﻿292 
  W. 
  H. 
  Twenhofel 
  — 
  The 
  Comanchean 
  and 
  

  

  eastern 
  South 
  Dakota 
  and 
  the 
  adjacent 
  parts 
  of 
  Nebraska 
  

   Meek 
  states 
  that 
  the 
  Dakota 
  has 
  a 
  thickness 
  of 
  400 
  feet 
  

   and 
  consists 
  of 
  sandstones 
  and 
  shales. 
  From 
  some 
  hori- 
  

   zon 
  or 
  horizons 
  of 
  this 
  sequence 
  have 
  been 
  collected 
  

   the 
  following 
  invertebrates 
  : 
  15 
  Arcopagellaf 
  macrodonta 
  

   Meek, 
  Cyrena 
  dakotensis 
  Meek, 
  Mactra 
  siouxensis 
  Meek, 
  

   Margaritana 
  nebraskensis 
  Meek, 
  Pharella 
  dakotaensis 
  

   Meek, 
  Trigonarca 
  siouxensis 
  Meek. 
  The 
  zone 
  containing 
  

   the 
  fossils 
  correlates 
  best 
  with 
  the 
  thin 
  sand 
  layers 
  in 
  

   the 
  upper 
  Dakota 
  of 
  Kansas. 
  The 
  shells 
  may 
  have 
  been 
  

   inhabitants 
  of 
  brackish 
  water, 
  but 
  not 
  of 
  fresh 
  water, 
  as 
  

   nearly 
  related 
  forms 
  occur 
  in 
  the 
  marine 
  Mentor. 
  

  

  In 
  the 
  Black 
  Hills 
  of 
  South 
  Dakota 
  Darton 
  found 
  16 
  that 
  

   the 
  lower 
  portion 
  of 
  what 
  had 
  previously 
  been 
  known 
  as 
  

   the 
  Dakota 
  in 
  reality 
  consisted 
  of 
  four 
  formations 
  or 
  

   members, 
  to 
  which, 
  from 
  the 
  base 
  upwards, 
  he 
  gave 
  the 
  

   names 
  of 
  Lakota, 
  Minnewaste, 
  Fuson 
  and 
  Dakota, 
  the 
  

   last 
  being 
  limited 
  to 
  those 
  strata 
  containing 
  the 
  leaves 
  

   of 
  dicotyledons 
  which 
  have 
  generally 
  been 
  considered 
  as 
  

   constituting 
  the 
  " 
  Dakota' 
  ' 
  flora. 
  The 
  Minnewaste 
  for- 
  

   mation 
  is 
  of 
  limestone. 
  No 
  marine 
  fossils 
  were 
  discov- 
  

   ered 
  in 
  any 
  of 
  the 
  formations, 
  so 
  it 
  is 
  assumed 
  that 
  none 
  

   is 
  present. 
  That 
  the 
  uppermost 
  unit 
  correlates 
  with 
  the 
  

   " 
  Dakota' 
  ' 
  of 
  Kansas 
  is 
  considered 
  very 
  probable. 
  The 
  

   three 
  lower 
  formations 
  are 
  regarded 
  as 
  of 
  Comanchean 
  

   age 
  ; 
  it 
  is 
  not 
  known, 
  however, 
  whether 
  they 
  have 
  a 
  time 
  

   equivalence 
  with 
  the 
  Washita. 
  

  

  Strata 
  which 
  are 
  the 
  equivalent 
  of 
  the 
  Washita 
  were 
  

   identified 
  about 
  fifteen 
  years 
  ago 
  in 
  Colorado 
  near 
  the 
  

   base 
  of 
  the 
  sandstones 
  which 
  had 
  previously 
  been 
  re- 
  

   ferred 
  to 
  the 
  " 
  Dakota. 
  " 
  1T 
  The 
  discovery 
  was 
  made 
  

   by 
  Stanton 
  and 
  Lee 
  in 
  a 
  section 
  on 
  the 
  Purgatoire 
  Eiver, 
  

   the 
  section 
  being 
  divided 
  and 
  correlated 
  as 
  follows 
  : 
  

  

  5. 
  Benton 
  shale 
  

  

  4. 
  Dakota 
  sandstone 
  100 
  feet 
  

  

  3. 
  Dark 
  shale 
  and 
  shaly 
  sandstone, 
  with 
  a 
  

  

  Washita 
  fauna 
  50-100 
  feet 
  

  

  2. 
  Coarse, 
  cross-laminated, 
  gray 
  sandstone 
  15-60 
  feet. 
  

  

  1. 
  Morrison 
  shale 
  

  

  15 
  F. 
  B. 
  Meek, 
  U. 
  S. 
  Geol. 
  Surv., 
  Terr., 
  9, 
  p. 
  XXV, 
  1876. 
  In 
  the 
  absence 
  

   of^ 
  definite 
  information 
  it 
  is 
  assumed 
  that 
  there 
  is 
  but 
  one 
  shell 
  zone 
  in 
  

   this 
  northern 
  occurrence. 
  This 
  assumption 
  may 
  be 
  in 
  error 
  and 
  there 
  may 
  

   be 
  more 
  than 
  one. 
  

  

  16 
  N. 
  H. 
  Darton, 
  U. 
  S. 
  Geol. 
  Surv., 
  21st 
  Ann. 
  Eept., 
  pt. 
  IV, 
  pp. 
  526-532, 
  

   1900. 
  

  

  17 
  T. 
  W. 
  Stanton, 
  Jour. 
  Geol., 
  13, 
  661-668, 
  1905. 
  

  

  