﻿372 
  PROCEEDINGS 
  OF 
  THE 
  GEOLOGICAL 
  SOCIETY. 
  [Apr. 
  27, 
  

  

  more 
  permeated 
  by 
  the 
  juices 
  of 
  the 
  polypes, 
  and 
  infinitely 
  more 
  

   fragile 
  than 
  the 
  deeper 
  parts 
  ; 
  they 
  are 
  covered 
  during 
  life 
  by 
  the 
  

   polype-tissues 
  which 
  determine 
  their 
  nutrition 
  and 
  growth, 
  and 
  

   these 
  processes 
  are 
  singularly 
  active 
  in 
  luxuriant 
  corals. 
  In 
  slow- 
  

   growing 
  corals 
  this 
  calicular 
  fragility 
  is 
  not 
  seen, 
  and 
  the 
  lower 
  

   parts 
  of 
  a 
  corallite 
  are 
  nearly 
  stony 
  in 
  hardness. 
  

  

  The 
  larger 
  Astrseans, 
  the 
  Isastrasans, 
  and 
  the 
  Stephanoccenians 
  

   were, 
  probably, 
  rapid 
  growers, 
  and 
  unfavourable 
  to 
  the 
  development 
  

   of 
  casts. 
  

  

  In 
  some 
  specimens 
  the 
  prefossil 
  influence 
  of 
  decomposing 
  mem- 
  

   brane 
  on 
  tender 
  calices 
  is 
  seen 
  by 
  the 
  fossilization 
  of 
  deformed 
  

   corallite-ends. 
  It 
  seems 
  reasonable 
  to 
  account 
  for 
  the 
  non-silicifi- 
  

   cation 
  and 
  consequent 
  non 
  - 
  preservation 
  of 
  the 
  superficial 
  hard 
  

   parts 
  by 
  admitting 
  the 
  destructive 
  influences 
  of 
  a 
  decomposing 
  

   membrane 
  upon 
  them, 
  their 
  molecular 
  adhesion 
  being 
  slight, 
  and 
  

   their 
  constitution 
  mainly 
  of 
  carbonate 
  of 
  lime. 
  

  

  The 
  sclerenchyma 
  thus 
  affected 
  might 
  be 
  strong 
  enough 
  to 
  last 
  

   out 
  the 
  filling 
  up 
  of 
  the 
  interseptal 
  loculi 
  with 
  carbonate 
  of 
  lime, 
  

   but 
  was 
  not 
  durable 
  enough 
  to 
  withstand 
  the 
  secondary 
  chemical 
  

   changes 
  which 
  terminated 
  in 
  the 
  replacement 
  of 
  the 
  carbonate 
  of 
  

   lime 
  in 
  the 
  loculi 
  by 
  silica. 
  

  

  Lower 
  down 
  the 
  corallum, 
  and 
  out 
  of 
  the 
  range 
  of 
  the 
  superficial 
  

   polype-tissues, 
  the 
  denser 
  sclerenchyma 
  was 
  gradually 
  replaced 
  by 
  

   a 
  homogeneous 
  dark 
  flint, 
  and 
  lost 
  but 
  little 
  of 
  its 
  details 
  during 
  the 
  

   process 
  ; 
  still 
  a 
  little 
  loss 
  is 
  noticed, 
  especially 
  in 
  prismatic 
  corallites. 
  

  

  To 
  account 
  for 
  this 
  loss, 
  of 
  granules 
  on 
  the 
  septa, 
  of 
  portions 
  of 
  

   the 
  septa 
  themselves, 
  of 
  endothecal 
  dissepiments, 
  and 
  of 
  more 
  or 
  

   less 
  of 
  the 
  corallite 
  -wall, 
  we 
  must 
  recognize 
  tbe 
  influence 
  of 
  the 
  

   decomposition 
  of 
  the 
  delicate 
  tissue 
  which 
  lines 
  these 
  parts, 
  even 
  

   deep 
  in 
  the 
  corallum. 
  

  

  The 
  loss 
  of 
  details 
  must 
  have 
  occurred 
  before 
  the 
  silicification 
  of 
  

   the 
  interspaces 
  (of 
  their 
  carbonate 
  of 
  lime), 
  because 
  the 
  space 
  

   formerly 
  occupied 
  by 
  the 
  deficient 
  hard 
  parts 
  becomes 
  filled 
  up 
  by 
  

   an 
  extension 
  of 
  the 
  infiltrated 
  carbonate, 
  which 
  is 
  rarely 
  so 
  perfectly 
  

   silicified 
  as 
  the 
  rest*. 
  

  

  The 
  most 
  remarkable 
  changes 
  induced 
  by 
  loss 
  of 
  sclerenchyma, 
  

   and 
  persistence 
  of 
  the 
  mineral 
  infiltrated 
  into 
  the 
  interspaces 
  of 
  a 
  

   coral, 
  are 
  observed 
  in 
  the 
  case 
  of 
  some 
  perforate 
  corals 
  at 
  Antigua 
  

   and 
  Jamaica. 
  The 
  corallum 
  resembles 
  that 
  of 
  an 
  Astraean 
  with 
  

   separate 
  corallites, 
  and 
  dentate 
  costae 
  in 
  regular 
  series 
  f 
  ; 
  really 
  the 
  

   dentations 
  are 
  casts 
  of 
  the 
  regular 
  spaces 
  between 
  the 
  network 
  of 
  

   the 
  original 
  corallite, 
  whose 
  walls 
  and 
  tissue 
  have 
  been 
  lost. 
  These 
  

   casts 
  are 
  not 
  perfectly 
  siliceous, 
  but 
  contain 
  much 
  carbonate 
  of 
  lime 
  ; 
  

   so 
  that 
  the 
  destruction 
  of 
  the 
  sclerenchyma, 
  which 
  developes 
  a 
  cast, 
  

   occurs 
  before 
  silicification 
  is 
  complete. 
  The 
  later 
  Tertiary 
  corals 
  are 
  

   constantly 
  found 
  without 
  their 
  septa, 
  or 
  with 
  their 
  columella? 
  and 
  part 
  

   of 
  their 
  septa 
  attached, 
  the 
  corallite 
  -wall 
  having 
  been 
  worn 
  away 
  : 
  

   this 
  is 
  produced 
  by 
  prefossil 
  wear 
  and 
  tear, 
  as 
  is 
  also 
  the 
  loss 
  of 
  

   the 
  details 
  of 
  some 
  dendroid 
  Astrseans 
  in 
  the 
  Jamaican 
  limestone. 
  

  

  * 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xix. 
  pi. 
  xiv. 
  fig. 
  7. 
  t 
  Op. 
  cit. 
  pi. 
  xiv. 
  fig. 
  4c. 
  

  

  