﻿204 
  H. 
  S. 
  Williams 
  — 
  Silurian- 
  Devonian 
  boundary 
  

  

  datum 
  planes 
  must 
  be 
  established, 
  whose 
  exact 
  correlation, 
  

   with 
  the 
  standard 
  section 
  in 
  the 
  country 
  where 
  the 
  time 
  

   boundary 
  in 
  each 
  case 
  was 
  first 
  defined, 
  is 
  known 
  with 
  pre- 
  

   cision. 
  Such 
  boundaries, 
  it 
  is 
  granted, 
  were 
  purely 
  arbitrary 
  

   in 
  the 
  first 
  place; 
  but 
  once 
  established 
  they 
  become, 
  like 
  

   Greenwich 
  or 
  the 
  Christian 
  era, 
  standards. 
  for 
  all 
  future 
  scien- 
  

   tific 
  uses. 
  

  

  The 
  fact 
  is 
  well 
  known 
  to 
  paleontologists 
  that 
  the 
  faunas 
  of 
  

   formations 
  of 
  the 
  same 
  geological 
  age 
  often 
  differ 
  widely 
  in 
  

   their 
  species 
  in 
  separate 
  regions 
  of 
  the 
  same 
  continent. 
  And 
  

   the 
  faunas 
  now 
  living 
  on 
  the 
  west 
  and 
  east 
  coasts 
  of 
  America, 
  

   under 
  like 
  conditions 
  of 
  temperature 
  and 
  environmental 
  con- 
  

   ditions, 
  contain 
  very 
  few 
  identical 
  species. 
  Even 
  in 
  the 
  same 
  

   sea 
  (as 
  the 
  fauna 
  off 
  Florida 
  and 
  that 
  off 
  New 
  England), 
  

   modern 
  faunas 
  differ 
  as 
  greatly 
  as 
  two 
  contiguous 
  faunas 
  of 
  

   the 
  geological 
  column. 
  These 
  facts 
  make 
  it 
  essential 
  for 
  the 
  

   student 
  of 
  evolution 
  to 
  distinguish 
  between 
  modifications 
  of 
  a 
  

   fauna 
  which 
  are 
  (a) 
  coordinate 
  with 
  the 
  change 
  of 
  place 
  (geo- 
  

   graphical 
  distribution), 
  and 
  those 
  (b) 
  coordinate 
  with 
  passage 
  of 
  

   time 
  (geological 
  range). 
  It 
  is 
  for 
  such 
  reasons 
  that 
  I 
  have 
  

   been 
  led 
  to 
  consider 
  the 
  determination 
  of 
  geological 
  boundaries 
  

   as 
  worthy 
  of 
  exhaustive 
  and 
  painstaking 
  investigation. 
  

  

  Such 
  methods 
  of 
  precision 
  were 
  applied 
  in 
  the 
  determination 
  

   of 
  the 
  American 
  equivalent 
  of 
  the 
  Cuboides 
  zone.* 
  And, 
  as 
  

   a 
  secondary 
  result 
  of 
  that 
  study 
  the 
  origin 
  of 
  the 
  Neodevonian 
  

   faunas 
  of 
  New 
  York 
  State 
  was 
  traced 
  to 
  a 
  northwestern 
  source, 
  

   while 
  the 
  Mesodevonian 
  faunas 
  were 
  found 
  to 
  be 
  more 
  closely 
  

   related 
  to 
  southern 
  faunas 
  (recognized 
  in 
  Brazil, 
  South 
  America) 
  

   and 
  not 
  identical 
  with 
  the 
  common 
  Mesodevonian 
  fauna 
  of 
  

   Europe. 
  

  

  Although 
  a 
  well-marked 
  biological 
  boundary 
  between 
  the 
  

   Devonian 
  and 
  Carboniferous 
  of 
  North 
  America 
  is 
  found, 
  

   where 
  Spirifer 
  disjunctus 
  is 
  replaced 
  by 
  Spirifer 
  marionensis, 
  

   and 
  Syringothyris 
  first 
  appears, 
  the 
  corresponding 
  boundary 
  in 
  

   standard 
  European 
  sections 
  is 
  so 
  indistinct 
  that 
  I 
  do 
  not 
  have 
  

   confidence 
  that 
  this 
  boundary 
  as 
  drawn 
  in 
  America 
  will 
  agree 
  

   precisely 
  with 
  the 
  European 
  standard. 
  

  

  The 
  discussion 
  of 
  the 
  Hercynian 
  problem 
  has 
  resulted 
  in 
  

   the 
  placing, 
  by 
  several 
  of 
  the 
  best 
  continental 
  European 
  pale- 
  

   ontologists, 
  of 
  the 
  Silurian-Devonian 
  boundary 
  below 
  the 
  

   Hercynian 
  fauna. 
  Kayser,f 
  followed 
  by 
  others, 
  has 
  adopted 
  

   the 
  opinion 
  that 
  the 
  American 
  equivalent 
  of 
  the 
  Hercynian 
  

   fauna 
  is 
  to 
  be 
  found 
  in 
  the 
  Lower 
  Helderberg. 
  

  

  * 
  The 
  Cuboides 
  Zone 
  and 
  its 
  Fauna: 
  a 
  discussion 
  of 
  Methods 
  of 
  Correlation. 
  

   Bull. 
  Geol. 
  Soc. 
  Amer., 
  vol. 
  x, 
  pp. 
  431-501, 
  pi. 
  xi-xiii, 
  1890. 
  

  

  f 
  Die 
  Fauna 
  der 
  altesten 
  Devon- 
  Ablagerungeu 
  der 
  Harzes. 
  Abhand. 
  geol. 
  

   special 
  Karte, 
  Preussen. 
  etc. 
  Band 
  ii, 
  heft 
  4, 
  pp. 
  284-285, 
  1878. 
  

  

  