﻿Yol. 
  53.] 
  OF 
  THE 
  CHALK 
  ROCK. 
  399 
  

  

  rare 
  or 
  wholly 
  absent 
  from 
  pelagic 
  deposits 
  situated 
  towards 
  the 
  

  

  centres 
  of 
  great 
  ocean 
  basins 
  It 
  is 
  also 
  present 
  in 
  

  

  samples 
  of 
  Globigerina-ooze 
  situated 
  at 
  no 
  great 
  dis- 
  

   tance 
  from 
  the 
  continents.' 
  'Glauconite 
  may 
  therefore 
  be 
  

   regarded 
  as 
  having 
  been 
  formed 
  in 
  deep 
  water 
  not 
  far 
  from 
  the 
  

   coasts, 
  or 
  in 
  shallow 
  water 
  where 
  no 
  large 
  quantity 
  of 
  continental 
  

   debris 
  was 
  deposited.' 
  Further, 
  at 
  the 
  present 
  day 
  glauconite 
  is 
  

   generally 
  associated 
  with 
  phosphate 
  of 
  lime 
  ; 
  this 
  is 
  what 
  we 
  find 
  

   too 
  in 
  the 
  Chalk 
  Rock 
  — 
  it 
  is 
  a 
  nodular 
  phosphatic 
  deposit, 
  containing 
  

   glauconite. 
  

  

  The 
  rarity 
  or 
  absence 
  of 
  aragonite-organisms, 
  even 
  in 
  the 
  form 
  

   of 
  casts, 
  in 
  the 
  Upper 
  Chalk, 
  is 
  considered 
  by 
  Mr. 
  P. 
  F. 
  Kendall 
  ! 
  

   to 
  be 
  due 
  to 
  the 
  fact 
  that 
  they 
  were, 
  like 
  the 
  shells 
  of 
  pteropods, 
  

   which 
  form 
  the 
  main 
  part 
  of 
  the 
  modern 
  Pteropod-ooze, 
  dissolved 
  

   at 
  a 
  particular 
  depth 
  — 
  not 
  less 
  than 
  1 
  £00 
  fathoms 
  ; 
  for 
  if 
  these 
  

   organisms 
  had 
  remained 
  for 
  a 
  short 
  time 
  on 
  the 
  floor 
  of 
  the 
  Chalk 
  

   sea, 
  some 
  of 
  them 
  would 
  have 
  left 
  traces 
  of 
  their 
  existence 
  in 
  the 
  

   form 
  of 
  casts, 
  since 
  the 
  soft 
  Chalk-ooze 
  would 
  easily 
  have 
  filled 
  up 
  

   the 
  interiors 
  of 
  the 
  shells. 
  If 
  this 
  explanation 
  be 
  correct, 
  then 
  I 
  

   think 
  we 
  may 
  safely 
  take 
  the 
  abundance 
  of 
  casts 
  of 
  aragonite- 
  

   organisms 
  (ammonoids, 
  Nautilus, 
  etc.) 
  in 
  the 
  Meussianmn-zone 
  as 
  

   an 
  indication 
  that 
  it 
  was 
  formed 
  in 
  water 
  of 
  less 
  depth 
  than 
  the 
  

   Upper 
  Chalk, 
  since 
  the 
  aragonite 
  shells 
  must 
  have 
  become 
  covered 
  up 
  

   with 
  sediment 
  and 
  were 
  not 
  dissolved 
  until 
  some 
  time 
  afterwards. 
  

  

  We 
  can 
  scarcely 
  doubt 
  that 
  the 
  Chalk 
  Eock 
  was 
  formed 
  beyond 
  

   the 
  limits 
  of 
  terrigenous 
  deposits, 
  and 
  therefore 
  presumably 
  at 
  a 
  

   depth 
  of 
  greater 
  than 
  about 
  100 
  fathoms. 
  Both 
  paleeontological 
  

   and 
  lithological 
  evidence 
  are 
  entirely 
  in 
  favour 
  of 
  this. 
  The 
  

   Benthos 
  within 
  the 
  100-fathom 
  line 
  would 
  have 
  been 
  much 
  richer 
  

   in 
  species 
  than 
  is 
  our 
  Reussianum-zone 
  ; 
  just 
  as 
  the 
  fauna 
  of 
  the 
  

   Planer-Kalk 
  of 
  Strehlen 
  — 
  particularly 
  the 
  gasteropoda 
  and 
  lamelli- 
  

   branchia 
  — 
  is 
  more 
  varied 
  than 
  that 
  of 
  the 
  Chalk 
  Hock. 
  If 
  we 
  

   remember 
  the 
  extremely 
  minute 
  quantity 
  of 
  detrital 
  material 
  which 
  

   is 
  present 
  in 
  the 
  Chalk 
  Eock, 
  and 
  if 
  we 
  compare 
  a 
  section 
  of 
  that 
  

   rock 
  with 
  sections 
  of 
  some 
  samples 
  of 
  hardened 
  Globigerina-ooze, 
  

   we 
  can 
  hardly 
  doubt 
  that 
  the 
  two 
  deposits 
  have 
  been 
  formed 
  in 
  the 
  

   same 
  way. 
  

  

  That 
  the 
  Reussianum-zone 
  was 
  not 
  laid 
  down 
  in 
  very 
  deep 
  water 
  

   is 
  shown 
  by 
  the 
  character 
  of 
  its 
  fauna 
  ; 
  not 
  only 
  does 
  it 
  contain 
  a 
  

   considerable 
  variety 
  of 
  forms, 
  but 
  the 
  number 
  of 
  individuals 
  of 
  any 
  

   species 
  is, 
  even 
  in 
  the 
  same 
  block 
  of 
  stone, 
  often 
  very 
  large. 
  This 
  

   is 
  contrary 
  to 
  what 
  is 
  met 
  with 
  in 
  abyssal 
  regions 
  at 
  the 
  present 
  

   day 
  ; 
  thus 
  in 
  the 
  deep-sea 
  Benthos 
  the 
  number 
  of 
  specimens 
  of 
  a 
  

   species, 
  as 
  well 
  as 
  the 
  number 
  of 
  species, 
  decreases 
  with 
  increasing 
  

   depth, 
  and, 
  apart 
  from 
  depth, 
  a 
  similar 
  diminution 
  in 
  numbers 
  is 
  

   noticed 
  in 
  tropical 
  and 
  subtropical 
  regions 
  as 
  the 
  distance 
  from 
  the 
  

   shore 
  becomes 
  greater. 
  Another 
  point 
  is 
  the 
  large 
  number 
  of 
  

   gasteropods 
  with 
  thick 
  shells, 
  often 
  of 
  large 
  size 
  (such 
  as 
  Turbo, 
  

   Pleurotomaria), 
  in 
  the 
  Reussianum-zone 
  ; 
  at 
  the 
  present 
  day 
  most 
  

   1 
  Eep. 
  Brit. 
  Assoc. 
  (Liverpool) 
  1896, 
  p. 
  791. 
  

  

  