﻿394 
  H. 
  S. 
  Williams 
  — 
  Southern 
  Devonian 
  formations. 
  

  

  Trenton 
  or 
  even 
  lower 
  rocks. 
  The 
  remarkable 
  nature 
  of 
  this 
  

   contrast 
  is 
  emphasized 
  by 
  the 
  fact 
  that 
  the 
  two 
  extremes 
  are 
  

   both 
  within 
  a 
  common 
  intercontinental 
  basin. 
  

  

  In 
  order 
  to 
  understand 
  the 
  nature 
  of 
  the 
  problem 
  it 
  is 
  essen- 
  

   tial 
  to 
  consider 
  the 
  geographical 
  conditions 
  which 
  prevailed 
  

   during 
  the 
  Devonian 
  era 
  in 
  this 
  eastern 
  part 
  of 
  North 
  America. 
  

   Referring 
  to 
  the 
  chart 
  on 
  the 
  opposite 
  page, 
  it 
  will 
  be 
  observed 
  

   that 
  the 
  main 
  part 
  of 
  the 
  eastern 
  half 
  of 
  the 
  continent 
  south 
  

   of 
  the 
  great 
  lakes 
  and 
  west 
  of 
  the 
  ridge 
  of 
  the 
  Appalachians 
  

   was 
  a 
  Mediterranean 
  sea. 
  The 
  northern 
  shore 
  extended 
  from 
  

   Minnesota 
  to 
  eastern 
  New 
  York. 
  The 
  land 
  to 
  the 
  north 
  (L), 
  

   which 
  was 
  chiefly 
  Archaean, 
  may 
  be 
  called 
  Laurentia. 
  On 
  the 
  

   east 
  (A) 
  was 
  a 
  shore 
  line 
  extending 
  from 
  eastern 
  New 
  York 
  

   to 
  central 
  Alabama, 
  the 
  land 
  of 
  which 
  may 
  be 
  called 
  Appa- 
  

   lachia. 
  Indications 
  in 
  Arkansas, 
  Indian 
  territory 
  and 
  Texas 
  

   point 
  to 
  land 
  surfaces 
  along 
  the 
  gulf-border 
  states 
  (K, 
  T), 
  but 
  

   their 
  exact 
  extent, 
  and 
  whether 
  islands 
  or 
  continuous 
  shores 
  on 
  

   the 
  southern 
  border 
  of 
  the 
  sea, 
  is 
  uncertain. 
  From 
  the 
  north 
  

   a 
  wide 
  open 
  oceanic 
  channel 
  swept 
  from 
  the 
  Mackenzie 
  river 
  

   valley 
  region 
  across 
  British 
  America, 
  the 
  Dakotas, 
  Nebraska, 
  

   and 
  Kansas 
  far 
  into 
  and 
  through 
  the 
  western 
  Texas 
  region 
  to 
  

   the 
  south. 
  This 
  channel 
  was 
  bounded 
  on 
  the 
  west 
  by 
  the 
  

   extensive 
  Archaean 
  islands 
  or 
  edges 
  of 
  lands 
  constituting 
  

   the 
  eastern 
  axis 
  of 
  the 
  present 
  Rocky 
  Mountains 
  (R), 
  and 
  it 
  may 
  

   be 
  called 
  the 
  Dakota 
  channel. 
  

  

  From 
  the 
  northern 
  shore 
  of 
  the 
  interior 
  continental 
  sea, 
  over 
  

   what 
  is 
  now 
  Wisconsin, 
  a 
  peninsula 
  (W), 
  extended 
  southward 
  

   in 
  the 
  early 
  part 
  and, 
  perhaps, 
  the 
  closing 
  part 
  of 
  Devonian 
  

   time, 
  which 
  may 
  have 
  constituted 
  shallows 
  as 
  far 
  as 
  the 
  Mis- 
  

   souri 
  island 
  (M). 
  This 
  Missouri 
  island 
  occupied 
  considerable 
  

   of 
  the 
  southeastern 
  part 
  of 
  Missouri, 
  made 
  to 
  be 
  of 
  greater 
  or 
  

   lesser 
  extent 
  with 
  the 
  oscillations 
  of 
  level 
  which 
  occurred 
  dur- 
  

   ing 
  the 
  Devonian 
  era. 
  In 
  the 
  midst 
  of 
  the 
  intercontinental 
  

   sea 
  thus 
  constituted 
  there 
  was 
  a 
  low 
  ridge 
  (C), 
  sometimes 
  land, 
  

   sometimes 
  partly 
  land 
  and 
  partly 
  shallow 
  ridge, 
  under 
  tide 
  

   level, 
  which 
  extended 
  more 
  or 
  less 
  continuously 
  from 
  western 
  

   Ontario 
  to 
  central 
  Alabama; 
  this 
  may 
  be 
  called 
  the 
  Cincinnati 
  

   plateau. 
  The 
  rocks 
  forming 
  the 
  surface 
  of 
  this 
  plateau 
  were, 
  

   chiefly, 
  Ordovician 
  limestones, 
  while 
  the 
  shores 
  of 
  Appalachia 
  

   and 
  Laurentia 
  were 
  chiefly 
  of 
  Archaean 
  rocks. 
  

  

  In 
  studying 
  the 
  Devonian 
  system 
  of 
  this 
  interior 
  continental 
  

   area 
  most 
  of 
  the 
  differences 
  in 
  stratigraphy, 
  met 
  with 
  along 
  

   the 
  northern 
  border 
  and 
  the 
  northern 
  part 
  of 
  the 
  Appalachian 
  

   shore, 
  are 
  readily 
  interpreted 
  by 
  the 
  application 
  of 
  two 
  general 
  

   geological 
  principles. 
  These 
  are 
  the 
  following 
  : 
  (1) 
  the 
  dif- 
  

   ferences 
  between 
  limestones 
  on 
  the 
  one 
  hand 
  and 
  clastic 
  rocks, 
  

   composed 
  of 
  fragments 
  of 
  argillaceous, 
  arenaceous 
  or 
  conglom- 
  

   erate 
  nature, 
  are 
  accounted 
  for 
  by 
  the 
  different 
  origins 
  of 
  the 
  

  

  