﻿368 
  PROCEEDINGS 
  OF 
  THE 
  GEOLOGICAL 
  SOCIETY. 
  [Apr. 
  27, 
  

  

  into 
  a 
  mineral 
  like 
  that 
  of 
  the 
  interspaces, 
  and 
  thus 
  a 
  polygonal 
  

   margin 
  surrounds 
  each 
  corallite*. 
  

  

  The 
  interspaces, 
  enlarged 
  by 
  the 
  loss 
  of 
  the 
  thickness 
  of 
  the 
  septa 
  

   and 
  walls, 
  look 
  very 
  prominent 
  in 
  their 
  dense 
  white 
  silica 
  ; 
  and 
  the 
  

   coral 
  may 
  readily 
  be 
  mistaken 
  for 
  one 
  with 
  pentagonal 
  walls, 
  thick 
  

   septa, 
  and 
  no 
  columella. 
  In 
  reality, 
  as 
  may 
  be 
  made 
  out 
  with 
  ease, 
  

   the 
  walls 
  are 
  circular, 
  the 
  septa 
  very 
  slender, 
  and 
  the 
  columella 
  

   large 
  and 
  occupying 
  much 
  space. 
  Examples 
  of 
  this 
  variety 
  of 
  

   fossilization 
  are 
  very 
  common 
  in 
  Antigua, 
  and 
  the 
  polygonal 
  shape 
  

   of 
  the 
  transverse 
  sections 
  of 
  the 
  corallites, 
  with 
  the 
  opaque-white 
  

   mineral 
  which 
  fills 
  up 
  the 
  interspaces, 
  are 
  very 
  characteristic. 
  The 
  

   more 
  the 
  sections 
  of 
  the 
  corallites 
  are 
  aslant, 
  and 
  not 
  at 
  right 
  angles 
  

   to 
  their 
  long 
  axis, 
  the 
  more 
  the 
  polygonal 
  form 
  is 
  developed 
  ; 
  it 
  is, 
  

   therefore, 
  requisite 
  to 
  notice 
  the 
  nature 
  of 
  the 
  section, 
  especially 
  in 
  

   the 
  specimens 
  where 
  the 
  destructive 
  silicification 
  has 
  affected 
  the 
  

   interspaces, 
  but 
  has 
  not 
  thinned 
  the 
  septa, 
  the 
  interseptal 
  loculi 
  

   being 
  filled 
  with 
  opalescent 
  silica. 
  One 
  large 
  specimen 
  of 
  Astrcea 
  

   megalaxona 
  affected 
  by 
  this 
  variety 
  has 
  the 
  central 
  spot 
  of 
  each 
  

   corallite, 
  as 
  seen 
  in 
  transverse 
  sections, 
  turned 
  into 
  a 
  white 
  form 
  of 
  

   silica. 
  The 
  columella 
  is, 
  therefore, 
  fossilized 
  white, 
  the 
  septa 
  are 
  

   of 
  transparent 
  and 
  rather 
  dark 
  silica, 
  the 
  interspaces 
  are 
  of 
  light 
  

   brown 
  silica, 
  and 
  all 
  are 
  indistinct, 
  except 
  the 
  white 
  central 
  spots. 
  

   Near 
  the 
  surface 
  these 
  spots 
  are 
  not 
  seen, 
  having 
  decayed 
  away, 
  so 
  

   that 
  the 
  coral, 
  instead 
  of 
  being 
  considered 
  a 
  species 
  closely 
  allied 
  to 
  

   Astrcea 
  Pleiades, 
  may 
  readily 
  be 
  taken 
  for 
  a 
  Palaeozoic 
  form. 
  

  

  5. 
  Siliceous 
  casts. 
  Variety 
  1. 
  — 
  The 
  sclerenchyma 
  is 
  deficient, 
  and 
  

   the 
  interspaces 
  are 
  filled 
  with 
  either 
  opaque 
  and 
  variously 
  coloured 
  

   silica 
  or 
  with 
  granular 
  silica, 
  which 
  is 
  white 
  or 
  redf. 
  

  

  Variety 
  2. 
  — 
  The 
  sclerenchyma 
  is 
  deficient 
  except 
  as 
  regards 
  the 
  

   septa, 
  which 
  are 
  seen 
  as 
  thin 
  radiating 
  siliceous 
  lines. 
  The 
  inter- 
  

   spaces 
  are 
  as 
  in 
  the 
  last 
  variety, 
  but 
  they 
  are 
  often 
  filled 
  with 
  

   transparent 
  chalcedony. 
  The 
  siliceous 
  casts 
  of 
  compound 
  corals, 
  so 
  

   common 
  in 
  European 
  Secondary 
  formations, 
  and 
  the 
  rarer 
  casts 
  of 
  

   simple 
  kinds 
  are 
  common 
  enough 
  in 
  the 
  "West 
  Indian 
  limestones. 
  

  

  As 
  the 
  genus 
  Isastrcea 
  is 
  so 
  usually 
  found 
  affected 
  by 
  this 
  form 
  of 
  

   fossilization 
  in 
  the 
  older 
  coral-formations, 
  so 
  some 
  genera 
  allied 
  to 
  

   it 
  in 
  the 
  polygonal 
  shape 
  of 
  their 
  calices 
  and 
  the 
  delicacy 
  of 
  their 
  

   walls 
  are 
  equally 
  affected 
  in 
  the 
  Antiguan 
  Chert 
  and 
  Marl. 
  

  

  The 
  casts 
  in 
  these 
  formations 
  are, 
  however, 
  on 
  a 
  very 
  great 
  scale 
  ; 
  

   and 
  not 
  only 
  are 
  the 
  surfaces 
  of 
  the 
  masses 
  found 
  as 
  casts, 
  but 
  the 
  

   whole 
  of 
  large 
  corals 
  are 
  thus 
  seen. 
  The 
  relation 
  of 
  the 
  second 
  

   variety 
  of 
  destructive 
  siliceous 
  fossilization 
  to 
  the 
  formation 
  of 
  casts 
  

   is 
  evident 
  ; 
  they 
  are 
  produced 
  by 
  the 
  absorption 
  and 
  destruction 
  of 
  

   the 
  sclerenchyma 
  subsequently 
  to 
  the 
  infiltration 
  of 
  the 
  interseptal 
  

   loculi 
  with 
  an 
  enduring 
  mineral. 
  The 
  genera 
  affected 
  by 
  the 
  most 
  

   perfect 
  destruction 
  of 
  the 
  sclerenchyma 
  are 
  those 
  whose 
  corallites 
  

   are 
  united 
  by 
  their 
  walls 
  without 
  any 
  intervening 
  costae 
  and 
  exo- 
  

  

  * 
  Astrcea 
  megalaxona, 
  nobis, 
  of 
  Antigua, 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xix. 
  

   pi. 
  xiii. 
  fig. 
  126. 
  

  

  t 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xix. 
  pi, 
  xiv. 
  figs. 
  C 
  & 
  8. 
  

  

  