﻿regarding 
  the 
  Periodic 
  Submergence 
  of 
  Europe. 
  37 
  

  

  similar 
  conditions 
  extending 
  over 
  a 
  lapse 
  of 
  time 
  of 
  a 
  truly 
  

   prodigious 
  duration. 
  At 
  the 
  time 
  of 
  the 
  deposition 
  of 
  the 
  

   higher 
  Astarte 
  limestone, 
  the 
  conditions 
  changed 
  almost 
  

   abruptly, 
  although 
  there 
  is 
  an 
  alternation 
  at 
  the 
  contact. 
  The 
  

   new 
  causes 
  which 
  brought 
  about 
  the 
  muddy 
  sediments 
  did 
  not 
  

   at 
  once 
  predominate 
  ; 
  there 
  was 
  a 
  struggle, 
  so 
  to 
  speak, 
  but 
  the 
  

   contrast 
  is 
  none 
  the 
  less 
  striking. 
  Without 
  doubt, 
  like 
  events 
  

   can 
  be 
  shown 
  within 
  the 
  same 
  stage 
  ; 
  but 
  when 
  they 
  are 
  accom- 
  

   panied 
  by 
  a 
  change 
  of 
  fauna 
  as 
  considerable 
  as 
  the 
  one 
  we 
  

   have 
  shown, 
  there 
  is 
  at 
  this 
  horizon 
  a 
  line 
  of 
  demarcation 
  of 
  a 
  

   certain 
  importance 
  (59). 
  

  

  On 
  oolite 
  formations. 
  — 
  Let 
  us 
  seek, 
  by 
  examining 
  what 
  

   takes 
  place 
  to-day 
  in 
  nature, 
  to 
  reconstruct 
  the 
  conditions 
  neces- 
  

   sary 
  for 
  the 
  formation 
  of 
  oolitic 
  limestones. 
  Waters 
  not 
  

   muddy, 
  saturated 
  with 
  carbonate 
  of 
  lime, 
  a 
  warm 
  climate, 
  

   abundant 
  evaporation 
  [now 
  known 
  to 
  be 
  due 
  to 
  the 
  action 
  of 
  

   denitrifying 
  bacteria], 
  precipitation 
  of 
  lime 
  at 
  the 
  surface, 
  

   either 
  around 
  little 
  particles 
  of 
  shells 
  or 
  sand, 
  or 
  even 
  about 
  

   little 
  crystals 
  of 
  carbonate 
  of 
  lime, 
  immense 
  areas 
  washed 
  

   periodically 
  by 
  slightly 
  agitated 
  seas, 
  a 
  constant 
  to-and-fro 
  

   movement 
  to 
  roll 
  the 
  little 
  bodies 
  while 
  they 
  are 
  being 
  en- 
  

   crusted 
  by 
  the 
  lime 
  : 
  we 
  may 
  figure 
  that 
  such 
  conditions 
  were 
  

   maintained 
  long 
  enough 
  for 
  the 
  accumulation 
  of 
  the 
  oolites 
  to 
  

   produce 
  masses 
  100 
  m. 
  in 
  thickness, 
  not 
  only 
  at 
  restricted 
  

   places, 
  as 
  can 
  be 
  observed 
  to-day 
  in 
  the 
  Antilles, 
  but 
  over 
  the 
  

   whole 
  extent 
  of 
  the 
  Paris 
  basin. 
  

  

  Then, 
  all 
  at 
  once, 
  the 
  scene 
  changes, 
  the 
  oolites 
  disappear, 
  

   the 
  waters 
  become 
  dirty, 
  deposit 
  mud, 
  and 
  nourish 
  an 
  abun- 
  

   dance 
  of 
  Panopeas, 
  Pholadomyas, 
  and 
  other 
  molluscs 
  to 
  which 
  

   the 
  new 
  conditions 
  are 
  more 
  favorable 
  ; 
  and 
  these 
  new 
  condi- 
  

   tions 
  continue 
  until 
  the 
  end 
  of 
  the 
  Jurassic, 
  long 
  enough 
  for 
  

   generations 
  of 
  species 
  to 
  succeed 
  each 
  other 
  in 
  the 
  midst 
  of 
  a 
  

   common 
  fauna 
  (66). 
  

  

  Secondary 
  crustal 
  movements 
  during 
  the 
  period 
  of 
  emer- 
  

   gence. 
  — 
  The 
  second 
  period 
  of 
  the 
  Jurassic, 
  which 
  we 
  have 
  

   named 
  the 
  period 
  of 
  elevation, 
  had 
  for 
  its 
  dominant 
  character 
  

   a 
  slow 
  elevation 
  of 
  the 
  crust 
  which 
  determined 
  the 
  progressive 
  

   retreat 
  of 
  the 
  sea, 
  so 
  that 
  the 
  consecutive 
  shores 
  which 
  we 
  can 
  

   follow 
  to-day 
  with 
  considerable 
  exactness 
  approach 
  more 
  and 
  

   more 
  the 
  center 
  of 
  the 
  basin. 
  But 
  just 
  as, 
  in 
  the 
  preceding 
  

   period, 
  the 
  general 
  subsidence 
  of 
  the 
  basin 
  was 
  not 
  the 
  result 
  

   of 
  a 
  uniform 
  movement, 
  but 
  there 
  were 
  times 
  of 
  arrest 
  and 
  

   oscillation 
  over 
  more 
  or 
  less 
  extensive 
  areas 
  ; 
  so, 
  in 
  this 
  second 
  

   period, 
  the 
  progressive 
  elevation 
  was 
  subject 
  to 
  the 
  same 
  irregu- 
  

   larities, 
  as 
  is 
  proved 
  by 
  the 
  state 
  of 
  the 
  sediments 
  of 
  the 
  epoch. 
  

  

  Thus, 
  the 
  Oxford 
  is 
  a 
  great 
  deposit 
  of 
  clay 
  or 
  more 
  or 
  less 
  

   marly 
  limestones, 
  i. 
  e., 
  of 
  sediments 
  laid 
  down 
  in 
  a 
  state 
  of 
  

  

  