﻿470 
  ME. 
  S. 
  S. 
  BfCIOIAX 
  OX 
  THE 
  

  

  Considering 
  the 
  very 
  great 
  development 
  of 
  the 
  Toarcian 
  limestone 
  

   in 
  the 
  Cotteswolds 
  it 
  may 
  be 
  advantageous 
  to 
  give 
  a 
  comparison 
  of 
  

   the 
  strata 
  of 
  that 
  district 
  with 
  a 
  section 
  of 
  the 
  strata 
  in 
  Dorset 
  (fig. 
  2, 
  

   facing 
  p. 
  456). 
  This 
  correlation 
  is 
  entirely 
  founded 
  on 
  the 
  position 
  of 
  

   the 
  different 
  species 
  of 
  Ammonites 
  ; 
  and 
  for 
  further 
  information 
  on 
  

   this 
  matter 
  the 
  reader 
  is 
  referred 
  to 
  my 
  Ponograph 
  on 
  " 
  Inferior- 
  

   Oolite 
  Ammonites," 
  Palaeontographical 
  Society, 
  p. 
  91. 
  

  

  The 
  horizontal 
  diagram 
  (fig. 
  3, 
  p. 
  468) 
  is 
  intended 
  to 
  represent 
  the 
  

   Inferior-Oolite 
  strata 
  north 
  and 
  south 
  of 
  the 
  Mendips, 
  with 
  their 
  

   geographical 
  extension 
  according 
  to 
  the 
  correlation 
  just 
  given, 
  on 
  a 
  

   line 
  from 
  Leckhampton 
  Hill 
  to 
  .Burton 
  Bradstock. 
  Apparently 
  the 
  

   Mendip 
  range 
  must 
  have 
  acted 
  as 
  a 
  barrier 
  between 
  the 
  areas 
  on 
  

   each 
  side 
  of 
  it 
  : 
  but 
  it 
  is 
  more 
  curious 
  to 
  find 
  that 
  a 
  change 
  in 
  litho- 
  

   logy 
  in 
  the 
  Sodbury 
  district, 
  a 
  change 
  which 
  I 
  have 
  demonstrated 
  in 
  

   the 
  case 
  of 
  the 
  sands, 
  should 
  coincide 
  with 
  an 
  outcrop 
  of 
  Carboniferous 
  

   Limestone. 
  This 
  is 
  part 
  of 
  a 
  patch 
  which 
  extends 
  to 
  Charfield 
  and 
  

   reappears 
  across 
  the 
  Severn 
  and 
  no 
  doubt 
  formed, 
  at 
  that 
  period, 
  a 
  

   subsidiary 
  range 
  of 
  hills 
  joining 
  the 
  Mendip 
  axis 
  at 
  right 
  angles. 
  

  

  However, 
  the 
  character 
  which 
  this 
  figure 
  brings 
  into 
  greatest 
  

   prominence 
  is 
  the 
  remarkable 
  persistence 
  of 
  a 
  hiatus 
  or 
  lacuna, 
  

   due 
  to 
  the 
  absence 
  of 
  certain 
  strata, 
  in 
  the 
  middle 
  of 
  what 
  is 
  called 
  

   Inferior 
  Oolite. 
  This 
  is 
  the 
  same 
  hiatus 
  which 
  Eugene 
  Deslongs- 
  

   champs 
  finds 
  in 
  Xormandy, 
  and 
  which 
  he 
  proposes 
  shall 
  mark 
  the 
  

   uppermost 
  limit 
  of 
  his 
  Infra-Oolitic 
  marls 
  ; 
  this 
  is 
  the 
  same 
  hiatus 
  

   which 
  Dr. 
  Yacek 
  says 
  occurs 
  over 
  the 
  continent 
  generally, 
  and 
  

   which 
  he 
  proposes 
  as 
  the 
  uppermost 
  limit 
  of 
  the 
  Lias 
  ; 
  this 
  is 
  the 
  

   hiatus 
  which 
  I, 
  following 
  d'Orbigny, 
  propose 
  to 
  take 
  as 
  the 
  dividing 
  

   line 
  between 
  the 
  " 
  Toarcien 
  " 
  and 
  the 
  "Bajocien" 
  ; 
  this 
  is 
  the 
  hiatus 
  

   which 
  is 
  accompanied 
  palaeontologically 
  by 
  the 
  sudden 
  exit 
  of 
  nearly 
  

   all 
  the 
  Hildoceratidae, 
  or 
  ' 
  true 
  Falciferi." 
  

  

  One 
  last 
  subject 
  remains 
  to 
  be 
  dealt 
  with, 
  namely, 
  the 
  matter 
  of 
  

   mapping. 
  In 
  connexion 
  herewith 
  the 
  proposed 
  classification 
  would 
  

   introduce 
  some 
  advantages, 
  and 
  would 
  certainly 
  get 
  rid 
  of 
  such 
  

   anomalies 
  as 
  the 
  same 
  zone 
  being 
  mapped 
  as 
  " 
  Mid 
  [ford 
  Sands" 
  

   in 
  one 
  county 
  and 
  as 
  Upper 
  Lias 
  Clay 
  in 
  another 
  : 
  or, 
  again, 
  as 
  

   " 
  Midford 
  Sands" 
  at 
  one 
  place 
  and 
  Inferior-Oolite 
  Limestone 
  some- 
  

   where 
  else. 
  It 
  would 
  probably 
  be 
  best 
  for 
  this 
  purpose 
  to 
  divide 
  

   the 
  Toarcian 
  into 
  Upper 
  and 
  Lower, 
  although 
  there 
  is 
  between 
  

   them, 
  lithologically, 
  no 
  marked 
  break 
  at 
  any 
  point, 
  and, 
  palaeonto- 
  

   logically, 
  only 
  a 
  small 
  one. 
  This 
  palaeontological 
  break 
  occurs 
  at 
  

   the 
  end 
  of 
  the 
  Opalinum-zone 
  with 
  the 
  extinction 
  of 
  the 
  genus 
  

   Grammoceras 
  — 
  only 
  one 
  species 
  of 
  which, 
  I 
  believe, 
  survives 
  into 
  the 
  

   Murchisonce-zone. 
  Therefore, 
  the 
  Falciferum-, 
  Commune-, 
  Jnrense-, 
  

   and 
  Opalinum-zon.es 
  form 
  the 
  Lower 
  Toarcian. 
  They 
  would 
  

   require 
  the 
  following 
  changes 
  among 
  others, 
  namely 
  the 
  abolition 
  of 
  

   the 
  dividing-line 
  now 
  drawn 
  between 
  Upper 
  Lias 
  * 
  and 
  " 
  Midford 
  

  

  * 
  Throughout 
  a 
  large 
  part 
  of 
  Dorset 
  and 
  Somerset 
  the 
  Upper 
  Lias 
  Clay 
  is 
  

   not 
  mapped, 
  although 
  to 
  a 
  great 
  extent 
  it 
  overlies 
  what 
  is 
  marked 
  as 
  "^2, 
  

   Middle 
  Lias." 
  For 
  instance, 
  at 
  South 
  Pethertom 
  it 
  is 
  exposed 
  to 
  a 
  depth 
  of 
  

   nearly 
  seven 
  feet 
  on 
  the 
  top 
  of 
  the 
  Marlstone 
  ; 
  but 
  nothing 
  is 
  said 
  about 
  it 
  on 
  

   the 
  map. 
  

  

  