﻿lviil 
  PKOCEEDINGS 
  OF 
  THE 
  GEOLOGICAL 
  SOCIETY. 
  

  

  The 
  general 
  result 
  of 
  these 
  numbers 
  is, 
  that 
  of 
  280 
  species 
  of 
  all 
  

   kinds 
  known 
  in 
  the 
  Lower 
  Greensand, 
  233 
  are 
  peculiar, 
  and 
  51, 
  or 
  

   only 
  about 
  18 
  per 
  cent., 
  pass 
  from 
  the 
  Lower 
  to 
  the 
  Upper 
  Cretaceous 
  

   strata. 
  It 
  is 
  important 
  to 
  observe 
  that 
  this 
  break 
  and 
  disappearance 
  

   of 
  so 
  many 
  species 
  in 
  succession 
  is 
  accompanied 
  by 
  a 
  stratigraphical 
  

   break 
  as 
  well 
  j 
  for, 
  round 
  the 
  Weald, 
  it 
  is 
  known 
  that 
  in 
  some 
  of 
  the 
  

   very 
  few 
  exposures 
  of 
  junctions, 
  the 
  Gault 
  has 
  been 
  seen 
  lying 
  on 
  

   eroded 
  surfaces 
  of 
  Lower 
  Greensand, 
  while 
  in 
  the 
  western 
  and 
  middle 
  

   parts 
  of 
  England, 
  on 
  the 
  west 
  and 
  north 
  of 
  the 
  great 
  Chalk 
  escarp- 
  

   ment, 
  the 
  frequent 
  and 
  sudden 
  overlaps 
  of 
  the 
  Lower 
  Greensand 
  by 
  

   the 
  Gault 
  leave 
  no 
  doubt 
  that 
  the 
  upper 
  formation 
  lies 
  actually 
  un- 
  

   conformably 
  on 
  the 
  lower, 
  and 
  the 
  time 
  occupied 
  by 
  the 
  denudation 
  

   has 
  been 
  with 
  us 
  unrepresented 
  by 
  any 
  stratified 
  formation. 
  

  

  I 
  must 
  refer 
  to 
  Table 
  V. 
  for 
  the 
  proportions 
  of 
  species 
  common 
  to 
  

   the 
  Gault, 
  Upper 
  Greensand, 
  Chalk 
  Marl, 
  Lower 
  Chalk, 
  and 
  Upper 
  

   Chalk; 
  and 
  an 
  attentive 
  consideration 
  of 
  the 
  numbers 
  will 
  show, 
  

   what 
  is 
  well 
  known, 
  that 
  between 
  any 
  of 
  these 
  subformations 
  there 
  

   is 
  no 
  approach 
  to 
  a 
  break 
  so 
  great 
  as 
  that 
  between 
  the 
  Lower 
  Green- 
  

   sand 
  and 
  the 
  Upper 
  Cretaceous 
  series, 
  whether 
  we 
  take 
  the 
  inferred 
  

   or 
  the 
  known 
  species. 
  This 
  fact 
  is 
  further 
  accompanied 
  by 
  the 
  cir- 
  

   cumstance 
  that, 
  comparatively 
  marked 
  as 
  the 
  lithological 
  distinctions 
  

   of 
  the 
  Upper 
  Cretaceous 
  subformations 
  are, 
  it 
  is 
  over 
  great 
  part 
  of 
  

   England 
  impossible 
  to 
  draw 
  a 
  precise 
  line 
  of 
  demarcation 
  between 
  

   them. 
  Thus, 
  in 
  places 
  surrounding 
  the 
  Wealden 
  area 
  there 
  is 
  appa- 
  

   rently 
  a 
  gradual 
  passage 
  from 
  Gault 
  to 
  Upper 
  Greensand, 
  for 
  the 
  

   Gault 
  becomes 
  pale 
  and 
  white 
  above, 
  and 
  the 
  Upper 
  Greensand 
  white 
  

   and 
  marly 
  below 
  ; 
  and 
  in 
  Buckinghamshire, 
  Bedfordshire, 
  and 
  Hert- 
  

   fordshire 
  it 
  is 
  difficult 
  to 
  draw 
  a 
  line 
  of 
  precise 
  value 
  between 
  the 
  

   top 
  of 
  the 
  Upper 
  Greensand 
  and 
  the 
  base 
  of 
  the 
  Chalk, 
  for 
  both 
  are 
  

   equally 
  white 
  and 
  the 
  greensand 
  is 
  exceedingly 
  calcareous. 
  The 
  

   Chalk-marl 
  is 
  a 
  local 
  variation 
  ; 
  and 
  though 
  Mr. 
  Whitaker 
  has, 
  over 
  

   a 
  considerable 
  area, 
  denned 
  a 
  line 
  between 
  Lower 
  and 
  Upper 
  Chalk 
  

   on 
  physical 
  grounds, 
  it 
  has 
  not 
  yet 
  been 
  universally 
  proved 
  for 
  Eng- 
  

   land. 
  Both 
  in 
  a 
  physical 
  and, 
  partly, 
  in 
  a 
  palaeontological 
  sense, 
  

   indeed, 
  the 
  whole 
  of 
  the 
  Upper 
  Cretaceous 
  divisions 
  may 
  almost 
  be 
  

   spoken 
  of 
  as 
  making 
  one 
  formation 
  lithologically 
  subdivided. 
  

  

  The 
  more 
  prominent 
  relations 
  of 
  the 
  fossils 
  in 
  these 
  subforma- 
  

   tions 
  are 
  as 
  follows 
  : 
  — 
  Of 
  Sponges, 
  Eoraminifera, 
  Corals, 
  Echinoder- 
  

   mata, 
  Articulata, 
  and 
  Polyzoa, 
  large 
  proportions 
  as 
  a 
  rule 
  pass 
  up- 
  

   wards 
  and 
  downwards. 
  

  

  Of 
  12 
  species 
  of 
  inferred 
  and 
  6 
  of 
  known 
  Brachiopoda, 
  none 
  are 
  

   peculiar 
  to 
  the 
  Gault 
  ; 
  of 
  26 
  inferred 
  and 
  23 
  known 
  species, 
  only 
  3 
  

   are 
  peculiar 
  to 
  the 
  Upper 
  Greensand 
  ; 
  of 
  23 
  inferred 
  and 
  15 
  known 
  

   forms, 
  none 
  are 
  peculiar 
  to 
  the 
  Chalk 
  Marl 
  ; 
  and 
  of 
  25 
  inferred 
  and 
  

   21 
  known 
  species, 
  only 
  3 
  are 
  peculiar 
  to 
  the 
  Lower 
  Chalk. 
  

  

  Of 
  20 
  inferred 
  and 
  15 
  known 
  Monomyarian 
  Mollusks 
  in 
  the 
  Gault, 
  

   only 
  2 
  are 
  confined 
  to 
  that 
  formation 
  ; 
  and 
  while, 
  out 
  of 
  60 
  known 
  

   species 
  in 
  the 
  Upper 
  Greensand, 
  38 
  are 
  peculiar 
  to 
  it, 
  not 
  one 
  known 
  

   species 
  is 
  peculiar 
  to 
  the 
  Chalk- 
  marl 
  ; 
  and 
  out 
  of 
  40 
  inferred 
  and 
  

   35 
  known 
  forms, 
  only 
  14 
  are 
  peculiar 
  to 
  the 
  Lower 
  Chalk. 
  

  

  