﻿336 
  0. 
  C. 
  Marsh 
  — 
  Geological 
  Horizons 
  as 
  

  

  Art. 
  XXXIII. 
  — 
  Geological 
  Horizons 
  as 
  determined 
  by 
  

   Vertebrate 
  Fossils 
  ;* 
  by 
  O. 
  C. 
  Marsh. 
  With 
  Plate 
  XII. 
  

  

  In 
  1877, 
  the 
  author 
  endeavored 
  to 
  bring 
  together 
  some 
  

   results 
  of 
  his 
  researches 
  in 
  the 
  Rocky 
  mountain 
  region 
  and 
  

   in 
  other 
  parts 
  of 
  the 
  country, 
  relating 
  to 
  the 
  succession 
  of 
  

   vertebrate 
  life.f 
  This 
  led 
  to 
  a 
  comparison 
  of 
  the 
  relative 
  value 
  

   of 
  the 
  three 
  different 
  groups 
  of 
  fossils 
  ; 
  plants, 
  invertebrates, 
  

   and 
  vertebrates, 
  in 
  marking 
  geological 
  time. 
  In 
  examining 
  

   the 
  subject 
  with 
  some 
  care, 
  the 
  author 
  found 
  that, 
  for 
  this 
  

   purpose, 
  plants 
  are 
  not 
  satisfactory 
  witnesses 
  ; 
  that 
  invertebrate 
  

   animals 
  are 
  much 
  better 
  ; 
  but 
  that 
  vertebrates 
  afford 
  the 
  most 
  

   reliable 
  evidence 
  of 
  climatic 
  and 
  other 
  geological 
  changes. 
  

   The 
  subdivisions 
  of 
  the 
  latter 
  group, 
  and, 
  in 
  fact, 
  all 
  forms 
  of 
  

   animal 
  life, 
  are 
  of 
  value 
  in 
  this 
  respect, 
  mainly 
  according 
  to 
  

   the 
  perfection 
  of 
  their 
  organization, 
  or 
  zoological 
  rank. 
  Fishes, 
  

   for 
  example, 
  are 
  but 
  slightly 
  affected 
  by 
  changes 
  that 
  would 
  

   destroy 
  Reptiles 
  or 
  Birds, 
  and 
  the 
  higher 
  Mammals 
  succumb 
  

   under 
  influences 
  that 
  the 
  lower 
  forms 
  pass 
  through 
  in 
  safety. 
  

   The 
  special 
  applications 
  of 
  this 
  general 
  law, 
  and 
  its 
  value 
  in 
  

   geology, 
  readily 
  suggest 
  themselves. 
  

  

  In 
  accordance 
  with 
  this 
  principle, 
  the 
  author 
  next 
  attempted 
  

   to 
  define 
  the 
  principal 
  geological 
  horizons 
  in 
  the 
  West 
  which 
  

   he 
  had 
  personally 
  investigated, 
  and 
  then 
  taking 
  in 
  each 
  the 
  

   largest 
  and 
  most 
  dominant 
  vertebrate 
  form 
  which 
  characterized 
  

   it, 
  used 
  the 
  name 
  for 
  the 
  horizon. 
  In 
  the 
  same 
  way, 
  some 
  of 
  

   the 
  principal 
  horizons 
  of 
  the 
  East 
  were 
  named, 
  and 
  the 
  whole 
  

   brought 
  together 
  in 
  a 
  section 
  to 
  illustrate 
  vertebrate 
  life 
  in 
  

   America.;): 
  

  

  The 
  names 
  thus 
  given 
  to 
  various 
  horizons 
  were 
  not 
  intended 
  

   to 
  replace 
  those 
  already 
  . 
  applied, 
  but 
  merely 
  to 
  supplement 
  

   them, 
  and 
  by 
  new 
  evidence, 
  to 
  clear 
  up 
  those 
  in 
  doubt. 
  The 
  

   same 
  principle 
  had 
  long 
  before 
  been 
  found 
  to 
  work 
  admirably 
  

   in 
  Europe, 
  where 
  certain 
  characteristic 
  invertebrate 
  fossils, 
  

   especially 
  Ammonites, 
  had 
  served 
  to 
  mark 
  definitely 
  various 
  

   subdivisions 
  of 
  a 
  single 
  formation. 
  The 
  wider 
  application 
  of 
  

   the 
  principle 
  to 
  vertebrate 
  fossils, 
  from 
  their 
  earliest 
  known 
  

   appearance 
  to 
  the 
  present 
  time, 
  has 
  already 
  helped 
  to 
  complete 
  

   the 
  record 
  of 
  vertebrate 
  life 
  in 
  America, 
  and 
  rendered 
  an 
  

   equal 
  service 
  to 
  systematic 
  geology. 
  

  

  Since 
  this 
  method 
  of 
  defining 
  geological 
  horizons 
  by 
  vertebrate 
  

   fossils 
  was 
  first 
  used 
  by 
  the 
  author 
  in 
  1877, 
  many 
  important 
  

  

  * 
  Abstract 
  of 
  Communication 
  made 
  to 
  the 
  International 
  Geological 
  Congress 
  

   Washington, 
  D. 
  C, 
  August 
  28th, 
  1891. 
  

  

  f 
  Introduction 
  and 
  Succession 
  of 
  Vertebrate 
  Life 
  in 
  America. 
  Address 
  before 
  

   the 
  American 
  Association 
  for 
  the 
  Advancement 
  of 
  Science, 
  Nashville, 
  Tenn., 
  

   August 
  30, 
  1877. 
  

  

  \ 
  The 
  same 
  address, 
  Frontispiece. 
  

  

  