﻿1 
  PEOCEEDINGS 
  OF 
  THE 
  GEOLOGICAL 
  SOCIETY. 
  

  

  Neither 
  is 
  there 
  any 
  marked 
  sudden 
  change 
  in 
  the 
  lithology 
  of 
  the 
  

   rocks, 
  except 
  between 
  the 
  brown 
  limestone 
  of 
  the 
  Marlstone 
  proper 
  

   and 
  the 
  Upper 
  Lias 
  clay. 
  

  

  Now 
  in 
  all 
  the 
  Lias 
  formations 
  of 
  England 
  there 
  are 
  about 
  243 
  

   genera 
  and 
  467 
  known 
  species 
  ; 
  and 
  if 
  we 
  construct 
  a 
  table 
  of 
  zones 
  

   distinguished 
  by 
  special 
  Ammonites, 
  after 
  the 
  manner 
  practised 
  by 
  

   Oppel 
  and 
  Dr. 
  Wright, 
  we 
  find 
  the 
  results 
  shown 
  in 
  the 
  following 
  

   Table, 
  — 
  premising 
  that 
  none 
  of 
  the 
  species 
  of 
  Ammonites 
  have 
  yet 
  

   been 
  observed 
  in 
  any 
  zone 
  lower 
  than 
  that 
  in 
  which 
  it 
  is 
  mentioned 
  

   as 
  commencing 
  ; 
  and 
  also 
  that 
  for 
  the 
  species 
  of 
  every 
  kind, 
  those 
  

   which 
  are 
  common 
  to 
  a 
  low 
  and 
  a 
  high 
  zone, 
  but 
  have 
  not 
  been 
  found 
  

   in 
  intermediate 
  subdivisions, 
  are 
  assumed 
  to 
  have 
  existed 
  during 
  the 
  

   deposition 
  of 
  the 
  intermediate 
  members 
  here 
  or 
  elsewhere. 
  For 
  

   instance, 
  a 
  species 
  passes 
  from 
  the 
  zone 
  of 
  A. 
  Planorbis 
  into 
  that 
  of 
  

   A. 
  spinatus, 
  the 
  upper 
  zone 
  of 
  the 
  Middle 
  Lias 
  ; 
  and 
  though 
  this 
  

   species 
  may 
  not 
  have 
  been 
  seen 
  or 
  catalogued 
  in 
  any 
  of 
  the 
  inter- 
  

   mediate 
  zones, 
  it 
  is 
  clear 
  that 
  it 
  must 
  have 
  existed 
  during 
  those 
  

   times, 
  because 
  no 
  one 
  believes 
  that 
  any 
  species, 
  having 
  once 
  passed 
  

   away, 
  has 
  been 
  re-created 
  or 
  redeveloped. 
  Furthermore, 
  in 
  the 
  

   Table 
  the 
  same 
  species 
  is 
  never 
  repeated 
  in 
  any 
  two 
  columns 
  through- 
  

   out 
  the 
  whole 
  series. 
  

  

  On 
  the 
  principles 
  stated 
  above, 
  it 
  appears 
  that 
  17 
  species, 
  or 
  rather 
  

   more 
  than 
  40 
  per 
  cent., 
  pass 
  upwards 
  from 
  the 
  zone 
  of 
  Ammonites 
  

   Planorbis, 
  all 
  of 
  which 
  must 
  have 
  lived 
  through 
  the 
  period 
  of 
  time 
  

   of 
  the 
  zone 
  of 
  A. 
  BucMandi. 
  From 
  the 
  zone 
  of 
  A. 
  Bucklandi 
  20 
  

   species, 
  or 
  18| 
  per 
  cent., 
  pass 
  upwards 
  ; 
  from 
  that 
  of 
  A. 
  Turneri 
  6 
  

   species, 
  or 
  more 
  than 
  50 
  per 
  cent., 
  pass 
  on 
  ; 
  from 
  that 
  of 
  A. 
  obtusus 
  2 
  

   species, 
  or 
  more 
  than 
  25 
  per 
  cent., 
  pass 
  ; 
  from 
  the 
  zone 
  of 
  A. 
  oxynotus 
  

   (which 
  has 
  only 
  yielded 
  6 
  species) 
  none 
  have 
  been 
  observed 
  higher 
  in 
  

   the 
  Lias, 
  while 
  from 
  that 
  of 
  A. 
  raricostatus, 
  which 
  forms 
  the 
  top 
  of 
  

   the 
  Lower 
  Lias, 
  6 
  species, 
  or 
  about 
  33 
  per 
  cent., 
  pass 
  upwards 
  into 
  

   the 
  Middle 
  Lias. 
  

  

  In 
  the 
  Middle 
  Lias, 
  from 
  the 
  zone 
  of 
  A. 
  Davosi, 
  or 
  A. 
  Henleyi, 
  31 
  

   species 
  pass 
  upwards, 
  or 
  very 
  nearly 
  38 
  per 
  cent. 
  ; 
  and 
  from 
  the 
  

   zone 
  of 
  A. 
  margaritatus 
  8 
  species 
  out 
  of 
  38, 
  or 
  21 
  per 
  cent., 
  go 
  

   into 
  higher 
  horizons. 
  From 
  the 
  zone 
  of 
  A. 
  spinatus 
  (the 
  top 
  of 
  the 
  

   Middle 
  Lias) 
  2 
  species 
  only, 
  or 
  about 
  5| 
  per 
  cent., 
  pass 
  into 
  the 
  

   Upper 
  Lias 
  ; 
  and 
  from 
  the 
  zone 
  of 
  A. 
  communis 
  (the 
  base 
  of 
  the 
  

   Upper 
  Lias) 
  15 
  species, 
  or 
  more 
  than 
  27 
  per 
  cent., 
  pass 
  upwards 
  into 
  

   the 
  zone 
  of 
  A. 
  Jurensis 
  or 
  the 
  Upper 
  Lias 
  sands. 
  From 
  the 
  Upper 
  

   Lias 
  13 
  species, 
  or 
  more 
  than 
  21 
  per 
  cent., 
  pass 
  into 
  the 
  Inferior 
  Oolite. 
  

  

  The 
  zone 
  of 
  A. 
  owynotus-, 
  in 
  which 
  only 
  6 
  species 
  are 
  recorded 
  in 
  

   England, 
  is 
  one 
  but 
  little 
  opened 
  for 
  economic, 
  purposes, 
  and 
  this 
  

   may 
  perhaps 
  account 
  for 
  the 
  scarcity 
  of 
  its 
  fossils. 
  Under 
  these 
  cir- 
  

   cumstances 
  it 
  is 
  scarcely 
  fair 
  to 
  argue 
  upon 
  it 
  ; 
  but, 
  with 
  this 
  ex- 
  

   ception, 
  I 
  am 
  on 
  the 
  whole 
  struck 
  with 
  two 
  main 
  facts, 
  the 
  first 
  of 
  

   which 
  has 
  been 
  well 
  stated 
  for 
  England 
  by 
  Dr. 
  "Wright, 
  namely, 
  

   that 
  as 
  regards 
  certain 
  species 
  of 
  Ammonites 
  the 
  lines 
  are 
  apt 
  to 
  be 
  

   trenchant, 
  though 
  other 
  species 
  in 
  a 
  limited 
  degree 
  pass 
  up 
  and 
  

   down 
  ; 
  and, 
  in 
  the 
  second 
  place, 
  a 
  large 
  proportion 
  of 
  the 
  common 
  

  

  