﻿KEWTON 
  : 
  GEOLOGICAL 
  ZONES 
  AND 
  THEIR 
  MOLLIJSCA. 
  283 
  

  

  It 
  is 
  necessary 
  to 
  understand 
  exactly 
  the 
  meaning 
  of 
  the 
  term 
  ' 
  zone 
  ' 
  

   in 
  its 
  geological 
  application, 
  because 
  some 
  authors 
  have 
  regarded 
  it 
  as 
  

   a 
  group 
  of 
  "organic 
  remains 
  of 
  which 
  one 
  abundant 
  and 
  characteristic 
  

   form 
  is 
  chosen 
  as 
  an 
  index 
  ",' 
  in 
  which 
  sense 
  the 
  zone 
  would 
  be 
  of 
  

   purely 
  zoological 
  value 
  ; 
  whereas 
  it 
  would 
  appear 
  from 
  Dr. 
  Man's 
  - 
  

   interpretation 
  that 
  this 
  term 
  should 
  be 
  regarded 
  as 
  a 
  geological 
  factor, 
  

   and 
  might 
  better 
  be 
  explained 
  thus 
  : 
  " 
  Zones 
  are 
  belts 
  of 
  strata, 
  each 
  

   of 
  which 
  is 
  characterized 
  by 
  an 
  assemblage 
  of 
  organic 
  remains 
  of 
  

   which 
  one 
  abundant 
  and 
  characteristic 
  form 
  is 
  chosen 
  as 
  an 
  index." 
  

  

  Mr. 
  Jukes-Browne,^ 
  f'lUy 
  approving 
  of 
  Dr. 
  Marr's 
  views, 
  agreed 
  

   also 
  with 
  the 
  general 
  opinion 
  that 
  the 
  zonal 
  methods 
  employed 
  in 
  the 
  

   classification 
  of 
  the 
  sedimentary 
  rocks 
  were 
  merely 
  a 
  development 
  

   of 
  William 
  Smith's 
  original 
  idea 
  that 
  formations 
  could 
  be 
  determined 
  

   by 
  their 
  organic 
  contents 
  — 
  "Just 
  as 
  one 
  stage 
  is 
  identified 
  and 
  

   distinguished 
  from 
  others 
  by 
  the 
  assemblage 
  of 
  fossils 
  it 
  contains, 
  so 
  

   in 
  its 
  turn 
  is 
  the 
  zone 
  identified 
  by 
  its 
  fossils." 
  

  

  Zones 
  are 
  frequently 
  of 
  wide 
  distribution 
  and 
  vary 
  much 
  in 
  thickness. 
  

   According 
  to 
  Mr. 
  H. 
  B. 
  Woodward* 
  "the 
  zone 
  of 
  Ammonites 
  anmdatuH 
  

   in 
  Yorkshire 
  is 
  some 
  30 
  feet 
  thick, 
  although 
  represented 
  in 
  other 
  

   parts 
  of 
  England 
  by 
  the 
  thin 
  laj-er 
  known 
  in 
  iN^orthamptonshire 
  as 
  the 
  

   'Transition 
  Bed', 
  which 
  is 
  very 
  fossiliferous, 
  but 
  only 
  a 
  few 
  inches 
  

   thick, 
  such 
  a 
  phenomenon 
  forming 
  a 
  good 
  ilhjstration 
  of 
  the 
  manner 
  

   in 
  which 
  a 
  bed 
  may 
  become 
  attenuated 
  in 
  its 
  range 
  over 
  distant 
  

   geographical 
  areas". 
  

  

  Geological 
  zoning 
  has 
  mostly 
  been 
  attempted 
  among 
  the 
  marine 
  

   formations 
  of 
  Palaeozoic 
  and 
  Mesozoic 
  age, 
  whereas 
  the 
  Cain-ozoic 
  

   rocks, 
  although 
  often 
  analytically 
  classified 
  into 
  beds, 
  have 
  not 
  yet 
  

   received 
  the 
  same 
  treatment 
  of 
  zonal 
  classification 
  as 
  exemplified 
  in 
  

   the 
  older 
  stratified 
  def)osits. 
  

  

  For 
  reference, 
  in 
  connection 
  with 
  the 
  zones 
  which 
  will 
  be 
  now 
  

   briefly 
  described, 
  a 
  chart 
  of 
  the 
  sedimentary 
  formations 
  has 
  been 
  

   introduced, 
  which 
  it 
  is 
  hoped 
  may 
  be 
  of 
  assistance 
  to 
  the 
  student 
  in 
  

   comprehending 
  the 
  positions 
  of 
  the 
  various 
  geological 
  horizons 
  that 
  

   will 
  be 
  alluded 
  to. 
  

  

  PALiEozoic 
  Zones. 
  

   In 
  the 
  older 
  Palaeozoic 
  rocks 
  where 
  molluscan 
  remains 
  are 
  often 
  

   obscure 
  and 
  sometimes 
  difficult 
  to 
  determine, 
  the 
  index-fossils 
  of 
  the 
  

   zones 
  have 
  been 
  selected 
  from 
  among 
  the 
  more 
  prolific 
  organisms 
  of 
  

   other 
  groups, 
  such 
  as 
  the 
  Trilobites 
  {Olenellus, 
  etc), 
  the 
  Graptolites, 
  

   or 
  the 
  Brachiopoda. 
  Among 
  Continental 
  palaeontologists, 
  Barrande 
  * 
  

   may 
  be 
  quoted 
  as 
  having 
  published 
  important 
  results 
  in 
  connection 
  

   with 
  his 
  examination 
  of 
  the 
  Mollusca 
  and 
  other 
  invertebrates 
  found 
  in 
  

   the 
  Lower 
  Palaeozoic 
  deposits 
  of 
  Bohemia, 
  which, 
  although 
  not 
  quite 
  

  

  ^ 
  H. 
  B. 
  Woodward, 
  "On 
  Geological 
  Zones": 
  Proc. 
  Geol. 
  Assoc, 
  vol. 
  xii, 
  

   p. 
  298, 
  1892. 
  

  

  * 
  The 
  Principles 
  of 
  Stratigrai:>hical 
  Geology, 
  1898, 
  p. 
  68. 
  

   ^ 
  Geological 
  Magazine, 
  1899, 
  pp. 
  216-19. 
  

  

  * 
  " 
  On 
  Geological 
  Zones 
  " 
  : 
  Proc. 
  Geol. 
  Assoc, 
  vol. 
  xii, 
  1892. 
  

  

  ^ 
  Systime 
  Silurien 
  clu 
  Centre 
  de 
  la 
  Boheme, 
  1865-81 
  (Cephalopoda, 
  Pteropoda, 
  

   and 
  Pelecypoda). 
  

  

  