﻿480 
  PKOF. 
  J. 
  W. 
  JTJDD 
  ON 
  THE 
  EOCENE 
  AND 
  OLIGOCENE 
  

  

  greatly 
  predominating 
  over 
  the 
  marine, 
  and 
  bands 
  of 
  lacustrine 
  marl 
  

   differing 
  in 
  no 
  respect 
  from 
  that 
  of 
  the 
  upper 
  and 
  lower 
  groups 
  "*. 
  

  

  That 
  brackish-water 
  shells 
  occur 
  in 
  certain 
  bands 
  in 
  the 
  strata 
  

   usually 
  ranked 
  as 
  the 
  Upper 
  and 
  Lower 
  Headon, 
  may 
  be 
  seen 
  by 
  

   an 
  inspection 
  of 
  the 
  sections 
  of 
  Dr. 
  Wright, 
  Prof. 
  Prestwich, 
  the 
  

   Geological 
  Survey, 
  and 
  Prof. 
  Blake 
  ; 
  and 
  these 
  brackish-water 
  bands 
  

   occasionally 
  contain 
  Cytherea 
  incrassata. 
  On 
  the 
  other 
  hand 
  the 
  

   central 
  portions 
  of 
  the 
  Headon 
  series 
  contain 
  so 
  few 
  purely 
  marine 
  

   forms, 
  and 
  such 
  as 
  do 
  occur 
  are 
  in 
  such 
  a 
  dwarfed 
  condition, 
  that 
  

   Mr. 
  G. 
  B. 
  Sowerby 
  strongly 
  insisted 
  on 
  the 
  inaccuracy 
  of 
  styling 
  it 
  

   a 
  marine 
  formation: 
  while 
  Sedgwick, 
  Forbes, 
  and 
  nearly 
  all 
  other 
  

   observers 
  have 
  admitted 
  that 
  nowhere 
  in 
  Headon 
  Hill 
  do 
  we 
  find 
  

   marine 
  conditions 
  so 
  strongly 
  marked 
  as 
  at 
  Col 
  well 
  Bay. 
  

  

  The 
  fact 
  is, 
  as 
  I 
  have 
  pointed 
  out 
  in 
  my 
  preceding 
  paper, 
  

   that 
  the 
  division 
  of 
  the 
  Headon 
  series 
  into 
  an 
  tipper 
  Freshwater, 
  a 
  

   Middle 
  Marine, 
  and 
  a 
  Lower 
  Freshwater 
  formation, 
  is 
  the 
  last 
  relic 
  

   of 
  a 
  classification 
  the 
  inapplicability 
  of 
  which 
  Sedgwick 
  had 
  begun 
  

   to 
  see 
  in 
  1S30, 
  and 
  which 
  was 
  rendered 
  quite 
  untenable 
  by 
  the 
  sub- 
  

   sequent 
  discoveries 
  of 
  Prestwich 
  and 
  Forbes. 
  The 
  limits 
  and 
  divi- 
  

   sions 
  of 
  this 
  supposed 
  Middle 
  Marine 
  Headon, 
  as 
  adopted 
  by 
  Messrs. 
  

   Keeping 
  and 
  Tawney, 
  are 
  quite 
  different 
  from 
  those 
  given 
  in 
  the 
  

   sections 
  of 
  the 
  Geological 
  Survey 
  : 
  and 
  while 
  the 
  former 
  authors 
  

   include 
  in 
  it 
  only 
  from 
  31 
  to 
  33 
  feet 
  of 
  strata, 
  Prof. 
  Blake 
  finds 
  

   the 
  brackish-water 
  beds 
  intercalated 
  with 
  about 
  45 
  feet 
  of 
  strata. 
  

   A 
  careful 
  study 
  of 
  the 
  published 
  sections 
  long 
  ago 
  convinced 
  me 
  

   that 
  different 
  authors 
  had 
  referred 
  to 
  different 
  parts 
  of 
  the 
  Headon 
  

   group 
  in 
  describing 
  the 
  Marine 
  or 
  Middle 
  Headon, 
  and 
  that 
  no 
  part 
  

   of 
  the 
  series 
  can 
  be 
  justly 
  separated 
  under 
  that 
  appellation. 
  

  

  The 
  great 
  difficulty 
  which 
  all 
  authors 
  have 
  found 
  is 
  to 
  correlate 
  

   the 
  strikingly 
  different 
  sections 
  exposed 
  in 
  Headon 
  Hill 
  and 
  Colwell 
  

   Bay. 
  

  

  The 
  method 
  which 
  was 
  adopted 
  by 
  Dr. 
  "Wright, 
  Mr. 
  Bristow 
  (in 
  

   one 
  of 
  his 
  sections 
  f 
  ), 
  and 
  myself, 
  was 
  to 
  select 
  what 
  appeared 
  to 
  be 
  

   the 
  most 
  constant 
  series 
  of 
  beds 
  in 
  the 
  two 
  sections, 
  and 
  to 
  bring 
  

   them 
  into 
  comparison. 
  

  

  All 
  through 
  Headon 
  Hill 
  there 
  can 
  be 
  traced 
  a 
  great 
  mass 
  of 
  

   sandstones 
  and 
  limestones 
  having 
  a 
  united 
  thickness 
  . 
  of 
  over 
  40 
  feet. 
  

   It 
  is 
  true 
  that, 
  when 
  examined 
  in 
  detail, 
  this 
  series 
  of 
  calcareo-are- 
  

   naccous 
  beds 
  exhibits 
  many 
  variations 
  ; 
  the 
  thick 
  masses 
  of 
  lime- 
  

   stone 
  which 
  are 
  seen 
  at 
  some 
  points 
  are 
  at 
  others 
  broken 
  up 
  into 
  

   several 
  thinner 
  ones 
  by 
  intervening 
  sands 
  or 
  clays 
  ; 
  and 
  in 
  some 
  places 
  

   the 
  limestone 
  contains 
  fewer 
  shells, 
  and 
  passes 
  into 
  a 
  sandstone 
  

   rock. 
  But, 
  in 
  spite 
  of 
  its 
  minor 
  variations 
  in 
  composition, 
  this 
  mass 
  of 
  

   strata 
  maintains 
  its 
  thickness 
  and 
  general 
  characters 
  for 
  a 
  distance 
  of 
  

   more 
  than 
  a 
  mile, 
  and 
  forms 
  the 
  most 
  conspicuous 
  feature 
  in 
  the 
  

  

  * 
  Proc. 
  Geol. 
  Soc. 
  vol. 
  ii. 
  p. 
  294. 
  

  

  t 
  The 
  Tertiary 
  Fluvio-marine 
  formation 
  of 
  the 
  Isle 
  of 
  Wight 
  (1856), 
  p. 
  132 
  ; 
  

   The 
  Geology 
  of 
  the 
  Isle 
  of 
  Wight 
  (1862), 
  p. 
  62. 
  In 
  the 
  table 
  of 
  Vertical 
  

   Section 
  No. 
  4, 
  the 
  difficulty 
  in 
  identifying 
  the 
  beds 
  of 
  Colwell 
  Bay 
  and 
  Headon 
  

   Hill 
  has 
  been 
  attempted 
  to 
  be 
  met 
  in 
  another 
  way 
  still. 
  

  

  