﻿100 
  H. 
  S. 
  Williams 
  — 
  Recurrence 
  of 
  Devonian 
  Fossils, 
  etc. 
  

  

  ciated 
  with 
  a 
  generalized 
  as 
  contrasted 
  with 
  a 
  specialized 
  fauna. 
  

  

  The 
  faunas 
  of 
  the 
  Devonian 
  and 
  Carboniferous 
  in 
  Nevada 
  

   and 
  California 
  are 
  peculiar 
  for 
  showing 
  a 
  very 
  long 
  continu- 
  

   ance 
  of 
  the 
  same 
  general 
  fauna, 
  with 
  changes 
  by 
  the 
  accession 
  

   of 
  new 
  species 
  as 
  time 
  progressed, 
  but 
  with 
  remarkable 
  persist- 
  

   ence 
  of 
  early 
  species 
  unchanged. 
  Walcott 
  called 
  attention 
  to 
  

   the 
  fact 
  in 
  his 
  Nevada 
  faunas. 
  The 
  great 
  thickness 
  of 
  Devon- 
  

   ian 
  rocks 
  shows 
  not 
  only 
  long 
  range 
  of 
  species 
  but 
  such 
  ming- 
  

   ling 
  as 
  to 
  bring 
  species 
  supposed 
  to 
  be 
  characteristic 
  of 
  Upper 
  

   Devonian 
  in 
  the 
  east 
  down 
  at 
  the 
  base 
  of 
  the 
  series 
  in 
  Nevada. 
  

   In 
  the 
  Appalachian 
  series 
  we 
  find 
  the 
  opposite 
  to 
  be 
  the 
  

   fact, 
  the 
  faunas 
  are 
  much 
  broken 
  up 
  into 
  distinct 
  zones, 
  with 
  

   very 
  few 
  species 
  tying 
  together 
  the 
  successive 
  faunas. 
  And 
  

   this 
  breaking 
  up 
  of 
  the 
  faunas 
  is 
  plainly 
  associated 
  with 
  

   oscillating 
  levels 
  and 
  general 
  passage 
  from 
  pure 
  marine 
  condi- 
  

   tions 
  up 
  to 
  brackish 
  water, 
  and 
  finally 
  by 
  the 
  closing 
  of 
  sedi- 
  

   mentation 
  upon 
  elevation 
  of 
  the 
  bottom 
  above 
  the 
  sea-level. 
  

   This 
  is 
  shown 
  by 
  the 
  passage 
  from 
  limestones 
  of 
  the 
  Lower 
  

   Devonian 
  into 
  shale, 
  sandstone 
  and 
  conglomerate 
  — 
  in 
  the 
  suc- 
  

   cession 
  of 
  deposits 
  that 
  followed. 
  

  

  Taking 
  the 
  first 
  full 
  appearance 
  of 
  a 
  characteristic 
  fauna 
  as 
  

   indicative 
  of 
  common 
  horizon 
  for 
  each 
  of 
  the 
  separate 
  prov- 
  

   inces, 
  we 
  observe 
  that 
  in 
  the 
  Appalachian 
  province 
  the 
  

   Devonian 
  species 
  are 
  more 
  closely 
  restricted 
  to 
  the 
  Devonian 
  

   age. 
  In 
  the 
  Mississippi 
  province, 
  after 
  the 
  Devonian 
  stage 
  is 
  

   sharply 
  closed, 
  this 
  case 
  of 
  recurrence 
  is 
  seen 
  well 
  up 
  in 
  Car- 
  

   boniferous 
  faunal 
  succession, 
  while 
  in 
  the 
  western 
  continental 
  

   border 
  there 
  was 
  a 
  mixture 
  of 
  Devonian 
  and 
  Carboniferous 
  

   species 
  all 
  along 
  the 
  way 
  till 
  the 
  characteristic 
  Carboniferous 
  

   species 
  were 
  present 
  in 
  full 
  force 
  in 
  the 
  prevalent 
  fauna. 
  

   This 
  later 
  and 
  later 
  removal 
  of 
  the 
  earlier 
  types 
  from 
  the 
  

   prevalent 
  faunas 
  as 
  we 
  pass 
  westward 
  across 
  the 
  continent 
  

   is 
  coordinate 
  with 
  the 
  continental 
  expansion 
  occurring 
  at 
  the 
  

   same 
  time. 
  The 
  conclusion 
  seems 
  evident 
  that 
  the 
  cutting 
  

   off 
  of 
  the 
  Devonian 
  species 
  was 
  in 
  some 
  way 
  associated 
  with 
  

   the 
  progress 
  of 
  the 
  continental 
  border 
  westward 
  during 
  later 
  

   Devonian 
  and 
  the 
  first 
  half 
  of 
  Carboniferous 
  time. 
  The 
  

   coal 
  measure 
  conditions, 
  were 
  as 
  early 
  as 
  the 
  Warsaw 
  and 
  

   St. 
  Louis 
  limestones 
  in 
  Pennsylvania, 
  if 
  not 
  still 
  earlier, 
  and 
  

   limestone 
  had 
  ceased 
  to 
  be 
  a 
  factor 
  in 
  the 
  deposits 
  in 
  northern 
  

   Pennsylvania 
  and 
  New 
  York, 
  before 
  the 
  Chemung 
  began. 
  

   In 
  the 
  Mississippi 
  Valley 
  limestones 
  continue 
  up 
  to 
  the 
  Ches- 
  

   ter, 
  and 
  then 
  the 
  sands, 
  conglomerate 
  and 
  coal 
  measure 
  con- 
  

   dition 
  suddenly 
  appear. 
  

  

  Before 
  we 
  reach 
  the 
  central 
  ridges 
  of 
  the 
  Rocky 
  Mountains 
  

   the 
  limestone 
  conditions 
  are 
  prevalent 
  till 
  the 
  upper 
  coal 
  meas- 
  

   ure 
  time, 
  and 
  still 
  further 
  west 
  there 
  are 
  no 
  indications 
  of 
  any 
  

  

  