﻿Xlvili 
  PROCEEDINGS 
  OF 
  TEE 
  GEOLOGICAL 
  SOCIETY. 
  

  

  middle 
  and 
  south 
  of 
  England 
  it 
  seems 
  to 
  be 
  perhaps 
  everywhere 
  con- 
  

   stant 
  between 
  the 
  top 
  of 
  the 
  New 
  Red 
  marl 
  and 
  the 
  base 
  of 
  the 
  

   undoubted 
  Lias. 
  

  

  The 
  number 
  of 
  fossils 
  from 
  these 
  beds 
  in 
  England 
  is 
  considerable, 
  

   comprising 
  32 
  genera 
  and 
  51 
  species 
  of 
  Shells, 
  6 
  genera 
  and 
  7 
  species 
  of 
  

   Eishes, 
  2 
  Reptiles, 
  and 
  one 
  Mammal; 
  in 
  all, 
  41 
  genera 
  and 
  61 
  species. 
  

   Of 
  these, 
  13 
  genera 
  of 
  Mollusca 
  are 
  common 
  to 
  the 
  St. 
  Cassian 
  beds, 
  

   and 
  18 
  to 
  the 
  Muschelkalk, 
  and 
  one 
  species 
  only, 
  Monotis 
  (Avicula) 
  

   decussata, 
  is 
  common 
  to 
  all 
  three 
  formations, 
  and 
  it 
  is 
  also 
  said 
  to 
  be 
  

   found 
  in 
  the 
  Lower 
  Lias. 
  Between 
  the 
  New 
  Red 
  marl 
  and 
  the 
  "West- 
  

   bury 
  or 
  Bone-beds 
  no 
  appearance 
  of 
  unconformity 
  has 
  been 
  noticed, 
  

   though 
  there 
  is 
  a 
  perfectly 
  sudden 
  break 
  lithologically 
  from 
  red 
  

   marl 
  to 
  black 
  shales 
  ; 
  but 
  as 
  Ave 
  have 
  no 
  fossil 
  Shells 
  in 
  England 
  

   in 
  the 
  marl, 
  and 
  no 
  Shells 
  at 
  all 
  by 
  which 
  to 
  compare 
  the 
  two 
  

   formations, 
  there 
  is 
  nothing 
  to 
  be 
  said 
  about 
  the 
  sudden 
  appearance 
  

   of 
  the 
  new 
  fauna, 
  except 
  that, 
  unless 
  some 
  strata 
  be 
  absent 
  inter- 
  

   mediate 
  in 
  date 
  between 
  our 
  Red 
  Marl 
  and 
  the 
  Westbury 
  (Kossen) 
  

   beds, 
  the 
  lithological 
  change 
  and 
  the 
  difference 
  in 
  colour 
  between 
  the 
  

   marl 
  and 
  the 
  shale 
  indicate 
  a 
  change 
  in 
  conditions 
  accompanied 
  by 
  

   a 
  fauna 
  that 
  migrated 
  hither 
  from 
  some 
  other 
  area. 
  Though 
  specifi- 
  

   cally 
  distinct, 
  yet 
  nearly 
  half 
  of 
  the 
  genera 
  of 
  that 
  fauna 
  being 
  

   common 
  to 
  the 
  Muschelkalk, 
  the 
  triassic 
  affinities 
  of 
  the 
  group 
  are 
  

   sufficiently 
  strong. 
  

  

  Lias. 
  — 
  It 
  is 
  remarkable, 
  however, 
  that 
  when 
  we 
  come 
  to 
  the 
  

   Lower 
  Lias, 
  only 
  one 
  species, 
  mentioned 
  above, 
  is 
  common 
  to 
  the 
  two 
  

   formations, 
  even 
  though 
  there 
  is 
  no 
  marked 
  lithological 
  break, 
  and 
  

   no 
  unconformity 
  has 
  been 
  observed 
  between 
  them. 
  The 
  exposed 
  

   sections, 
  however, 
  are 
  so 
  scarce 
  (the 
  junction 
  only 
  having 
  been 
  

   observed 
  in 
  the 
  southern 
  half 
  of 
  England), 
  that 
  it 
  is 
  not 
  impossible 
  

   that 
  overlaps 
  may 
  exist 
  in 
  places. 
  

  

  The 
  Lower 
  Lias 
  averages 
  from 
  600 
  to 
  900 
  feet 
  in 
  thickness, 
  and 
  has 
  

   been 
  always 
  mapped 
  as 
  one 
  indivisible 
  series 
  of 
  clays, 
  shales, 
  and 
  

   limestones. 
  Nevertheless 
  palaeontologists, 
  led 
  by 
  Quenstedt 
  and 
  

   Oppel 
  (whose 
  views 
  have 
  in 
  England 
  been 
  worked 
  out 
  by 
  Strick- 
  

   land, 
  Wright, 
  and 
  others), 
  have 
  attempted, 
  not 
  unsuccessfully, 
  to 
  

   divide 
  it 
  into 
  six 
  zones, 
  each 
  of 
  which 
  is 
  partly 
  marked 
  by 
  its 
  own 
  

   group 
  of 
  fossils, 
  especially 
  by 
  certain 
  Ammonites. 
  

  

  The 
  Middle 
  Lias, 
  or 
  Marlstone 
  series, 
  in 
  like 
  manner 
  has 
  been 
  

   divided 
  (not 
  mapped) 
  into 
  three 
  zones 
  ; 
  and 
  

  

  The 
  Upper 
  Lias 
  into 
  two 
  zones, 
  the 
  lower 
  being 
  the 
  Upper 
  Lias 
  clay, 
  

   and 
  the 
  upper 
  the 
  sand, 
  the 
  latter 
  of 
  which, 
  until 
  the 
  appearance 
  

   of 
  Dr. 
  Wright's 
  paper, 
  was 
  considered 
  a 
  subdivision 
  of 
  the 
  Inferior 
  

   Oolite. 
  

  

  From 
  Devon 
  and 
  Dorsetshire 
  to 
  Yorkshire 
  all 
  of 
  these 
  divisions 
  are 
  

   said 
  to 
  be 
  constant, 
  except 
  where, 
  from 
  accidental 
  circumstances, 
  

   any 
  one 
  of 
  them 
  may 
  chance 
  to 
  lie 
  on 
  some 
  protruding 
  boss 
  of 
  

   palaeozoic 
  rock 
  ; 
  and, 
  from 
  bottom 
  to 
  top 
  of 
  the 
  whole 
  series, 
  we 
  

   cannot 
  assert 
  that 
  anywhere 
  is 
  there 
  actual 
  unconformity 
  between 
  

   any 
  two 
  of 
  the 
  subdivisions, 
  whether 
  they 
  be 
  analyzed 
  minutely 
  or 
  

   taken 
  as 
  three 
  grand 
  divisions 
  of 
  Lower, 
  Middle, 
  and 
  Upper 
  Lias. 
  

  

  