﻿H. 
  S. 
  Williams 
  — 
  Southern 
  Devonian 
  formations. 
  403 
  

  

  occasionally, 
  also 
  new 
  genera 
  to 
  arise 
  as 
  the 
  result 
  of 
  such 
  

   disturbance. 
  

  

  From 
  these 
  considerations 
  it 
  becomes 
  evident 
  that 
  geological 
  

   phenomena 
  affecting 
  the 
  topographical 
  conditions 
  of 
  the 
  surface 
  

   of 
  the 
  earth 
  have, 
  from 
  a 
  biological 
  point 
  of 
  view, 
  varying 
  

   values. 
  

  

  There 
  have 
  been, 
  in 
  the 
  course 
  of 
  geological 
  time, 
  move- 
  

   ments 
  of 
  the 
  earth's 
  crust 
  in 
  one 
  direction, 
  continuously, 
  for 
  

   sufficiently 
  long 
  periods 
  to 
  lift 
  to 
  mountain 
  proportions 
  the 
  

   edges 
  of 
  continents. 
  These 
  may 
  properly 
  be 
  called 
  geological 
  

   revolutions, 
  and 
  mark 
  such 
  grand 
  changes 
  as 
  separate 
  the 
  

   Paleozoic 
  from 
  the 
  Mesozoic, 
  and 
  the 
  Mesozoic 
  from 
  the 
  Ter- 
  

   tiary. 
  They 
  probably 
  disturbed 
  the 
  biological 
  equilibrium 
  

   of 
  the 
  marine 
  faunas 
  throughout 
  the 
  greater 
  part 
  of 
  the 
  earth's 
  

   surface. 
  

  

  There 
  occurred 
  lesser 
  changes, 
  such 
  as 
  the 
  supposed 
  case 
  of 
  

   the 
  Oriskany, 
  which 
  disturbed 
  the 
  equilibrium 
  of 
  the 
  faunas 
  

   within 
  confined 
  provinces, 
  destroying 
  old 
  species, 
  introducing 
  

   new 
  species, 
  and 
  otherwise 
  inducing 
  modification 
  of 
  the 
  species 
  

   which 
  survived. 
  These 
  were 
  geological 
  crises 
  and 
  were 
  local 
  

   in 
  their 
  effects. 
  

  

  The 
  lesser 
  changes, 
  resulting 
  in 
  rearranging 
  the 
  local 
  environ- 
  

   ment 
  along 
  a 
  common 
  shore, 
  or 
  in 
  a 
  common 
  basin, 
  were 
  of 
  less 
  

   consequence 
  and 
  were 
  of 
  more 
  or 
  less 
  continuous 
  operation, 
  

   causing 
  the 
  shifting 
  of 
  faunas 
  back 
  and 
  forth 
  with 
  resulting 
  

   slight 
  changes 
  in 
  the 
  varietal 
  and, 
  possibly, 
  specific 
  character 
  of 
  

   the 
  species, 
  but 
  chiefly 
  expressed 
  in 
  rearrangement 
  of 
  the 
  

   dominant 
  and 
  secondary 
  species, 
  in 
  the 
  abundance, 
  and 
  some- 
  

   times 
  the 
  presence 
  and 
  absence 
  of 
  species 
  from 
  a 
  fauna 
  whose 
  

   general 
  features 
  remained 
  intact. 
  

  

  To 
  apply 
  these 
  biological 
  principles 
  to 
  the 
  interpretation 
  of 
  

   geological 
  events 
  it 
  is 
  proposed 
  that, 
  while 
  occasional 
  modifica- 
  

   tion 
  of 
  forms 
  of 
  specific 
  value 
  may 
  have 
  occurred 
  during 
  the 
  

   ordinary 
  shifting 
  of 
  local 
  conditions 
  marking 
  different 
  epochs, 
  

   the 
  crises 
  which 
  disturbed 
  the 
  biological 
  equilibrium 
  of 
  exist- 
  

   ing 
  marine 
  faunas 
  were 
  the 
  times 
  when 
  the 
  chief 
  modifications 
  

   of 
  specific 
  and 
  generic 
  value 
  took 
  place 
  ; 
  the 
  interval 
  between 
  

   two 
  such 
  crises 
  represents 
  a 
  biological 
  period. 
  Those 
  changes, 
  

   marked 
  by 
  the 
  extinction 
  of 
  genera 
  and 
  the 
  introduction 
  of 
  

   new 
  types 
  of 
  family 
  or 
  higher 
  rank 
  among 
  marine 
  species, 
  

   were 
  of 
  the 
  nature 
  of 
  revolutions, 
  in 
  which 
  vast 
  changes 
  were 
  

   made 
  in 
  the 
  surface 
  conditions 
  of 
  the 
  earth, 
  particularly 
  in 
  the 
  

   way 
  of 
  lifting 
  tracts 
  of 
  the 
  ocean 
  bottom 
  to 
  a 
  permanent 
  sub- 
  

   aqueous 
  position 
  and 
  in 
  separating 
  the 
  great 
  geological 
  eras 
  from 
  

   one 
  another. 
  Hence 
  in 
  the 
  study 
  of 
  the 
  geological 
  history 
  of 
  

   the 
  earth 
  the 
  crises, 
  as 
  well 
  as 
  the 
  revolutions, 
  become 
  of 
  prime 
  

   importance, 
  and 
  in 
  studying 
  the 
  relations 
  of 
  fossil 
  faunas 
  it 
  is 
  

   important 
  to 
  discover 
  the 
  territory 
  affected 
  by 
  a 
  crisis, 
  and 
  the 
  

   particular 
  locality 
  where 
  the 
  effect 
  of 
  the 
  crisis 
  was 
  centered. 
  

  

  