﻿3Q6 
  PROCEEDINGS 
  OF 
  THE 
  GEOLOGICAL 
  SOCIETY. 
  [Apr. 
  27, 
  

  

  The 
  calices 
  usually 
  have 
  the 
  interseptal 
  loculi 
  filled 
  with 
  white 
  por- 
  

   celain-looking 
  silica, 
  the 
  septa 
  being* 
  grey. 
  

  

  In 
  a 
  small 
  Astraean 
  * 
  from 
  the 
  chert 
  of 
  Antigua, 
  the 
  septa, 
  endo- 
  

   theca, 
  and 
  walls 
  are 
  of 
  homogeneous 
  flint, 
  and 
  the 
  interseptal 
  

   loculi 
  and 
  exothecal 
  cells 
  contain 
  opal 
  and 
  porcellanous 
  silica. 
  The 
  

   result 
  is 
  the 
  production 
  of 
  a 
  fossil 
  second 
  to 
  none 
  in 
  beauty. 
  

  

  3. 
  /Siliceous 
  and 
  Crystalline. 
  Variety 
  1. 
  — 
  The 
  selerenchyma 
  con- 
  

   sists 
  of 
  silica 
  resembling 
  the 
  original 
  tissue 
  in 
  colour 
  ; 
  it 
  is 
  sometimes 
  

   transparent, 
  and 
  often 
  opaque 
  and 
  white. 
  The 
  interspaces 
  contain 
  

   crystalline 
  quartz 
  ; 
  the 
  endothecal 
  spaces 
  are 
  more 
  or 
  less 
  empty; 
  

   but 
  the 
  costae, 
  the 
  ccenenchymal 
  spaces, 
  and 
  the 
  cells 
  of 
  the 
  exo- 
  

   theca 
  are 
  replaced 
  by 
  radiating 
  crystals 
  of 
  quartz. 
  

  

  Variety 
  -2. 
  — 
  The 
  selerenchyma 
  is 
  like 
  that 
  of 
  the 
  last 
  variety, 
  

   but 
  the 
  costse 
  and 
  exotheca 
  exist 
  ; 
  the 
  corallites 
  are 
  often 
  much 
  de- 
  

   formed, 
  and 
  the 
  septa 
  are 
  generally 
  deficient. 
  The 
  exothecal 
  and 
  

   ccenenchymal 
  spaces 
  are 
  empty, 
  whilst 
  the 
  endothecal 
  spaces 
  and 
  

   the 
  interior 
  of 
  the 
  corallites 
  are 
  filled 
  with 
  crystals 
  of 
  quartz, 
  ac- 
  

   companied 
  often 
  by 
  homogeneous 
  silica. 
  

  

  The 
  first 
  variety 
  is 
  seen 
  in 
  the 
  Astraeans 
  of 
  the 
  lower 
  part 
  of 
  the 
  

   Antiguan 
  marl, 
  and 
  the 
  second 
  particularly 
  affects 
  the 
  species 
  As~ 
  

   trcea 
  radiata, 
  var. 
  intermedia, 
  at 
  Antigua 
  ; 
  both 
  have 
  a 
  relation 
  to 
  

   the 
  next 
  form 
  of 
  mineralization. 
  

  

  4. 
  Siliceous 
  and 
  Destructive. 
  Variety 
  1. 
  — 
  The 
  selerenchyma 
  is 
  

   much 
  thinner 
  than 
  normal, 
  the 
  details 
  are 
  very 
  defective, 
  and 
  the 
  

   endotheca 
  is 
  usually 
  wanting. 
  The 
  silica 
  is 
  variously 
  coloured, 
  and 
  

   is 
  either 
  transparent 
  and 
  grey, 
  or 
  opaque 
  and 
  white, 
  but 
  it 
  is 
  often 
  

   black. 
  The 
  interspaces 
  are 
  filled 
  with 
  transparent, 
  opalescent, 
  or 
  

   opaque- 
  white 
  silica 
  ; 
  they 
  are 
  larger 
  than 
  is 
  normal 
  from 
  the 
  loss 
  of 
  

   the 
  substance 
  of 
  the 
  original 
  hard 
  parts. 
  The 
  corallites 
  are 
  usually 
  

   much 
  deformed. 
  

  

  Variety 
  2. 
  — 
  The 
  selerenchyma 
  is 
  as 
  in 
  the 
  last 
  variety, 
  but 
  in 
  

   some 
  parts 
  more 
  or 
  less 
  of 
  the 
  walls 
  and 
  septa 
  are 
  wanting. 
  The 
  

   interspaces 
  are 
  as 
  in 
  the 
  last 
  variety. 
  

  

  Variety 
  3. 
  — 
  The 
  selerenchyma 
  is 
  very 
  imperfect, 
  and 
  only 
  re- 
  

   mains 
  here 
  and 
  there 
  in 
  a 
  large 
  silicified 
  corallum. 
  The 
  silica 
  may 
  

   be 
  opaque 
  or 
  nearly 
  transparent. 
  The 
  interspaces 
  are 
  filled 
  with 
  

   chalcedony, 
  agate, 
  or 
  more 
  or 
  less 
  crystalline 
  silica. 
  In 
  some 
  

   specimens 
  a 
  simple 
  dimple 
  in 
  the 
  siliceous 
  mass 
  indicates 
  a 
  former 
  

   corallite. 
  

  

  Variety 
  4. 
  — 
  The 
  walls, 
  septa, 
  and 
  endotheca 
  are 
  formed 
  of 
  

   rough 
  granular 
  silica, 
  and 
  all 
  the 
  minuter 
  details 
  are 
  absent, 
  whilst 
  

   the 
  endothecal 
  structures 
  are 
  greatly 
  deficient. 
  The 
  endothecal 
  

   spaces 
  are 
  filled 
  with 
  homogeneous 
  flint 
  and 
  granular 
  silica. 
  

  

  In 
  a 
  very 
  well-marked 
  example 
  of 
  the 
  third 
  variety 
  of 
  this 
  form 
  

   of 
  silicification, 
  the 
  destruction 
  of 
  the 
  corallites 
  and 
  their 
  replace- 
  

   ment 
  by 
  semitransparent 
  chalcedony 
  cannot 
  be 
  accounted 
  for 
  by 
  

   the 
  prefossil 
  loss 
  of 
  details 
  ; 
  for 
  the 
  silicification 
  has 
  so 
  altered 
  the 
  

   original 
  substance 
  of 
  the 
  corallum, 
  that 
  the 
  corallites 
  can 
  only 
  be 
  re- 
  

   cognized 
  as 
  dimple-shaped 
  depressions 
  in 
  the 
  siliceous 
  mass. 
  In 
  one 
  

  

  * 
  Astrcsa 
  ccllulosa, 
  nobis. 
  

  

  