﻿18S 
  W. 
  T. 
  Lee 
  — 
  Type 
  Section 
  of 
  Morrison 
  Formation. 
  

  

  European 
  stratigraphers 
  refer 
  to 
  the 
  Upper 
  Cretaceous. 
  

   If 
  the 
  latter 
  reference 
  be 
  conceded, 
  the 
  Purgatoire 
  for- 
  

   mation 
  and 
  " 
  lower 
  Dakota" 
  of 
  Rocky 
  Mountain 
  locali- 
  

   ties 
  will 
  fall 
  at 
  the 
  base 
  of 
  the 
  Upper 
  Cretaceous. 
  The 
  

   abrupt 
  change 
  from 
  the 
  conglomeratic 
  "lower 
  Dakota" 
  

   to 
  the 
  underlying 
  Morrison 
  (even 
  at 
  the 
  town 
  of 
  Morri- 
  

   son, 
  if 
  the 
  Saurian 
  conglomerate 
  is 
  "lower 
  Dakota") 
  

   is 
  in 
  harmony 
  with 
  this 
  suggestion. 
  This 
  disposition 
  

   would 
  make 
  a 
  thin 
  but 
  complex 
  group 
  of 
  sandstones 
  and 
  

   shales 
  in 
  the 
  lower 
  part 
  of 
  the 
  redefined 
  Upper 
  Creta- 
  

   ceous 
  series. 
  

  

  Whatever 
  may 
  be 
  the 
  final 
  reference 
  of 
  the 
  group 
  

   after 
  revision 
  of 
  the 
  Kiowa 
  fauna 
  and 
  the 
  Dakota 
  flora, 
  

   it 
  now 
  seems 
  necessary 
  to 
  refer 
  it 
  wholly 
  to 
  one 
  series 
  

   or 
  to 
  the 
  other 
  rather 
  than 
  referring 
  some 
  of 
  the 
  mem- 
  

   bers 
  to 
  Lower 
  Cretaceous 
  and 
  others 
  to 
  Upper 
  Creta- 
  

   ceous. 
  It 
  has 
  been 
  the 
  custom 
  of 
  geologists, 
  including 
  

   myself, 
  to 
  postulate 
  a 
  withdrawal 
  of 
  the 
  Lower 
  Creta- 
  

   ceous 
  sea 
  from 
  the 
  Rocky 
  Mountain 
  region 
  at 
  the 
  end 
  

   of 
  Lower 
  Cretaceous 
  time 
  and 
  a 
  re-advance 
  at 
  the 
  begin- 
  

   ning 
  of 
  Upper 
  Cretaceous. 
  This 
  interpretation 
  depends 
  

   almost 
  wholly 
  on 
  the 
  acceptance 
  of 
  the 
  age 
  relations 
  as 
  

   determined 
  by 
  the 
  Kiowa 
  invertebrates. 
  From 
  a 
  con- 
  

   sideration 
  of 
  all 
  the 
  localities 
  known 
  to 
  me 
  in 
  Colorado 
  

   and 
  New 
  Mexico, 
  the 
  stratigraphic 
  relations 
  seem 
  best 
  

   explained 
  on 
  the 
  postulate 
  that 
  the 
  Dakota 
  group 
  re- 
  

   sulted 
  from 
  the 
  halting 
  and 
  perhaps 
  oscillating 
  advance 
  

   of 
  the 
  Cretaceous 
  sea. 
  As 
  such 
  it 
  constitutes 
  the 
  basal 
  

   part 
  of 
  the 
  Upper 
  Cretaceous 
  series. 
  This 
  classification 
  

   of 
  the 
  formations 
  would 
  restore 
  the 
  original 
  usage 
  of 
  the 
  

   term 
  Dakota 
  for 
  Colorado 
  localities 
  and 
  would 
  leave 
  the 
  

   restricted 
  Morrison 
  unconformably 
  below 
  Dakota 
  and 
  

   unconformable 
  above 
  Jurassic, 
  the 
  only 
  representative 
  

   of 
  the 
  long 
  epoch 
  between 
  Jurassic 
  and 
  Upper 
  Creta- 
  

   ceous. 
  

  

  