﻿162 
  Key 
  €8 
  — 
  Carboniferous 
  and 
  Devonian 
  Strata. 
  

  

  from 
  Louisiana 
  these 
  shales 
  rapidly 
  become 
  thicker. 
  At 
  Han- 
  

   nibal 
  they 
  measure 
  20 
  feet 
  in 
  thickness 
  ; 
  at 
  Keokuk, 
  probably 
  

   not 
  less 
  than 
  50 
  feet 
  ; 
  beyond, 
  they 
  merge 
  with 
  the 
  Hannibal 
  

   shales. 
  

  

  The 
  Grassy 
  black 
  shales* 
  are 
  only 
  four 
  feet 
  thick 
  at 
  Louis- 
  

   iana. 
  They 
  attain 
  a 
  greater 
  vertical 
  measurement 
  northward. 
  

   Before 
  disappearing 
  below 
  river-level 
  in 
  the 
  Keokuk 
  syncline, 
  

   they 
  reach 
  a 
  thickness 
  of 
  30 
  feet. 
  In 
  well-sections 
  at 
  Keokuk 
  

   they 
  have 
  not 
  been 
  definitely 
  recognized 
  or 
  separated 
  from 
  

   the 
  associated 
  shales. 
  At 
  Morning 
  Sun, 
  north 
  of 
  Burling- 
  

   ton, 
  they 
  are 
  distinctly 
  present 
  in 
  a 
  number 
  of 
  deep-well 
  

   sections. 
  They 
  have 
  been 
  traced 
  farther 
  north 
  to 
  beyond 
  

   Muscatine, 
  where 
  Uddenf 
  has 
  given 
  them 
  the 
  title 
  of 
  Sweet- 
  

   land 
  beds. 
  Here 
  they 
  are 
  45 
  feet 
  thick; 
  rest 
  in 
  notable 
  uncon- 
  

   formity 
  upon 
  the 
  Cedar 
  limestones; 
  and 
  have 
  resting 
  upon 
  

   them 
  unconformably 
  the 
  Des 
  Moines 
  coal 
  measures. 
  

  

  Below 
  the 
  black 
  shales 
  there 
  are 
  still 
  other 
  blue 
  shales. 
  

   They 
  are 
  not 
  exposed 
  above 
  river-level 
  at 
  either 
  end 
  of 
  the 
  

   syncline 
  ; 
  but 
  as 
  shown 
  in 
  deep-well 
  sections, 
  at 
  Keokuk, 
  there 
  

   are 
  at 
  least 
  125 
  feet 
  referable 
  to 
  them 
  ; 
  at 
  Burlington, 
  about 
  

   100 
  feet 
  ; 
  and 
  at 
  Morning 
  Sun, 
  50 
  feet. 
  When 
  the 
  Iowa;); 
  and 
  

   Missouri 
  § 
  reports 
  were 
  printed 
  it 
  was 
  surmised 
  that 
  this 
  part 
  of 
  

   the 
  great 
  shale 
  section 
  at 
  Burlington 
  rested 
  directly 
  upon, 
  or 
  

   was 
  an 
  integral 
  portion 
  of 
  the 
  shales 
  called 
  farther 
  north 
  the 
  

   Lime 
  Creek 
  formation. 
  Since 
  that 
  time 
  this 
  view 
  has 
  proved 
  

   to 
  be 
  really 
  correct. 
  The 
  shales 
  in 
  question 
  actually 
  continue 
  

   in 
  full 
  development 
  to 
  the 
  Minnesota 
  boundary. 
  They 
  rest 
  

   on 
  the 
  Callaway 
  limestone 
  in 
  Missouri, 
  which 
  appears 
  to 
  be 
  

   the 
  exact 
  equivalent 
  of 
  the 
  Cedar 
  limestone 
  in 
  Iowa. 
  

  

  The 
  Grassy 
  shales 
  are 
  of 
  exceptional 
  interest 
  since, 
  in 
  spite 
  

   of 
  their 
  associated 
  faunal 
  affinities, 
  they 
  probably 
  represent 
  the 
  

   basal 
  member 
  of 
  the 
  Carboniferous 
  section 
  of 
  the 
  Upper 
  

   Mississippi 
  region. 
  At 
  Louisiana 
  these 
  shales 
  recline 
  directly 
  

   upon 
  Silurian 
  limestones. 
  A 
  few 
  miles 
  away 
  they 
  lie 
  im- 
  

   mediately 
  upon 
  the 
  Callaway 
  (Devonian) 
  limestone. 
  Farther 
  

   on 
  the 
  Lime 
  Creek 
  shales 
  are 
  found 
  immediately 
  beneath. 
  At 
  

   their 
  base, 
  therefore, 
  a 
  marked 
  unconformity 
  exists, 
  which 
  is 
  

   also 
  well 
  displayed 
  at 
  the 
  north, 
  above 
  Muscatine. 
  

  

  The 
  present 
  correlation 
  of 
  the 
  Grassy 
  black 
  shales 
  seems 
  to 
  

   set 
  at 
  rest 
  several 
  moot 
  questions. 
  They, 
  doubtless, 
  represent 
  

   the 
  Chattanooga 
  black 
  shales 
  which 
  in 
  the 
  south 
  constitute, 
  

   according 
  to 
  Schuchert,|] 
  the 
  base 
  of 
  the 
  Mississippian 
  section. 
  

   They 
  are 
  not 
  to 
  be 
  regarded 
  as 
  Devonian 
  in 
  age, 
  as 
  suggested 
  

  

  *Proc. 
  Towa 
  Acad. 
  Sci., 
  vol. 
  v, 
  p. 
  60, 
  1898. 
  

  

  flowa 
  Geol. 
  Surv., 
  vol. 
  ix, 
  p. 
  289, 
  1899. 
  

  

  \ 
  Iowa 
  Geol. 
  Surv., 
  vol. 
  i, 
  p. 
  55, 
  1893. 
  

  

  § 
  Missouri 
  Geol. 
  Surv., 
  vol. 
  iv, 
  p. 
  56, 
  1894. 
  

  

  || 
  Bull. 
  Geol. 
  Soc. 
  America, 
  vol. 
  xx, 
  p. 
  548, 
  1910. 
  

  

  