﻿36 
  C. 
  Schuchert—Hebert's 
  Views 
  of 
  1857 
  

  

  deposited 
  changed 
  in 
  a 
  rather 
  marked 
  manner 
  at 
  the 
  boundary 
  

   between 
  the 
  two 
  epochs. 
  The 
  beds 
  of 
  the 
  Upper 
  Lias, 
  with 
  

   their 
  numerous 
  cephalopoda, 
  tell 
  of 
  waters 
  which 
  were 
  quite 
  

   deep. 
  The 
  disappearance 
  of 
  these 
  animals 
  at 
  the 
  base 
  of 
  the 
  

   Lower 
  Oolite, 
  and 
  the 
  presence 
  of 
  many 
  little 
  polyps 
  in 
  the 
  

   east, 
  and 
  of 
  rounded 
  pebbles 
  in 
  the 
  west, 
  indicate 
  the 
  nearness 
  

   of 
  the 
  shore, 
  or 
  waters 
  which 
  were 
  very 
  shallow. 
  There 
  was, 
  

   then, 
  at 
  the 
  end 
  of 
  the 
  Lias, 
  in 
  the 
  Paris 
  basin, 
  an 
  elevation 
  of 
  

   the 
  land, 
  and 
  when 
  the 
  waters, 
  which 
  had 
  become 
  deep, 
  were 
  

   again 
  inhabited 
  by 
  cephalopods, 
  the 
  species 
  were 
  entirely 
  dif- 
  

   ferent 
  (page 
  23). 
  

  

  Lower 
  Oolite-Upper 
  Oolite 
  boundary. 
  — 
  In 
  both 
  east 
  and 
  

   west 
  we 
  find 
  the 
  Lower 
  Oolite 
  represented 
  by 
  an 
  entirely 
  simi- 
  

   lar 
  fauna 
  and 
  bearing 
  on 
  its 
  upper 
  surface 
  the 
  traces 
  of 
  denu- 
  

   dation 
  and 
  other 
  phenomena 
  indicating 
  a 
  time 
  of 
  arrest 
  in 
  

   sedimentation 
  and 
  an 
  upward 
  movement 
  of 
  the 
  crust. 
  At 
  

   either 
  limit, 
  the 
  millet-like 
  oolite 
  is 
  an 
  infallible 
  horizon 
  

   marker. 
  Between 
  the 
  two 
  formations, 
  in 
  the 
  east, 
  is 
  found 
  the 
  

   Fuller's 
  Earth, 
  with 
  its 
  maximum 
  thickness. 
  In 
  the 
  west, 
  on 
  

   the 
  contrarj 
  T 
  , 
  the 
  absence 
  in 
  the 
  Sarthe 
  of 
  any 
  kind 
  of 
  sedi- 
  

   ments 
  shows 
  that 
  during 
  a 
  very 
  considerable 
  time 
  — 
  long 
  

   enough 
  for 
  the 
  accumulation 
  of 
  the 
  thick 
  beds 
  of 
  Ostrea 
  acu- 
  

   7ninata 
  and 
  other 
  fossils 
  — 
  the 
  land 
  remained 
  emergent 
  (31). 
  

  

  Great 
  Oolite-Oxford 
  boundary. 
  — 
  At 
  the 
  time 
  of 
  the 
  Great 
  

   Oolite, 
  the 
  seas 
  throughout 
  the 
  Paris 
  basin 
  were 
  constantly 
  

   shallow, 
  and 
  it 
  happened 
  many 
  times 
  that 
  in 
  one 
  place 
  or 
  

   another 
  more 
  or 
  less 
  extensive 
  areas 
  were 
  raised 
  so 
  as 
  to 
  be 
  at 
  

   the 
  level 
  of 
  the 
  sea. 
  But 
  these 
  partial 
  elevations 
  did 
  not 
  

   hinder 
  the 
  dominantly 
  downward 
  movement, 
  as 
  a 
  result 
  of 
  

   which 
  the 
  seas 
  rose 
  more 
  and 
  more 
  along 
  the 
  shore 
  of 
  the 
  basin. 
  

   This 
  movement, 
  whose 
  origin 
  goes 
  back 
  to 
  the 
  Trias, 
  continued 
  

   to 
  the 
  end 
  of 
  the 
  Great 
  Oolite, 
  interrupted 
  momentarily, 
  either 
  

   in 
  a 
  general 
  way, 
  as 
  at 
  the 
  end 
  of 
  the 
  Lias, 
  the 
  Lower 
  Oolite 
  or 
  

   the 
  Great 
  Oolite, 
  or 
  in 
  a 
  local 
  way, 
  of 
  which 
  the 
  Great 
  Oolite 
  

   offers 
  us 
  numerous 
  examples 
  (33). 
  

  

  On 
  isostasy. 
  — 
  During 
  the 
  first 
  half 
  of 
  the 
  Jurassic 
  [i. 
  e., 
  

   previous 
  to 
  the 
  Oxfordian] 
  the 
  sediments 
  that 
  accumulated 
  on 
  

   the 
  bottom 
  of 
  the 
  basin 
  helped 
  by 
  their 
  weight 
  to 
  increase 
  the 
  

   depth 
  of 
  it 
  [the 
  older 
  idea 
  of 
  isostatic 
  balance] 
  ; 
  however, 
  this 
  

   overloading 
  could 
  not 
  explain 
  the 
  widespread 
  sinking 
  of 
  the 
  

   shores, 
  accompanied 
  by 
  the 
  intermittent 
  elevations 
  which 
  

   divided 
  the 
  Jurassic 
  into 
  distinct 
  epochs, 
  each 
  characterized 
  by 
  

   a 
  particular 
  fauna, 
  Without 
  doubt, 
  although 
  in 
  reality 
  this 
  

   overloading 
  did 
  produce 
  results 
  which 
  must 
  be 
  taken 
  into 
  

   account, 
  it 
  was 
  not 
  the 
  principal 
  cause 
  of 
  these 
  movements 
  of 
  

   the 
  land. 
  That 
  cause 
  is 
  more 
  general 
  (40). 
  

  

  Coral 
  Raa-Kimmeridge 
  boundary. 
  — 
  The 
  characters 
  of 
  the 
  

   Diceras 
  limestone 
  indicate 
  a 
  shore 
  deposit 
  laid 
  down 
  under 
  

  

  