﻿210 
  GEOLOGY. 
  

  

  plants 
  or 
  animals 
  of 
  existing 
  species, 
  unmixed 
  with 
  any 
  

   that 
  are 
  extinct." 
  

  

  Some 
  geologists 
  include 
  under 
  the 
  Quaternary 
  division 
  

   some 
  of 
  the 
  upper 
  strata 
  of 
  the 
  Tertiary; 
  but 
  in 
  a 
  more 
  

   restricted 
  and 
  accepted 
  sense, 
  it 
  will 
  be 
  used 
  here 
  to 
  

   comprehend 
  only 
  the 
  Post 
  Pliocene 
  deposits, 
  and 
  will 
  

   include 
  the 
  modern 
  diluvian 
  or 
  northern 
  drift, 
  which 
  

   here 
  separates 
  the 
  Modern 
  group 
  from 
  the 
  Tertiary 
  ; 
  the 
  

   ferruginous 
  sands 
  and 
  clays, 
  presumed 
  to 
  have 
  had 
  a 
  

   contemporaneous 
  origin 
  ; 
  the 
  recent 
  lacustrine 
  marls, 
  

   the 
  series 
  closing 
  with 
  the 
  alluvium 
  of 
  the 
  surface. 
  

  

  The 
  foregoing 
  description 
  will 
  be 
  better 
  understood 
  by 
  

   referring 
  to 
  the 
  section 
  of 
  geological 
  strata 
  represented 
  in 
  

   Plate 
  IX., 
  in 
  which, 
  however, 
  many 
  of 
  the 
  subordinate 
  

   strata, 
  according 
  to 
  the 
  different 
  systems, 
  have 
  been 
  

   omitted 
  as 
  not 
  essential 
  to 
  the 
  illustration. 
  

  

  Although 
  all 
  the 
  foregoing 
  principal 
  divisions 
  or 
  for- 
  

   mations 
  are 
  found 
  to 
  exist 
  in 
  all 
  the 
  countries 
  that 
  have 
  

   been 
  geologically 
  explored, 
  it 
  is 
  not 
  to 
  be 
  inferred 
  that 
  

   all 
  the 
  strata 
  of 
  which 
  they 
  are 
  composed 
  pervade 
  the 
  

   whole 
  globe. 
  Many 
  of 
  them, 
  on 
  the 
  contrary, 
  are 
  absent 
  

   in 
  different 
  countries, 
  owing 
  to 
  the 
  relative 
  position 
  of 
  

   the 
  earth's 
  surface, 
  and 
  the 
  distribution 
  of 
  land 
  and 
  

   water 
  at 
  the 
  period 
  of 
  their 
  deposition. 
  They 
  are 
  repre- 
  

   sented, 
  therefore, 
  in 
  the 
  order 
  they 
  assume 
  in 
  point 
  of 
  

   time, 
  being 
  governed 
  by 
  the 
  mutations 
  which 
  the 
  earth 
  

   has 
  undergone, 
  and 
  the 
  paroxysms 
  of 
  upheaval 
  and 
  sub- 
  

   sidence, 
  more 
  or 
  less 
  active 
  and 
  general, 
  which, 
  at 
  remote 
  

   and 
  widely 
  separated 
  periods, 
  have 
  repeatedly 
  occurred. 
  

  

  But, 
  although 
  important 
  strata, 
  or 
  even 
  entire 
  groups, 
  

   may 
  be 
  missing 
  or 
  non-existent 
  in 
  some 
  quarters, 
  in 
  none 
  

   is 
  their 
  relative 
  order 
  inverted 
  or 
  transposed; 
  but 
  they 
  

   maintain 
  an 
  undeviating 
  succession 
  consistent 
  with 
  the 
  

  

  