﻿172 
  T. 
  E. 
  Savage 
  — 
  Devonian 
  Formation 
  of 
  Illinois. 
  

  

  Lower 
  Devonian. 
  

  

  Helderbergian 
  Series. 
  

  

  The 
  oldest 
  rocks 
  belonging 
  to 
  the 
  Devonian 
  system 
  in 
  

   Illinois 
  are 
  of 
  Helderbergian 
  age 
  and 
  correspond 
  in 
  time 
  

   to 
  some 
  part 
  of 
  the 
  New 
  Scotland 
  formation 
  of 
  the 
  New 
  

   York 
  section. 
  These 
  strata 
  consist 
  of 
  rather 
  thin- 
  

   bedded, 
  cherty, 
  argillaceous 
  limestones, 
  about 
  90 
  feet 
  

  

  Fig. 
  1. 
  

  

  Fig. 
  1. 
  — 
  Helderberg 
  (Bailey) 
  limestone 
  in 
  the 
  east 
  bluff 
  of 
  the 
  Missis- 
  

   sippi 
  river, 
  near 
  the 
  northwest 
  corner 
  of 
  Union 
  Co., 
  Illinois. 
  

  

  thick. 
  Strata 
  of 
  corresponding 
  age 
  have 
  also 
  been 
  de- 
  

   scribed 
  by 
  Weller 
  3 
  in 
  Ste. 
  Genevieve 
  County, 
  Missouri. 
  

   In 
  places 
  in 
  Illinois 
  these 
  rocks 
  are 
  unconformably 
  

   overlain 
  by 
  rather 
  thick-bedded 
  c 
  v, 
  sely 
  crystalline 
  

   limestones, 
  which 
  are 
  probably 
  equivai^. 
  + 
  o 
  some 
  part 
  

   of 
  the 
  Becraft 
  limestone 
  of 
  New 
  York, 
  ana 
  thick- 
  

  

  3 
  Weller, 
  Stuart: 
  BuU. 
  Geol. 
  Soc. 
  America, 
  25, 
  135-136, 
  l[ 
  : 
  l\ 
  

  

  