﻿28 
  GEOLOGICAL 
  MEMOIRS. 
  

  

  eludes 
  as 
  follows 
  : 
  — 
  " 
  The 
  question, 
  after 
  all, 
  is 
  not 
  very 
  important 
  ; 
  

   the 
  main 
  point 
  is 
  the 
  stratigraphical 
  position 
  of 
  the 
  formation, 
  on 
  

   which 
  all 
  are 
  agreed. 
  Systems 
  are 
  more 
  or 
  less 
  artificial 
  arrange- 
  

   ments 
  necessary 
  for 
  the 
  general 
  classification 
  of 
  formations, 
  and 
  

   which 
  ought, 
  as 
  far 
  as 
  possible, 
  to 
  comprise 
  in 
  the 
  same 
  group 
  those 
  

   formations 
  of 
  which 
  the 
  faunas 
  are 
  the 
  most 
  analogous. 
  At 
  the 
  

   same 
  time, 
  in 
  proportion 
  as 
  the 
  gaps 
  between 
  recognized 
  systems 
  are 
  

   filled 
  up 
  by 
  new 
  discoveries, 
  the 
  lines 
  of 
  demarcation 
  become 
  more 
  

   vague 
  and 
  uncertain 
  

  

  " 
  Now 
  as 
  the 
  fauna 
  of 
  a 
  formation 
  does 
  not 
  everywhere 
  consist 
  of 
  

   the 
  same 
  species, 
  but 
  varies 
  considerably 
  in 
  different 
  localities, 
  it 
  is 
  

   not 
  impossible 
  that 
  the 
  fauna 
  of 
  one 
  of 
  these 
  transition-formations 
  

   should 
  in 
  one 
  district 
  have 
  a 
  greater 
  analogy 
  with 
  those 
  of 
  the 
  over- 
  

   lying 
  beds, 
  whilst 
  in 
  another 
  locality 
  it 
  might 
  have 
  the 
  greatest 
  

   affinities 
  with 
  the 
  fauna 
  of 
  the 
  underlying 
  beds. 
  I 
  will 
  not, 
  there- 
  

   fore, 
  give 
  any 
  positive 
  opinion 
  respecting 
  the 
  Triassic 
  or 
  Liassic 
  age 
  

   of 
  the 
  Rhoetic 
  beds 
  ; 
  I 
  only 
  wish 
  to 
  point 
  out 
  what 
  are 
  the 
  prevailing 
  

   affinities 
  in 
  the 
  region 
  which 
  I 
  have 
  studied." 
  

  

  The 
  author 
  then 
  shows 
  that 
  the 
  21 
  genera 
  hitherto 
  found 
  in 
  the 
  

   Rhgetic 
  beds 
  of 
  the 
  Yaudois 
  Alps 
  have 
  a 
  much 
  greater 
  affinity 
  with 
  the 
  

   Liassic 
  and 
  Jurassic 
  beds 
  than 
  with 
  the 
  Trias, 
  inasmuch 
  as 
  there 
  

   are 
  only 
  2 
  genera 
  common 
  to 
  it 
  and 
  the 
  Trias, 
  and 
  13 
  genera 
  com- 
  

   mon 
  to 
  the 
  Rheetic 
  and 
  the 
  overlying 
  formations 
  ; 
  so 
  that, 
  as 
  far 
  as 
  

   concerns 
  the 
  Yaudois 
  Alps, 
  he 
  considers 
  the 
  Rhsetic 
  formation 
  as 
  

   belonging 
  rather 
  to 
  the 
  Liassic 
  system. 
  This 
  result 
  is 
  intermediate 
  

   between 
  the 
  two 
  contending 
  theories. 
  With 
  the 
  one, 
  he 
  recognizes 
  

   the 
  independence 
  of 
  the 
  two 
  formations, 
  Hettangien 
  (Infralias) 
  and 
  

   Rhaetic 
  ; 
  and 
  with 
  the 
  other, 
  he 
  is 
  disposed 
  to 
  consider 
  the 
  Rhaatic 
  

   as 
  belonging 
  to 
  the 
  Liassic 
  rather 
  than 
  to 
  the 
  Triassic 
  system. 
  

  

  [W.J.H.] 
  

  

  The 
  Avicula-contorta 
  Zone 
  : 
  its 
  Extent 
  and 
  Organic 
  Contents. 
  

   By 
  Dr. 
  A. 
  von 
  Dittmar. 
  

  

  [Die 
  Contorta-Zone 
  (Zone 
  der 
  Avicula 
  contort 
  a, 
  Portl., 
  ihre 
  Yerbreitung, 
  und 
  ihre 
  

   organischen 
  Einschliisse, 
  von 
  Dr. 
  Alphons 
  v. 
  Dittmar. 
  Miinchen, 
  1864.] 
  

  

  In 
  the 
  year 
  1828, 
  when 
  L. 
  von 
  Buch 
  noticed 
  the 
  beds 
  containing 
  

   Avicula 
  incequivalvis 
  and 
  Gervillia 
  pernoides, 
  we 
  had 
  the 
  first 
  indi- 
  

   cation 
  of 
  a 
  geological 
  horizon 
  which, 
  in 
  extent 
  and 
  importance, 
  is 
  

   scarcely 
  surpassed 
  by 
  any 
  other. 
  It 
  was 
  not, 
  however, 
  until 
  the 
  year 
  

   1856 
  that 
  attention 
  was 
  called, 
  for 
  the 
  first 
  time, 
  to 
  the 
  perfect 
  

   accordance 
  of 
  the 
  bivalves 
  and 
  other 
  fossils 
  of 
  the 
  Swabian 
  " 
  Bone- 
  

   bed-sandstone 
  " 
  with 
  those 
  of 
  the 
  " 
  Gervillia-beds 
  " 
  of 
  Kbssen. 
  

  

  A 
  new 
  impulse 
  was 
  then 
  given 
  to 
  the 
  study 
  of 
  the 
  equivalents 
  

   of 
  the 
  Kossen 
  beds 
  ; 
  and 
  this 
  zone, 
  to 
  which 
  so 
  little 
  attention 
  had 
  

   hitherto 
  been 
  paid, 
  was 
  carefully 
  examined 
  and 
  compared 
  in 
  dif- 
  

   ferent 
  districts, 
  and 
  sought 
  for 
  in 
  new 
  localities. 
  Then 
  it 
  was 
  that 
  

   the 
  controversy 
  regarding 
  the 
  geological 
  position 
  of 
  these 
  beds 
  

   became 
  more 
  animated, 
  and 
  began 
  to 
  command 
  greater 
  attention. 
  

  

  