﻿1864.] 
  DUNCAN 
  WEST 
  INDIAN 
  COKALS. 
  373 
  

  

  It 
  is 
  singular, 
  and 
  as 
  yet 
  not 
  explained, 
  that 
  the 
  epitheca 
  or 
  

   general 
  envelope 
  of 
  many, 
  but 
  not 
  all, 
  of 
  the 
  genera 
  is 
  wanting 
  

   in 
  the 
  fossil 
  specimens. 
  Doubtless 
  many 
  of 
  the 
  siliceous 
  masses 
  in 
  

   which 
  corallites 
  can 
  be 
  detected 
  have 
  been 
  rolled 
  since 
  their 
  fossili- 
  

   zation, 
  but 
  there 
  are 
  no 
  evidences 
  of 
  epitheca 
  in 
  very 
  perfect 
  

   Astrseans 
  which 
  have 
  not 
  been 
  rolled. 
  Finally, 
  it 
  may 
  be 
  taken 
  as 
  

   a 
  rule 
  that 
  the 
  most 
  perfect 
  replacement 
  of 
  the 
  carbonate 
  of 
  lime 
  of 
  

   the 
  corals 
  by 
  silica 
  is 
  found 
  in 
  the 
  oldest 
  beds. 
  

  

  § 
  Y. 
  Conclusion. 
  

  

  I 
  believe 
  that 
  there 
  is 
  no 
  truth 
  in 
  the 
  hypothesis 
  which 
  asserts 
  that 
  

   the 
  silicification 
  of 
  the 
  West 
  Indian 
  fossil 
  corals 
  was 
  the 
  result 
  of 
  a 
  

   volcanic 
  outburst, 
  which 
  poured 
  siliceous 
  solutions 
  over 
  the 
  depressed 
  

   reefs 
  of 
  the 
  Miocene 
  age. 
  The 
  corals 
  of 
  the 
  Inclined 
  Beds 
  and 
  of 
  

   the 
  Chert 
  of 
  Antigua 
  are 
  silicified, 
  so 
  are 
  the 
  Woods 
  and 
  Mollusca 
  ; 
  

   but 
  the 
  lower 
  beds 
  of 
  the 
  Marl 
  contain 
  both 
  perfectly 
  silicified 
  

   Astraeans 
  and 
  calcareo-siliceous 
  corals 
  also, 
  whilst 
  the 
  upper 
  part 
  

   of 
  the 
  formation 
  yields 
  fossils 
  in 
  all 
  stages 
  of 
  siliceous 
  and 
  calcareous 
  

   mineralization. 
  The 
  neighbouring 
  Barbudan 
  limestone 
  has 
  no 
  sili- 
  

   ceous 
  corals, 
  yet 
  it 
  is 
  of 
  the 
  same 
  age 
  as 
  the 
  Antiguan 
  Marl. 
  The 
  San 
  

   Domingan 
  fossils 
  are 
  usually 
  calcareous, 
  but 
  here 
  and 
  there 
  siliceous 
  

   specimens 
  are 
  met 
  with 
  in 
  the 
  same 
  beds 
  with 
  the 
  others. 
  There 
  is 
  

   no 
  evidence 
  to 
  prove 
  that 
  coral-growth 
  ever 
  ceased 
  from 
  the 
  Miocene 
  

   time 
  to 
  the 
  present 
  in 
  the 
  Caribbean 
  Sea, 
  although 
  there 
  has 
  been 
  a 
  

   change 
  in 
  the 
  facies 
  of 
  the 
  fauna. 
  These 
  facts 
  and 
  remarks 
  are 
  very 
  

   antagonistic 
  to 
  the 
  hypothesis, 
  and 
  rather 
  tend 
  to 
  prove 
  that 
  the 
  

   silicification 
  of 
  the 
  corals 
  has 
  been 
  a 
  slow 
  process, 
  which 
  has 
  had 
  no 
  

   other 
  origin 
  than 
  in 
  those 
  chemical 
  operations 
  which 
  are 
  still 
  in 
  

   action, 
  and 
  that 
  their 
  greater 
  or 
  less 
  intensity 
  in 
  certain 
  favourable 
  

   localities 
  has 
  produced 
  siliceous 
  fossils 
  amongst 
  those 
  affected 
  by 
  the 
  

   calcareous 
  form 
  of 
  mineralization. 
  Wherever 
  a 
  highly 
  aerated 
  sea, 
  

   containing 
  silica 
  in 
  solution, 
  acts 
  on 
  calcareous 
  fossils 
  at 
  consider- 
  

   able 
  depth, 
  and, 
  therefore, 
  under 
  considerable 
  pressure, 
  there 
  would 
  

   appear 
  to 
  result 
  a 
  chemical 
  transposition 
  ; 
  and 
  the 
  presence 
  of 
  

   crystals 
  of 
  quartz, 
  of 
  homogeneous 
  flint, 
  of 
  the 
  hydrates 
  of 
  silica, 
  

   and 
  their 
  coloured 
  varieties 
  is 
  due 
  to 
  chemical 
  influences 
  which 
  

   bear 
  a 
  relation 
  to 
  the 
  geological 
  changes 
  in 
  and 
  about 
  the 
  reefs. 
  

   It 
  must 
  be 
  remembered 
  that 
  there 
  is 
  a 
  small 
  amount 
  of 
  silica 
  in 
  

   corals 
  ; 
  and 
  it 
  will 
  be 
  observed 
  in 
  microscopic 
  sections 
  of 
  An- 
  

   tiguan 
  corals, 
  in 
  which 
  silicification 
  is 
  incomplete, 
  that 
  the 
  silica 
  is 
  

   usually 
  deposited 
  in 
  molecules 
  in 
  the 
  centre 
  of 
  the 
  calcareous 
  mass, 
  and 
  

   not 
  on 
  its 
  superficies. 
  The 
  process 
  of 
  siliceous 
  transposition 
  is 
  doubt- 
  

   less 
  very 
  slow 
  ; 
  it 
  is 
  not 
  always 
  perfect, 
  for 
  the 
  silica 
  would 
  appear 
  

   to 
  have 
  an 
  affinity 
  for 
  bodies 
  formerly 
  organized, 
  and 
  often 
  to 
  destroy 
  

   the 
  former 
  tissues. 
  Thus 
  there 
  is 
  abundant 
  carbonate 
  of 
  lime 
  in 
  

   which 
  transposition 
  is 
  not 
  going 
  on 
  around 
  the 
  siliceous 
  fossils, 
  and 
  

   the 
  persistence 
  of 
  some 
  animal 
  or 
  vegetable 
  organized 
  tissue, 
  decom- 
  

   posing 
  more 
  or 
  less 
  slowly, 
  appears 
  to 
  determine 
  the 
  presence 
  of 
  

   certain 
  forms 
  of 
  silica 
  ; 
  moreover, 
  the 
  details 
  of 
  corals 
  are 
  often 
  so 
  

  

  