled 
  with 
  opaque 
  light-grey 
  granular 
  silica, 
  the 
  sclerenchyma 
  

   having 
  been 
  thinned 
  down 
  to 
  the 
  utmost 
  ; 
  probably 
  both 
  by 
  pre- 
  

   fossil 
  wear 
  and 
  tear 
  and 
  by 
  destructive 
  silicification. 
  The 
  walls 
  and 
  

   septa 
  of 
  the 
  corallites 
  have 
  lost 
  half 
  their 
  thickness, 
  the 
  columella 
  

   has 
  not 
  lost 
  much 
  ; 
  but 
  all 
  traces 
  of 
  endotheca 
  and 
  of 
  costse 
  have 
  

   gone 
  : 
  what 
  remains 
  is 
  either 
  transparent 
  and 
  pale 
  grey, 
  or 
  opaque 
  

   and 
  jet-black. 
  The 
  exothecal 
  cells 
  have 
  been 
  filled 
  and 
  changed 
  

  

  * 
  The 
  best 
  distinctions 
  between 
  the 
  loss 
  of 
  sclerenchyma 
  from 
  prefossil 
  wear 
  

   and 
  tear, 
  and 
  from 
  destructive 
  silicification, 
  are 
  as 
  follows 
  : 
  — 
  the 
  latter 
  is 
  more 
  

   decided 
  in 
  its 
  effects, 
  it 
  acts 
  unequally 
  upon 
  parts 
  closely 
  attached, 
  and 
  its 
  effects 
  

   are 
  such 
  as 
  would 
  tend, 
  if 
  the 
  homogeneous 
  flint 
  or 
  opalescent 
  silica 
  were 
  re- 
  

   moved, 
  to 
  the 
  falling 
  to 
  pieces 
  of 
  the 
  corallum. 
  Both 
  conditions 
  have 
  a 
  relation 
  

   to 
  the 
  effects 
  of 
  decomposing 
  tissue 
  on 
  the 
  sclerenchyma. 
  

  

  2 
  D 
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