﻿174 
  T. 
  E. 
  Savage 
  — 
  Devonian 
  Formation 
  of 
  Illinois. 
  

  

  Devonian 
  sediments 
  were 
  laid 
  down. 
  Concealed 
  patches 
  

   of 
  these 
  rocks 
  may 
  be 
  present 
  in 
  southern 
  Illinois, 
  but 
  

   there 
  is 
  no 
  known 
  place 
  in 
  the 
  state 
  where 
  they 
  are 
  

   exposed. 
  

  

  Middle 
  Devonian. 
  

  

  Ulsterian 
  Series. 
  

  

  Clear 
  Creek 
  Chert. 
  — 
  In 
  places 
  in 
  Illinois 
  the 
  Clear 
  

   Creek 
  chert 
  rests 
  unconformably 
  upon 
  the 
  Back-bone 
  

   limestone, 
  and 
  in 
  others 
  it 
  lies 
  upon 
  the 
  Bailey 
  limestone. 
  

   It 
  corresponds 
  in 
  age 
  to 
  the 
  Camden 
  chert 
  of 
  Tennessee, 
  

   and 
  in 
  this 
  region 
  consists 
  in 
  large 
  part 
  of 
  a 
  succession 
  

   of 
  chert 
  layers 
  or 
  of 
  alternating 
  chert 
  and 
  limestone 
  

   layers, 
  3 
  to 
  8 
  inches 
  thick. 
  At 
  certain 
  levels 
  in 
  the 
  upper 
  

   half 
  of 
  the 
  formation 
  numerous 
  casts 
  and 
  molds 
  of 
  fos- 
  

   sils, 
  mostly 
  brachiopods, 
  occur 
  along 
  the 
  bedding 
  planes, 
  

   but 
  in 
  the 
  lower 
  part 
  of 
  the 
  formation 
  the 
  chert 
  is 
  more 
  

   massive 
  and 
  contains 
  few 
  or 
  no 
  fossils. 
  The 
  total 
  thick- 
  

   ness 
  of 
  this 
  chert 
  deposit 
  exceeds 
  300 
  feet. 
  The 
  more 
  

   common 
  species 
  of 
  fossils 
  in 
  the 
  formation 
  are 
  the 
  fol- 
  

   lowing: 
  Pholidops 
  terminate, 
  Schuchertella 
  pandora, 
  

   Chonostrophia 
  reversa, 
  Eodevonaria 
  arcuata, 
  Anoplia 
  

   nucleata, 
  Rhipidomella 
  cf. 
  musculosa, 
  Anoplotheca 
  fla- 
  

   bellites, 
  Centronella 
  glans-fagea, 
  Amphigenia 
  curta, 
  

   Spirifer 
  duodenarius, 
  S. 
  liemicyclus, 
  8. 
  macrothyris, 
  8. 
  

   ivorthenanus, 
  Reticularia 
  fimbriata, 
  Metaplasia 
  pyxi- 
  

   data, 
  Acidaspis 
  tuberculata 
  and 
  Pliacops 
  cristata. 
  The 
  

   uppermost 
  layers 
  of 
  chert 
  are 
  in 
  places 
  interbedded 
  with 
  

   sandstone 
  layers 
  containing 
  Onondaga 
  fossils 
  ; 
  the 
  chert 
  

   passing 
  up 
  into 
  the 
  sandstone 
  without 
  a 
  sedimentary 
  

   break. 
  In 
  a 
  previous 
  paper 
  by 
  the 
  writer 
  5 
  the 
  chert 
  

   formation 
  was 
  thought 
  to 
  be 
  of 
  Upper 
  Oriskany 
  age, 
  

   but 
  Dunbar 
  6 
  has 
  shown 
  that 
  in 
  western 
  Tennessee 
  the 
  

   Camden 
  chert, 
  which 
  is 
  equivalent 
  in 
  age 
  to 
  the 
  Clear 
  

   Creek 
  formation, 
  occurs 
  above 
  the 
  typical 
  Upper 
  Oris- 
  

   kany 
  strata 
  and 
  separated 
  from 
  them 
  by 
  a 
  sedimentary 
  

   break 
  of 
  considerable 
  magnitude. 
  Many 
  of 
  the 
  species 
  

   of 
  fossils 
  occurring 
  in 
  the 
  chert 
  are 
  peculiar 
  to 
  this 
  lower 
  

   Mississippi 
  embayment, 
  and 
  are 
  of 
  South 
  American 
  

   origin. 
  

  

  However, 
  there 
  are 
  a 
  few 
  species 
  in 
  this 
  fauna 
  that 
  

   can 
  be 
  considered 
  characteristic 
  of 
  the 
  Onondaga, 
  and 
  

  

  ' 
  5 
  This 
  Journal 
  (4), 
  25, 
  436, 
  1908. 
  

   c 
  Op. 
  cit., 
  pp. 
  749-753. 
  

  

  