﻿402 
  H. 
  S. 
  Williams 
  — 
  Southern 
  Devonian 
  formations. 
  

  

  3d. 
  The 
  catastrophic 
  effects, 
  upon 
  the 
  sedimentation, 
  and 
  

   upon 
  the 
  inhabitants 
  of 
  a 
  more 
  or 
  less 
  inclosed 
  marine 
  basin, 
  

   produced 
  by 
  the 
  passing 
  below 
  tide 
  level 
  of 
  a, 
  portion 
  of 
  the 
  

   rim 
  of 
  such 
  a 
  basin. 
  — 
  In 
  the 
  case 
  of 
  the 
  Oriskany 
  the 
  open- 
  

   ing 
  of 
  such 
  a 
  passage 
  from 
  the 
  interior 
  basin 
  to 
  the 
  northeast- 
  

   ward, 
  will 
  account 
  for 
  the 
  sudden 
  wash 
  of 
  a 
  great 
  mass 
  of 
  

   indiscriminate 
  sediments, 
  from 
  the 
  tide-swept, 
  depressed 
  land 
  

   into 
  the 
  basin 
  during 
  the 
  depression 
  which 
  was 
  marked 
  before 
  

   and 
  after 
  by 
  limestone-accumulations 
  throughout 
  the 
  northeast 
  

   bay. 
  This 
  will 
  explain 
  the 
  purity 
  of 
  the 
  Oriskany 
  fauna 
  and 
  

   sediments 
  only 
  in 
  the 
  northeast 
  corner 
  of 
  the 
  basin, 
  and 
  their 
  

   entire 
  absence 
  along 
  the 
  western 
  extension 
  of 
  the 
  great 
  basin, 
  

   from 
  which 
  direction, 
  in 
  the 
  absence 
  of 
  such 
  a 
  depression, 
  we 
  

   should 
  have 
  expected 
  them 
  to 
  arrive 
  into 
  the 
  waters 
  of 
  the 
  basin. 
  

  

  In 
  addition 
  to 
  these 
  three 
  geological 
  principles 
  there 
  are 
  a 
  

   few 
  biological 
  principles 
  which 
  have 
  been 
  adopted 
  as 
  working 
  

   hypotheses, 
  the 
  formulation 
  of 
  which 
  may 
  be 
  appropriate 
  in 
  

   this 
  connection. 
  

  

  A. 
  It 
  is 
  assumed 
  that 
  in 
  a 
  common 
  intercontinental 
  marine 
  

   basin, 
  such 
  as 
  evidently 
  existed 
  during 
  Devonian 
  time 
  over 
  

  

  . 
  this 
  region, 
  the 
  various 
  faunas 
  living 
  in 
  the 
  basin, 
  after 
  having 
  

   attained 
  a 
  biological 
  equilibrium 
  to 
  their 
  conditions 
  of 
  environ- 
  

   ment 
  and 
  to 
  each 
  other, 
  will 
  preserve 
  their 
  integrity 
  so 
  long 
  as 
  

   the 
  conditions 
  of 
  environment 
  remain 
  constant. 
  

  

  B. 
  The 
  passage 
  from 
  one 
  formation 
  to 
  another, 
  involving 
  a 
  

   change 
  of 
  fauna, 
  is, 
  therefore, 
  assumed 
  to 
  indicate 
  a 
  change 
  

   in 
  the 
  conditions 
  of 
  environment. 
  This 
  change 
  is 
  usually 
  ex- 
  

   pressed 
  in 
  terms 
  of 
  difference 
  in 
  the 
  character 
  of 
  the 
  sediments. 
  

  

  C. 
  These 
  changes 
  in 
  the 
  conditions 
  of 
  environment 
  may 
  be 
  

   of 
  two 
  kinds. 
  One 
  of 
  them 
  consists 
  in 
  oscillations 
  of 
  level, 
  

   gradually 
  shifting 
  the 
  shore 
  conditions, 
  but 
  consistent 
  with 
  a 
  

   movement 
  of 
  the 
  species 
  to 
  adjust 
  themselves 
  to 
  the 
  slightly 
  

   changing 
  conditions. 
  In 
  this 
  way 
  the 
  local 
  differences 
  of 
  a 
  

   common 
  general 
  fauna 
  may 
  be 
  explained. 
  In 
  this 
  case 
  it 
  is 
  

   probable 
  that 
  the 
  faunas 
  would 
  not 
  acquire 
  any 
  new 
  genera 
  not 
  

   already 
  living 
  in 
  the 
  basin, 
  but 
  the 
  readjustment 
  of 
  biological 
  

   equilibrium 
  might 
  result 
  in 
  the 
  evolution 
  of 
  new 
  varieties 
  

   and 
  possibly 
  new 
  species. 
  

  

  D. 
  A 
  second 
  kind 
  of 
  environmental 
  change 
  would 
  result 
  

   when 
  from 
  any 
  cause 
  the 
  relations 
  between 
  the 
  basins 
  as 
  a 
  

   whole 
  and 
  the 
  ocean 
  outside 
  were 
  changed. 
  Such 
  change 
  

   might 
  occur 
  by 
  the 
  opening 
  of 
  a 
  new 
  passage 
  to 
  the 
  exterior 
  or 
  

   by 
  the 
  closure 
  of 
  established 
  passages 
  resulting 
  in 
  deflection 
  of 
  

   currents 
  of 
  the 
  ocean. 
  But 
  the 
  biological 
  effect 
  would 
  be 
  of 
  a 
  

   different 
  nature 
  to 
  that 
  in 
  the 
  previous 
  case. 
  The 
  general 
  

   biological 
  equilibrium 
  would 
  be 
  broken. 
  Not 
  only 
  would 
  new 
  

   species 
  and 
  genera 
  be 
  introduced, 
  but 
  the 
  breaking 
  of 
  the 
  

   established 
  equilibrium 
  among 
  the 
  species 
  would 
  result 
  in 
  a 
  

   specific 
  advance 
  in 
  evolution 
  all 
  around 
  ; 
  and 
  we 
  might 
  expect, 
  

  

  

  