﻿VON 
  DITTMAR 
  AVICULA 
  -CONTORTA 
  ZONE. 
  29 
  

  

  This 
  bed, 
  having 
  the 
  same 
  abundance 
  of 
  characteristic 
  fossils, 
  was 
  

   found 
  all 
  through 
  Germany, 
  in 
  England, 
  Norway, 
  Sweden, 
  Bohemia, 
  

   Hungary, 
  Lombardy, 
  and 
  the 
  south-east 
  of 
  France. 
  In 
  all 
  the 
  

   northern 
  localities 
  it 
  occurs 
  as 
  a 
  thin 
  zone, 
  which 
  might 
  be 
  easily 
  

   overlooked; 
  but 
  in 
  the 
  Alps 
  and 
  in 
  Lombardy 
  it 
  is 
  of 
  immense 
  

   thickness, 
  and 
  forms 
  entire 
  mountains. 
  Nowhere 
  is 
  it 
  wanting 
  in 
  

   organic 
  remains 
  : 
  here, 
  as 
  in 
  Wurtemberg 
  and 
  England, 
  we 
  find 
  

   the 
  accumulated 
  fragments 
  of 
  numerous 
  teeth 
  and 
  of 
  spines 
  of 
  Fish 
  

   and 
  Saurians 
  ; 
  there, 
  as 
  in 
  the 
  Bavarian 
  Alps, 
  Lombardy, 
  "Wurtem- 
  

   berg, 
  and 
  France, 
  it 
  is 
  crowded 
  with 
  Bivalves, 
  especially 
  Avicula, 
  

   Gervillia, 
  and 
  Pecten 
  ; 
  elsewhere, 
  as 
  in 
  Sweden, 
  Franconia, 
  and 
  per- 
  

   haps 
  also 
  in 
  the 
  " 
  Gresten-beds 
  " 
  of 
  the 
  Alps, 
  remains 
  of 
  Plants 
  

   abound. 
  

  

  Everywhere 
  we 
  find, 
  forming 
  a 
  safe 
  guide 
  and 
  sign-post, 
  the 
  Avi- 
  

   cula 
  contorta, 
  Portl., 
  whence 
  the 
  beds 
  in 
  question 
  may 
  justly 
  be 
  

   designated 
  as 
  the 
  "Zone 
  of 
  Avicula 
  contorta" 
  or 
  the 
  " 
  Contorta- 
  

   beds." 
  All 
  the 
  other 
  names 
  which 
  these 
  strata 
  have 
  received 
  the 
  

   author 
  considers 
  as 
  one-sided, 
  and 
  consequently 
  unsatisfactory, 
  and 
  

   he 
  gives 
  reasons 
  for 
  so 
  regarding 
  them. 
  

  

  The 
  object 
  of 
  the 
  author 
  in 
  this 
  work 
  is 
  to 
  point 
  out 
  the 
  extent 
  

   of 
  these 
  beds, 
  to 
  portray 
  and 
  compare 
  their 
  different 
  forms 
  of 
  deve- 
  

   lopment, 
  and, 
  lastly, 
  to 
  give 
  his 
  contribution 
  towards 
  the 
  solution 
  of 
  

   the 
  question 
  whether 
  they 
  are 
  Triassic 
  or 
  Jurassic. 
  

  

  Dr. 
  von 
  Dittmar 
  considers 
  in 
  some 
  detail 
  the 
  geographical 
  exten- 
  

   sion 
  of 
  this 
  zone, 
  and 
  points 
  out 
  the 
  diversity, 
  both 
  in 
  its 
  lithological 
  

   and 
  palseontological 
  characters, 
  which 
  is 
  met 
  with 
  in 
  tracing 
  it 
  over 
  

   a 
  large 
  area 
  — 
  a 
  diversity 
  such 
  as 
  we 
  find 
  under 
  similar 
  circumstances 
  

   in 
  almost 
  every 
  other 
  formation. 
  

  

  In 
  the 
  northern 
  and 
  southern 
  subalpine 
  regions 
  this 
  Contorta-zone 
  

   consists 
  of 
  grey 
  limestones 
  and 
  dark-coloured 
  marls, 
  forming 
  at 
  times 
  

   entire 
  mountain-chains. 
  Outside 
  this 
  district 
  it 
  occurs 
  on 
  a 
  much 
  

   smaller 
  scale 
  as 
  a 
  more 
  or 
  less 
  brown- 
  coloured 
  ferruginous 
  sandstone, 
  

   which 
  only 
  in 
  certain 
  localities 
  attains 
  to 
  a 
  thickness 
  of 
  100 
  feet 
  or 
  

   more. 
  

  

  In 
  France, 
  where, 
  on 
  the 
  right 
  bank 
  of 
  the 
  Saone 
  and 
  Bhone, 
  it 
  

   follows 
  for 
  a 
  considerable 
  distance 
  the 
  upper 
  surface 
  of 
  the 
  Keuper 
  

   Marls, 
  or 
  lies 
  directly 
  on 
  the 
  granite 
  of 
  the 
  Cote 
  d'Or, 
  it 
  is 
  con- 
  

   stantly 
  accompanied 
  by 
  certain 
  bands 
  of 
  a 
  granitic 
  conglomerate, 
  

   and 
  is 
  there 
  usually 
  designated 
  " 
  Arkose." 
  

  

  In 
  Wiirtemberg, 
  where 
  there 
  is 
  no 
  trace 
  of 
  this 
  conglomerate, 
  it 
  

   occurs 
  as 
  a 
  light 
  grey 
  or 
  yellow 
  fine-grained 
  sandstone, 
  from 
  1 
  to 
  30 
  

   feet 
  thick, 
  with 
  thin 
  layers 
  of 
  yellow 
  loam 
  or 
  greyish 
  clay. 
  This 
  is 
  

   sharply 
  marked 
  off 
  from 
  the 
  lowest 
  dark 
  limestone-bands 
  of 
  the 
  Lias, 
  

   and 
  is 
  accompanied, 
  near 
  the 
  upper 
  and 
  lower 
  surface, 
  by 
  a 
  hard 
  

   sandstone-breccia, 
  which 
  abounds 
  in 
  numerous 
  fragments 
  of 
  Fish- 
  

   teeth, 
  bones, 
  &c. 
  This 
  latter 
  is 
  the 
  " 
  Schwabische 
  Kloake," 
  of 
  Quen- 
  

   stedt, 
  and 
  the 
  " 
  Bone-bed 
  " 
  of 
  English 
  geologists. 
  Such 
  a 
  bone-bed 
  

   is 
  altogether 
  absent 
  in 
  the 
  Alps, 
  and 
  only 
  traces 
  of 
  it 
  occur 
  in 
  the 
  

   French 
  " 
  Arkose." 
  

  

  In 
  Franconia, 
  the 
  Saxon 
  Principalities, 
  and 
  Hanover, 
  this 
  zone 
  

  

  