﻿regarding 
  the 
  Periodic 
  Submergence 
  of 
  Europe. 
  41 
  

  

  In 
  each 
  of 
  these 
  stages 
  [he 
  names 
  six 
  for 
  the 
  Jurassic] 
  we 
  

   recognize 
  constant 
  fossiliferous 
  horizons 
  in 
  the 
  whole 
  basin, 
  

   belonging 
  to 
  systems 
  of 
  beds 
  often 
  easily 
  distinguished 
  by 
  

   their 
  mineralogical 
  characters, 
  although 
  there 
  may 
  not 
  be 
  

   between 
  them 
  sharply 
  defined 
  limits, 
  either 
  mineralogical 
  or 
  

   paleontological 
  ; 
  these 
  are 
  the 
  formations 
  which 
  make 
  up 
  the 
  

   stages.. 
  Their 
  characters 
  depend 
  also 
  on 
  the 
  physical 
  and 
  me- 
  

   chanical 
  conditions 
  which 
  presided 
  over 
  their 
  deposition 
  ; 
  they 
  

   differ 
  especially 
  because 
  of 
  the 
  variable 
  depth 
  of 
  the 
  seas, 
  but 
  

   they 
  pass 
  from 
  one 
  into 
  another 
  because 
  they 
  were 
  formed 
  dur- 
  

   ing 
  the 
  same 
  secondary 
  oscillation. 
  They 
  are 
  themselves 
  sub- 
  

   divided 
  into 
  beds, 
  sometimes 
  very 
  numerous, 
  whose 
  characters 
  

   maybe 
  maintained 
  for 
  great 
  distances, 
  like 
  the 
  bed 
  with 
  Amm. 
  

   primordialis, 
  or 
  may 
  vary 
  at 
  neighboring 
  points. 
  This 
  fourth 
  

   method 
  of 
  division, 
  however, 
  although 
  indispensable 
  for 
  local 
  

   descriptions, 
  cannot 
  enter 
  into 
  the 
  classification 
  of 
  a 
  more 
  

   extended 
  area. 
  

  

  