﻿142 
  Williams 
  — 
  Devonian 
  Interval 
  in 
  northern 
  Arkansas. 
  

  

  of 
  the 
  St. 
  Clair 
  limestoiLe. 
  The 
  Sylaraore 
  limestone 
  (A2) 
  

   wears 
  away 
  faster 
  than 
  the 
  limestone 
  and 
  shows 
  a 
  retreat 
  in 
  the 
  

   ledge 
  of 
  exposed 
  rocks. 
  When 
  examined, 
  it 
  is 
  found 
  to 
  be 
  

   the 
  typical 
  Sylamore 
  sandstone 
  composed 
  of 
  polished 
  sand 
  

   grains, 
  and 
  pebbles 
  of 
  black 
  shale 
  and 
  fish-bone 
  fragments. 
  

   Immediately 
  above 
  it 
  comes, 
  without 
  apparent 
  change 
  in 
  the 
  

   plane 
  of 
  bedding, 
  the 
  St. 
  Joe 
  marhle 
  (A3) 
  containing 
  Carbon- 
  

   iferous 
  fossils. 
  

  

  Still 
  further 
  west 
  is 
  the 
  St. 
  Joe 
  section. 
  

  

  No. 
  1238. 
  St. 
  Joe, 
  Searcy 
  Co. 
  

  

  In 
  this 
  section, 
  the 
  green 
  shale, 
  associated 
  with 
  the 
  Sylamore 
  

   sandstone 
  (A4: 
  and 
  5), 
  contains 
  nodules 
  like 
  those 
  in 
  the 
  sand- 
  

   stone 
  ; 
  and 
  the 
  Lingulas 
  contained 
  in 
  them, 
  and 
  fragments 
  of 
  

   Lingulas 
  in 
  the 
  black 
  nodules, 
  point 
  to 
  a 
  common 
  origin 
  for 
  the 
  

   varying 
  kinds 
  of 
  deposit, 
  i. 
  e., 
  black 
  shale, 
  Sylamore 
  sandstone, 
  

   and 
  this 
  green 
  shale 
  containing 
  the 
  black 
  nodules. 
  A 
  piece 
  of 
  

   large 
  fish-plate 
  indicates 
  the 
  age 
  to 
  be 
  as 
  late 
  as 
  Devonian, 
  but, 
  

   in 
  itself, 
  does 
  not 
  make 
  certain 
  that 
  it 
  may 
  not 
  have 
  been 
  late 
  

   Devonian, 
  though 
  the 
  probability 
  is 
  that 
  it 
  belongs 
  below 
  the 
  

   latest 
  Devonian. 
  At 
  this 
  point 
  the 
  termination 
  of 
  the 
  Silurian 
  

   terrane 
  (A2) 
  is 
  Trenton 
  limestone 
  (Polk 
  Bayou 
  limestone). 
  

   The 
  interval 
  material 
  is, 
  first, 
  a 
  few 
  feet 
  of 
  green, 
  gritty 
  mud 
  

   shaly 
  rock 
  (A 
  4), 
  including 
  a 
  few 
  black, 
  'rounded, 
  hard 
  nodules, 
  

   its 
  upper 
  part 
  shaly, 
  followed 
  by 
  four 
  inches 
  of 
  Sylamore 
  

   sandstone 
  (A5), 
  with 
  its 
  characteristic 
  polished 
  siliceous 
  grains 
  

   and 
  black 
  worn 
  nodules. 
  The 
  nodules 
  are 
  slightly 
  calcareous, 
  

   due 
  either 
  to 
  infiltration 
  of 
  caleite, 
  or 
  included 
  fragments 
  of 
  

   shell. 
  Some 
  of 
  the 
  nodules 
  contain 
  pieces, 
  and 
  whole 
  shells 
  

   of 
  Lingulas, 
  but 
  the 
  structure 
  of 
  the 
  nodules 
  suggests 
  aggluti- 
  

   nation. 
  In 
  one 
  of 
  them 
  a 
  grain 
  of 
  the 
  polished 
  silica 
  is 
  

   enclosed 
  ; 
  and 
  in 
  another 
  the 
  nodule 
  is 
  made 
  up 
  of 
  fragments 
  

   of 
  shells 
  and 
  oolitic 
  grains. 
  The 
  sandstone 
  is 
  followed 
  above 
  

   by 
  a 
  compact, 
  red 
  Carboniferous 
  limestone 
  (A6) 
  of 
  thirty 
  feet 
  

   thickness. 
  The 
  under 
  surface 
  of 
  this 
  limestone, 
  where 
  it 
  came 
  

   in 
  contact 
  with 
  the 
  interval 
  sandstone, 
  is 
  penetrated 
  by, 
  or 
  con- 
  

   tains 
  black 
  nodules 
  like 
  those 
  below 
  ; 
  two 
  specimens 
  were 
  

   obtained 
  partially 
  surrounded 
  by 
  the 
  crystalline 
  limestone. 
  

   The 
  Lingula 
  is 
  like 
  a 
  minute 
  Lingula 
  spatulata 
  of 
  the 
  New 
  

   York 
  black 
  shales, 
  but 
  is 
  rather 
  shorter 
  and 
  more 
  solidly 
  

   formed 
  than 
  the 
  ordinary 
  type 
  of 
  L. 
  spatulata. 
  However, 
  the 
  

   variability 
  expressed 
  in 
  the 
  few 
  specimens 
  examined 
  leaves 
  no 
  

   mark 
  for 
  differentiating 
  it 
  specifically 
  from 
  Z. 
  s^patulata. 
  as 
  

   seen 
  in 
  sundry 
  localities 
  in 
  the 
  north. 
  

  

  In 
  one 
  piece 
  of 
  the 
  Sylamore 
  sandstone, 
  a 
  large 
  fragment 
  

   of 
  a 
  fish-plate 
  was 
  found, 
  reminding 
  one 
  of 
  the 
  plates 
  of 
  

   Dinichthys 
  of 
  the 
  Black 
  shales 
  of 
  Ohio. 
  But 
  the 
  piece 
  is 
  

   worn 
  and 
  crushed, 
  and 
  its 
  original 
  outlines 
  are 
  indistinguish- 
  

  

  