﻿286 
  PROCEKDINGS 
  OF 
  THK 
  JIALACOLOGICAL 
  SOCII/IT. 
  

  

  Further 
  valuable 
  work 
  lias 
  been 
  accomplished 
  within 
  the 
  last 
  few 
  

   3'ears 
  towards 
  more 
  completely 
  zoning 
  the 
  same 
  series 
  of 
  rocks 
  in 
  

   Great 
  Britain, 
  but 
  the 
  index-fossils 
  in 
  connection 
  therewith 
  have 
  been 
  

   chosen, 
  not 
  from 
  among 
  the 
  Mollusca, 
  but 
  mostly 
  from 
  the 
  Brachiopoda 
  

   and 
  Corals, 
  so 
  that 
  its 
  consideration 
  does 
  not 
  come 
  within 
  the 
  scope 
  of 
  

   the 
  present 
  account. 
  

  

  Mesozoic 
  Zonks. 
  

  

  The 
  Mesozoic 
  rocks 
  have 
  received 
  a 
  large 
  share 
  of 
  study 
  in 
  connection 
  

   with 
  their 
  classification 
  into 
  zones. 
  

  

  First 
  of 
  all 
  there 
  is 
  the 
  zone 
  of 
  Avicula 
  contorta, 
  established 
  by 
  

   Dr. 
  Wright 
  in 
  I860,' 
  characterizing 
  the 
  Rhaetic 
  beds, 
  which 
  he 
  

   regarded 
  as 
  the 
  equivalent 
  of 
  the 
  ' 
  Kossener 
  Schichten 
  ' 
  and 
  the 
  Upper 
  

   St. 
  Cassian 
  strata 
  of 
  the 
  Tyrol 
  and 
  other 
  European 
  districts, 
  and 
  

   consequently 
  belonging 
  either 
  to 
  the 
  topmost 
  series 
  of 
  the 
  Triassic 
  

   deposits 
  or 
  the 
  basal 
  beds 
  of 
  the 
  Liassic 
  system. 
  The 
  Pelecypod 
  

   which 
  gives 
  its 
  name 
  to 
  this 
  zone 
  was 
  originally 
  described 
  by 
  Captain 
  

   Portlock 
  - 
  from 
  Ireland, 
  but 
  the 
  best 
  section 
  of 
  the 
  lihaetic 
  formation 
  

   is 
  undoubtedly 
  to 
  be 
  seen 
  in 
  the 
  cliffs 
  from 
  Penarth 
  Head 
  to 
  Lavernoch 
  

   Point, 
  to 
  the 
  south 
  of 
  Cardiff. 
  It 
  is 
  here 
  that 
  the 
  Avicula 
  contorta 
  

   occurs 
  in 
  greatest 
  abundance, 
  and 
  usually 
  in 
  a 
  crushed 
  condition, 
  

   making 
  up 
  the 
  black 
  shaly 
  beds 
  and 
  shelly 
  limestones 
  of 
  this 
  

   formation, 
  accompanied 
  by 
  numerous 
  examples 
  of 
  Cardium 
  rhceticum, 
  

   in 
  a 
  similar 
  state 
  of 
  preservation. 
  

  

  According 
  to 
  Mr. 
  H. 
  B. 
  Woodward,^ 
  the 
  Ehsetic 
  beds 
  are 
  situated 
  

   between 
  the 
  lied 
  Marls 
  of 
  the 
  Keuper 
  Series 
  and 
  the 
  Lower 
  Lias, 
  and 
  

   extend 
  across 
  England 
  from 
  near 
  Eedcar 
  in 
  Yoi'kshire 
  to 
  near 
  Lyme 
  

   Regis 
  and 
  Axmouth 
  on 
  the 
  coast 
  of 
  Devonshire. 
  

  

  For 
  most 
  exhaustive 
  accounts 
  of 
  the 
  Avicula 
  contorta 
  zone 
  we 
  are 
  

   indebted 
  to 
  the 
  writings 
  of 
  Thomas 
  AVright 
  * 
  and 
  Charles 
  Moore." 
  

  

  In 
  much 
  more 
  modern 
  years 
  the 
  Trias 
  rocks 
  of 
  European 
  and 
  Asiatic 
  

   countries 
  (India, 
  etc.) 
  have 
  been 
  carefully 
  zoned 
  according 
  to 
  the 
  

   Ammonites 
  by 
  such 
  authorities 
  as 
  Mojsisovics, 
  Suess, 
  Diener, 
  and 
  

   Waagen.^ 
  

  

  The 
  Liassic 
  rocks 
  which 
  follow 
  the 
  Rhaetic 
  deposits 
  have 
  been 
  

   specially 
  adapted 
  for 
  zoning 
  on 
  account 
  of 
  the 
  numerous 
  Ammonites, 
  

   which, 
  commencing 
  in 
  the 
  lowest 
  beds, 
  continue 
  throughout 
  the 
  whole 
  

   of 
  the 
  Mesozoic 
  system. 
  Some 
  of 
  the 
  earliest 
  remarks 
  on 
  this 
  subject 
  

   were 
  made 
  by 
  Louis 
  Hunton 
  '^ 
  in 
  1838, 
  who, 
  studying 
  Lias 
  sections 
  

   near 
  AVhitby, 
  observed 
  that 
  "of 
  all 
  organic 
  remains 
  the 
  Ammonoids 
  

   aiford 
  the 
  most 
  beautiful 
  illustration 
  of 
  the 
  subdivision 
  of 
  strata, 
  

   for 
  they 
  appear 
  to 
  have 
  been 
  the 
  least 
  able, 
  of 
  all 
  the 
  Lias 
  genera, 
  to 
  

   conform 
  to 
  a 
  change 
  of 
  external 
  circumstances". 
  He 
  also 
  noted 
  the 
  

  

  ^ 
  Quart. 
  Journ. 
  Geol. 
  Soc, 
  vol. 
  xvi, 
  p. 
  374, 
  1860. 
  

  

  ^ 
  Report 
  Geology 
  Londonderry 
  , 
  etc., 
  1843, 
  p. 
  126. 
  

  

  ^ 
  The 
  Geology 
  of 
  England 
  and 
  Wales, 
  2nd 
  ed., 
  pp. 
  245, 
  246, 
  1887. 
  

  

  ■* 
  Quart. 
  Journ. 
  Geol. 
  Soc, 
  vol. 
  xvi, 
  p. 
  374, 
  1860. 
  

  

  •' 
  Ibid., 
  vol. 
  xvii, 
  p. 
  483, 
  1861. 
  

  

  I 
  Sitz. 
  Akad. 
  AViss. 
  Wien, 
  1895, 
  vol. 
  cix. 
  

  

  '' 
  Trans. 
  Geol. 
  Soc. 
  London, 
  ser. 
  II, 
  vol. 
  v, 
  p. 
  215, 
  1838. 
  

  

  