﻿Vol. 
  53-] 
  THE 
  CRETACEOUS 
  STRATA. 
  OF 
  COUNTY 
  ANTRIM. 
  603 
  

  

  England, 
  mention 
  may 
  be 
  made 
  of 
  the 
  Plocoscyphia 
  mceanch-ina- 
  

   layer, 
  lying 
  immediately 
  above 
  the 
  Chloritic 
  Marl 
  with 
  its 
  rolled 
  

   phosphatic 
  nodules. 
  With 
  this 
  exception, 
  throughout 
  the 
  whole 
  of 
  

   the 
  Cenomanian 
  and 
  Turonian 
  periods, 
  no 
  such 
  colonial 
  assemblage 
  

   is 
  again 
  met 
  with, 
  until, 
  at 
  the 
  very 
  summit 
  of 
  the 
  strata 
  of 
  the 
  

   latter 
  age, 
  the 
  Chalk 
  Rock 
  displays 
  evidence 
  of 
  partial 
  elevation. 
  

   Analyses 
  of 
  the 
  lower 
  strata 
  generally 
  show 
  no 
  trace 
  of 
  glauconitic 
  

   casts 
  of 
  sponge-spicules, 
  yet 
  the 
  Chalk 
  Rock 
  and 
  Holaster 
  planus- 
  

   beds 
  of 
  Devon, 
  Eastbourne, 
  and 
  the 
  Midlands, 
  have 
  without 
  

   exception 
  yielded 
  these 
  as 
  the 
  most 
  important 
  members 
  of 
  the 
  

   residue. 
  It 
  is 
  very 
  interesting 
  to 
  find 
  that 
  precisely 
  the 
  same 
  fact 
  

   holds 
  good 
  for 
  the 
  Inoceramus-zone 
  in 
  Ireland, 
  which 
  I 
  have 
  regarded 
  

   as 
  partly 
  synchronous 
  with 
  the 
  Chalk 
  Rock 
  of 
  England. 
  In 
  Antrim 
  

   the 
  abundance 
  of 
  these 
  glauconitic 
  casts 
  of 
  spicules 
  is 
  noticeable 
  so 
  

   long 
  as 
  the 
  limestones 
  contain 
  small 
  fragments 
  of 
  quartz 
  and 
  other 
  

   detrital 
  materials, 
  but 
  the 
  sponges 
  attain 
  their 
  maximum 
  develop- 
  

   ment 
  (forming 
  definite 
  bands) 
  at 
  the 
  point 
  where 
  the 
  detrital 
  

   minerals 
  become 
  rare, 
  and 
  pure 
  white 
  limestone 
  is 
  commencing 
  to 
  be 
  

   formed. 
  In 
  other 
  words, 
  the 
  most 
  favourable 
  locality 
  for 
  the 
  forma- 
  

   tion 
  of 
  a 
  sponge-bed 
  appears 
  to 
  be 
  that 
  one 
  where 
  currents 
  are 
  

   carrying 
  only 
  the 
  very 
  finest 
  particles 
  in 
  suspension, 
  the 
  sediment 
  

   forming 
  on 
  the 
  ocean-floor 
  being 
  almost 
  purely 
  calcareous. 
  But 
  

   although 
  definitely 
  recognizable 
  sponge-beds 
  cease 
  at 
  this 
  level 
  (the 
  

   base 
  of 
  the 
  White 
  Chalk), 
  the 
  continued 
  presence 
  of 
  these 
  organisms 
  

   in 
  the 
  chalk 
  is 
  evidenced 
  by 
  the 
  abundance 
  of 
  flints. 
  

  

  If 
  any 
  one 
  be 
  yet 
  inclined 
  to 
  dispute 
  the 
  connexion 
  between 
  flints 
  

   and 
  sponges, 
  I 
  would 
  call 
  special 
  attention 
  to 
  a 
  memoir 
  by 
  Prof. 
  

   Sollas, 
  1 
  which 
  had 
  not 
  come 
  under 
  my 
  notice 
  when 
  discussing 
  

   these 
  questions 
  in 
  previous 
  papers. 
  He 
  remarks 
  (op. 
  cit. 
  p. 
  438) 
  : 
  — 
  ■ 
  

   ' 
  The 
  Trimingham 
  flints 
  afford 
  evidence 
  straight 
  to 
  the 
  point 
  ; 
  

   for 
  not 
  only 
  are 
  sponge-spicules 
  intimately 
  associated 
  with 
  them 
  and 
  

   in 
  great 
  numbers, 
  but 
  these 
  spicules 
  afford 
  us 
  clear 
  proof 
  of 
  the 
  

   previous 
  existence 
  of 
  a 
  great 
  mass 
  of 
  other 
  spicules 
  of 
  which 
  they 
  

   are 
  themselves 
  but 
  a 
  miserable 
  remnant.' 
  Considering 
  the 
  aggre- 
  

   gation 
  of 
  flints 
  in 
  layers, 
  he 
  concludes 
  that 
  drifting 
  has 
  not 
  occurred 
  

   to 
  any 
  great 
  extent, 
  if 
  present 
  being 
  only 
  sufficient 
  to 
  help 
  in 
  

   mixing 
  the 
  different 
  sponge-spicules 
  together 
  (for, 
  as 
  Sollas, 
  Hinde, 
  

   Wright, 
  and 
  others 
  have 
  shown, 
  spicules 
  of 
  many 
  genera 
  have 
  been 
  

   preserved 
  intermingled 
  in 
  flints), 
  but 
  not 
  to 
  sort 
  them 
  out 
  into 
  any 
  

   distinct 
  layer. 
  His 
  belief 
  is, 
  and 
  personally 
  I 
  am 
  inclined 
  to 
  adopt 
  

   the 
  conclusion, 
  that 
  the 
  area 
  where 
  the 
  spicules 
  are 
  now 
  found 
  was 
  

   once 
  a 
  sponge-bed 
  ; 
  with 
  death 
  and 
  dissolution 
  of 
  the 
  organisms 
  the 
  

   spicules 
  were 
  set 
  free 
  from 
  different 
  adjacent 
  sponges, 
  and, 
  falling 
  

   into 
  the 
  same 
  deposit, 
  naturally 
  mixed 
  together, 
  movements 
  of 
  the 
  

   surrounding 
  sea-water 
  helping 
  to 
  render 
  the 
  mixture 
  more 
  complete. 
  

   Thus 
  a 
  chalky 
  ooze 
  would 
  be 
  produced, 
  crammed 
  with 
  sponge-spicules 
  

   of 
  all 
  sorts 
  and 
  sizes. 
  Such 
  sponges 
  as 
  possessed 
  skeletons 
  compact 
  

   enough 
  to 
  maintain 
  their 
  general 
  form 
  after 
  death 
  would 
  become 
  

   filled 
  with 
  this 
  ooze, 
  and, 
  undergoing 
  silicification, 
  would 
  furnish 
  us 
  

  

  1 
  Ann. 
  & 
  Mag. 
  Nat. 
  Hist. 
  ser. 
  5, 
  vol. 
  vi. 
  (1880) 
  pp. 
  384-395, 
  437-461. 
  

  

  