﻿370 
  PROCEEDINGS 
  OF 
  THE 
  GEOLOGICAL 
  SOCIETY. 
  [^-P 
  r 
  - 
  27, 
  

  

  three 
  varieties, 
  owing 
  to 
  the 
  intensity 
  of 
  the 
  siliceous 
  change 
  being- 
  

   most 
  decided 
  both 
  at 
  the 
  calicular 
  surface 
  of 
  the 
  corallum 
  and 
  in 
  

   the 
  centre 
  of 
  the 
  deeply 
  seated 
  corallites. 
  There 
  are 
  examples 
  of 
  

   large 
  Astraeans 
  in 
  which 
  only 
  the 
  original 
  sclerenchyma 
  is 
  silicified, 
  

   the 
  calcareous 
  and 
  infiltrated 
  salts 
  of 
  the 
  interspaces 
  still 
  existing 
  : 
  

   in 
  others 
  the 
  silicification 
  of 
  these 
  last 
  is 
  more 
  or 
  less 
  decided, 
  and, 
  

   in 
  a 
  specimen 
  in 
  the 
  Museum 
  of 
  Practical 
  Geology, 
  perfect 
  general 
  

   silicification 
  has 
  occurred. 
  The 
  tendency 
  of 
  every 
  grade 
  of 
  calcareo- 
  

   siliceous 
  mineralization 
  is 
  towards 
  perfect 
  silicification 
  ; 
  and, 
  as 
  this 
  

   is 
  more 
  or 
  less 
  attained, 
  the 
  destruction 
  of 
  the 
  minute 
  details 
  of 
  the 
  

   coral 
  is 
  very 
  constantly 
  noticed. 
  

  

  The 
  nature 
  of 
  these 
  forms 
  of 
  mineralization 
  is 
  best 
  seen 
  in 
  trans- 
  

   verse 
  section 
  of 
  large 
  corals. 
  The 
  septa 
  are 
  white 
  and 
  saccharoid- 
  

   looking 
  in 
  some 
  corallites, 
  where 
  much 
  carbonate 
  of 
  lime 
  still 
  

   exists*, 
  and 
  dark 
  granules 
  in 
  the 
  septal 
  structures 
  indicate 
  the 
  

   presence 
  of 
  silica; 
  in 
  other 
  corallites 
  the 
  hard 
  parts 
  are 
  of 
  black 
  

   ordinary 
  flint, 
  the 
  interspaces 
  being 
  filled 
  with 
  light- 
  coloured 
  car- 
  

   bonate 
  of 
  lime 
  and 
  with 
  granules 
  of 
  dark 
  silicaf, 
  whilst 
  in 
  some 
  

   perfect 
  corals 
  the 
  reverse 
  of 
  the 
  last 
  case 
  is 
  seen 
  — 
  the 
  septa 
  being 
  

   grey, 
  and 
  the 
  interspaces 
  black. 
  

  

  Between 
  these 
  conditions 
  there 
  are 
  several 
  which 
  are 
  determined 
  

   by 
  the 
  greater 
  or 
  less 
  amount 
  of 
  silica 
  in 
  the 
  sclerenchyma, 
  and 
  its 
  

   peculiar 
  colour. 
  

  

  In 
  some 
  specimens 
  the 
  opaque-red, 
  orange, 
  opal, 
  and 
  black 
  colours 
  

   of 
  the 
  silica 
  of 
  the 
  septa 
  are 
  rendered 
  most 
  prominent 
  by 
  the 
  grey 
  

   of 
  the 
  interspaces, 
  whilst 
  in 
  others 
  the 
  similarity 
  of 
  colour 
  between 
  

   the 
  interseptal 
  loculi 
  and 
  the 
  septa 
  is 
  very 
  confusing. 
  

  

  In 
  a 
  few 
  instances 
  both 
  the 
  original 
  hard 
  parts 
  and 
  the 
  inter- 
  

   spaces 
  are 
  of 
  the 
  same 
  yellow-ochre 
  tint, 
  and 
  the 
  details 
  are 
  per- 
  

   fectly 
  undistinguishable 
  before 
  nitric 
  acid 
  is 
  used 
  to 
  clear 
  away 
  the 
  

   excess 
  of 
  the 
  remaining 
  carbonate 
  of 
  lime. 
  

  

  The 
  silicification 
  of 
  the 
  original 
  hard 
  parts 
  is 
  always 
  in 
  advance 
  

   of 
  that 
  of 
  the 
  salts 
  of 
  the 
  interspaces 
  in 
  the 
  central 
  parts 
  of 
  a 
  

   corallum 
  ; 
  and 
  it 
  can 
  be 
  seen 
  that 
  the 
  silicification 
  commences 
  and 
  

   radiates 
  from 
  the 
  central 
  parts 
  of 
  septa, 
  and 
  from 
  the 
  midst 
  of 
  the 
  

   infiltrated 
  salts. 
  

  

  The 
  employment 
  of 
  nitric 
  acid 
  is, 
  therefore, 
  safe 
  ; 
  for 
  the 
  inter- 
  

   spaces 
  are 
  more 
  or 
  less 
  cleared 
  by 
  it, 
  except 
  when 
  their 
  contents 
  are 
  

   perfectly 
  silicified. 
  

  

  7. 
  Calcareo 
  -siliceous 
  and 
  Destructive. 
  Variety 
  1. 
  — 
  The 
  scleren- 
  

   chyma 
  resembles 
  the 
  condition 
  of 
  the 
  various 
  grades 
  of 
  the 
  third 
  

   variety 
  of 
  the 
  last 
  form 
  of 
  mineralization 
  ; 
  but 
  the 
  minuter 
  struc- 
  

   tural 
  details 
  are 
  absent, 
  and 
  the 
  septa 
  are 
  thin. 
  The 
  interspaces 
  

   are 
  filled 
  as 
  in 
  the 
  third 
  variety 
  just 
  mentioned, 
  but 
  they 
  occupy 
  a 
  

   little 
  more 
  space. 
  

  

  Variety 
  2. 
  — 
  The 
  sclerenchyma 
  is 
  much 
  thinner 
  ; 
  the 
  walls 
  and 
  

   exotheca 
  are 
  often 
  wanting 
  ; 
  the 
  mineral 
  is 
  white 
  in 
  colour, 
  and 
  is 
  

   nearly 
  pure 
  silica. 
  The 
  interspaces 
  are 
  filled 
  with 
  carbonate 
  of 
  

  

  * 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xix. 
  pi. 
  xiii. 
  figs, 
  la, 
  2a. 
  

   t 
  Ibid. 
  figs. 
  3, 
  4, 
  6. 
  

  

  