﻿286 
  W. 
  H. 
  Tivenhofel 
  — 
  The 
  Comanchean 
  and 
  

  

  one, 
  with 
  perhaps 
  a 
  greater 
  tendency 
  toward 
  aridity. 
  

   The 
  invading 
  Comanchean 
  sea 
  covered 
  this 
  plain 
  and 
  

   great 
  banks 
  of 
  Gryphceas 
  flourished 
  in 
  the 
  shallow 
  

   waters. 
  The 
  advance 
  appears 
  to 
  have 
  been 
  oscillatory, 
  

   as 
  from 
  time 
  to 
  time 
  there 
  were 
  conditions 
  giving 
  rise 
  

   to 
  deposits 
  of 
  black 
  mud, 
  perhaps 
  laid 
  down 
  in 
  lagoons 
  

   developed 
  by 
  rivers 
  on 
  the 
  borders 
  of 
  the 
  sea. 
  In 
  these 
  

   black 
  muds, 
  decomposition 
  of 
  organic 
  matter, 
  probably 
  

   through 
  the 
  agency 
  of 
  sulphur 
  bacteria, 
  gave 
  rise 
  to 
  sul- 
  

   phur 
  gases 
  — 
  now 
  shown 
  in 
  the 
  abundant 
  nodules 
  of 
  

   pyrite 
  in 
  some 
  of 
  the 
  beds 
  — 
  bringing 
  about 
  the 
  elimina- 
  

   tion 
  of 
  the 
  bottom 
  life 
  which 
  had 
  existed 
  in 
  the 
  clearer 
  

   waters. 
  As 
  the 
  sea 
  advanced 
  farther 
  northward, 
  black 
  

   mud 
  deposition 
  decreased 
  on 
  this 
  portion 
  of 
  the 
  bottom 
  

   and 
  calcareous 
  sediments 
  attained 
  a 
  greater 
  importance. 
  

   As 
  the 
  sea 
  retired, 
  the 
  Spring 
  Creek 
  clays 
  and 
  Greenleaf 
  

   sandstones 
  were 
  deposited, 
  while, 
  on 
  the 
  succeeding 
  land, 
  

   streams 
  extended 
  their 
  channels 
  across 
  the 
  late 
  sea- 
  

   bottom 
  to 
  the 
  receding 
  waters 
  and 
  laid 
  down 
  the 
  Kirby 
  

   clays 
  and 
  Reeder 
  sandstones. 
  A 
  complete 
  cycle 
  of 
  sedi- 
  

   mentation 
  had 
  taken 
  place, 
  beginning 
  with 
  terrestrial 
  

   deposits, 
  running 
  through 
  the 
  gamut 
  of 
  marine 
  sedi- 
  

   ments, 
  and 
  ending 
  with 
  those 
  of 
  terrestrial 
  origin. 
  

  

  The 
  Mentor-Dakota 
  Sequence. 
  

  

  This 
  sequence 
  makes 
  its 
  first 
  appearance 
  north 
  of 
  the 
  

   great 
  tongue 
  of 
  overlapping 
  Tertiary 
  which 
  extends 
  

   across 
  the 
  southern 
  portion 
  of 
  the 
  western 
  half 
  of 
  Kan- 
  

   sas, 
  and 
  its 
  uppermost 
  portion 
  extends 
  northward 
  into 
  

   Dakota. 
  In 
  the 
  southern 
  limits 
  there 
  are 
  several 
  levels 
  

   of 
  marine 
  deposits 
  carrying 
  marine 
  fossils. 
  With 
  dis- 
  

   tance 
  northward 
  these 
  gradually 
  drop 
  out, 
  but 
  at 
  least 
  

   one 
  marine 
  horizon 
  extends 
  as 
  far 
  north 
  as 
  southeastern 
  

   South 
  Dakota. 
  The 
  origin 
  and 
  sequence 
  of 
  strata 
  in 
  this 
  

   portion 
  of 
  the 
  state 
  are 
  as 
  follows, 
  the 
  data 
  relating 
  to 
  

   the 
  uppermost 
  portions 
  being 
  almost 
  wholly 
  compiled 
  

   from 
  the 
  work 
  of 
  other 
  students 
  to 
  whom 
  citations 
  will 
  

   be 
  made 
  on 
  succeeding 
  pages. 
  

  

  Base 
  of 
  Benton, 
  marine. 
  

   Upper 
  Dakota 
  

  

  Gypsiferous 
  shales, 
  salt 
  lake 
  or 
  arm 
  of 
  sea 
  and 
  

  

  "terrestrial 
  deposits, 
  "Dakota" 
  flora 
  ( 
  ?) 
  10-20 
  feet 
  

  

  Saliferous 
  shales, 
  partly 
  and 
  perhaps 
  entirely 
  

  

  marine, 
  Washita 
  fauna 
  15-30 
  feet 
  

  

  

  