﻿64 
  PROCEEDINGS 
  OF 
  THE 
  GEOLOGICAL 
  SOCIETY. 
  

  

  territories 
  have 
  come 
  into 
  contact 
  and 
  require 
  to 
  be 
  denned. 
  If 
  good 
  

   natural 
  barriers, 
  such 
  as 
  a 
  mountain-range 
  or 
  a 
  broad 
  river 
  inter- 
  

   vene, 
  these 
  have 
  generally 
  been 
  taken 
  as 
  frontiers; 
  but 
  if 
  not, 
  

   frequently 
  after 
  much 
  dispute, 
  some 
  less 
  denned 
  line 
  has 
  been 
  

   adopted. 
  Substitute 
  for 
  colonies 
  strata 
  and 
  for 
  the 
  physical 
  boun- 
  

   daries 
  of 
  rivers 
  and 
  mountains 
  the 
  equally 
  well-marked 
  limits 
  cor- 
  

   responding 
  to 
  petrological 
  change, 
  often 
  accompanied 
  by 
  uncon- 
  

   formity, 
  and 
  the 
  history 
  of 
  the 
  repartition 
  of 
  the 
  stratigraphical 
  

   sequence 
  into 
  systems 
  and 
  stages 
  corresponds 
  to 
  the 
  subdivision 
  of 
  

   geographical 
  areas 
  into 
  states. 
  The 
  story 
  is 
  a 
  story 
  of 
  haphazard, 
  

   in 
  which 
  scientific 
  induction 
  has 
  played 
  a 
  subordinate 
  part, 
  and 
  the 
  

   idea 
  of 
  rendering 
  the 
  divisions 
  equal 
  has 
  probably 
  never 
  occurred 
  

   to 
  any 
  one. 
  

  

  There 
  is, 
  however, 
  this 
  difference 
  between 
  countries 
  and 
  strata. 
  

   The 
  mapping 
  out 
  of 
  the 
  former 
  covers 
  the 
  whole 
  area, 
  whilst 
  the 
  

   latter 
  in 
  any 
  and 
  every 
  country 
  can 
  only 
  represent 
  a 
  certain 
  portion, 
  

   probably 
  a 
  very 
  small 
  portion, 
  of 
  geological 
  time. 
  The 
  gaps 
  that 
  

   are 
  unrepresented 
  are 
  an 
  unknown 
  quantity, 
  but 
  are 
  undoubtedly 
  

   in 
  any 
  given 
  area 
  in 
  excess 
  of 
  that 
  portion 
  of 
  the 
  scale 
  that 
  is 
  re- 
  

   presented 
  by 
  strata. 
  Therefore 
  even 
  if 
  the 
  thickness 
  of 
  strata 
  in 
  

   any 
  subdivision 
  were 
  directly 
  proportional 
  to 
  the 
  duration 
  of 
  time 
  

   occupied 
  in 
  the 
  formation 
  of 
  that 
  subdivision 
  — 
  in 
  other 
  words, 
  if 
  the 
  

   thickness 
  of 
  a 
  system 
  varied 
  directly 
  with 
  the 
  duration 
  of 
  the 
  cor- 
  

   responding 
  period, 
  wbich 
  it 
  most 
  certainly 
  does 
  not, 
  the 
  duration 
  of 
  

   the 
  corresponding 
  division 
  on 
  the 
  time-scale 
  would 
  still 
  be 
  unknown, 
  

   as 
  the 
  time 
  corresponding 
  to 
  the 
  breaks 
  in 
  the 
  sequence 
  is 
  undeter- 
  

   minable. 
  

  

  2. 
  Under 
  these 
  circumstances, 
  and 
  seeing 
  that 
  anything 
  like 
  an 
  

   equalized 
  scale 
  is 
  hopeless 
  at 
  present, 
  there 
  is 
  a 
  fair 
  reason 
  for 
  

   accepting, 
  as 
  has 
  been 
  done 
  in 
  the 
  map 
  of 
  Europe, 
  those 
  periods 
  

   that 
  are 
  best 
  known. 
  In 
  the 
  ILesozoic 
  era, 
  for 
  instance, 
  in 
  which 
  

   the 
  relations 
  of 
  the 
  periods 
  are 
  simpler 
  than 
  in 
  the 
  Caanozoic 
  or 
  

   Palaeozoic, 
  there 
  can 
  be 
  very 
  little 
  question 
  that 
  a 
  division 
  of 
  both 
  

   Cretaceous 
  and 
  Jurassic 
  into 
  two 
  would 
  produce 
  periods 
  much 
  more 
  

   nearly 
  equal 
  to 
  the 
  Trias 
  in 
  palseontological 
  importance 
  (the 
  only 
  

   approach 
  to 
  a 
  time-test 
  that 
  we 
  possess) 
  than 
  are 
  the 
  great 
  

   Cretaceous 
  and 
  Jurassic 
  periods 
  as 
  at 
  present 
  usually 
  adopted. 
  

   This 
  subdivision 
  of 
  the 
  Mesozoic 
  era 
  into 
  five 
  periods, 
  Cretaceous, 
  

   Infra-cretaceous, 
  Oolitic, 
  Liassic, 
  and 
  Triassic, 
  has 
  been 
  adopted 
  by 
  

   several 
  geologists, 
  amongst 
  others 
  by 
  Renevier 
  and 
  by 
  De 
  Lapparent 
  

  

  