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  DR. 
  J. 
  CROLL 
  ON 
  PREVAILING 
  MISCONCEPTIONS 
  REGARDING 
  

  

  13. 
  On 
  prevailing 
  Misconceptions 
  regarding 
  the 
  Evidence 
  wlxich 
  

   ive 
  ought 
  to 
  expect 
  of 
  Former 
  Glacial 
  Periods. 
  By 
  James 
  

   Croll, 
  LL.D., 
  F.R.S. 
  (Read 
  January 
  23, 
  1889.) 
  

  

  (Communicated 
  by 
  Prof. 
  Bonney, 
  D.Sc, 
  LL.D., 
  F.E.S., 
  F.G.S.) 
  

  

  Within 
  the 
  whole 
  range 
  of 
  geological 
  science 
  there 
  is 
  perhaps 
  not 
  

   a 
  point 
  on 
  which 
  a 
  greater 
  amount 
  of 
  misapprehension 
  prevails 
  than 
  

   in 
  regard 
  to 
  the 
  evidence 
  which 
  we 
  ought 
  to 
  expect 
  of 
  former 
  

   glacial 
  periods. 
  The 
  imperfection 
  of 
  geological 
  records 
  is 
  far 
  

   greater 
  than 
  is 
  generally 
  believed 
  — 
  so 
  great, 
  indeed, 
  that 
  the 
  mere 
  

   absence 
  of 
  direct 
  geological 
  evidence 
  can 
  hardly 
  be 
  regarded 
  as 
  

   sufficient 
  proof 
  that 
  the 
  conclusions 
  derived 
  from 
  astronomical 
  and 
  

   physical 
  considerations 
  regarding 
  former 
  ice-periods 
  are 
  improbable. 
  

   Nor 
  is 
  this 
  all. 
  Not 
  only 
  are 
  the 
  geological 
  records 
  of 
  ancient 
  

   glacial 
  conditions 
  imperfect, 
  but 
  this 
  imperfection 
  follows 
  as 
  a 
  

   natural 
  consequence 
  from 
  the 
  principles 
  of 
  geology 
  itself. 
  There 
  

   are 
  not 
  merely 
  so 
  many 
  blanks 
  or 
  gaps 
  in 
  the 
  records, 
  but 
  a 
  reason 
  

   exists 
  in 
  the 
  very 
  nature 
  of 
  geological 
  evidence 
  why 
  such 
  breaks 
  in 
  

   the 
  record 
  might 
  naturally 
  be 
  expected 
  to 
  occur. 
  

  

  The 
  evidence 
  of 
  Grlaciation 
  is 
  to 
  be 
  found 
  chiefly 
  on 
  Land-surfaces. 
  — 
  

   It 
  is 
  on 
  a 
  land-surface 
  that 
  the 
  principal 
  traces 
  of 
  the 
  action 
  of 
  ice 
  

   during 
  a 
  glacial 
  epoch 
  are 
  left, 
  for 
  it 
  is 
  there 
  that 
  the 
  stones 
  are 
  

   chiefly 
  striated, 
  the 
  rocks 
  ground 
  down, 
  and 
  the 
  Boulder-clay 
  formed. 
  

   But 
  where 
  are 
  all 
  our 
  ancient 
  land-surfaces 
  ? 
  They 
  are 
  not 
  to 
  be 
  

   found. 
  The 
  total 
  thickness 
  of 
  the 
  stratified 
  rocks 
  of 
  Great 
  Britain 
  

   is, 
  according 
  to 
  Professor 
  Ramsay, 
  nearly 
  fourteen 
  miles. 
  But 
  from 
  

   the 
  top 
  to 
  the 
  bottom 
  of 
  this 
  enormous 
  pile 
  of 
  deposits 
  there 
  is 
  

   hardly 
  a 
  single 
  land-surface 
  to 
  be 
  detected. 
  Patches 
  of 
  real 
  old 
  

   land-surfaces 
  of 
  a 
  local 
  character 
  may, 
  indeed, 
  be 
  found, 
  as, 
  for 
  

   example, 
  the 
  dirt-beds 
  of 
  Portland 
  ; 
  but, 
  with 
  the 
  exception 
  of 
  coal- 
  

   seams, 
  every 
  general 
  formation 
  has 
  been 
  accumulated 
  under 
  water, 
  

   and 
  none 
  but 
  the 
  under-clays 
  ever 
  existed 
  as 
  a 
  land-surface. 
  And 
  

   it 
  is 
  here, 
  in 
  a 
  general 
  formation, 
  that 
  the 
  geologist 
  has 
  to 
  collect 
  

   all 
  his 
  information 
  regarding 
  the 
  existence 
  of 
  former 
  glacial 
  epochs. 
  

   The 
  entire 
  stratified 
  rocks 
  of 
  the 
  globe, 
  with 
  the 
  exception 
  of 
  the 
  

   coal-beds 
  and 
  under-clays 
  (in 
  neither 
  of 
  which 
  would 
  one 
  expect 
  to 
  

   find 
  traces 
  of 
  ice-action), 
  consist 
  almost 
  wholly 
  of 
  a 
  series 
  of 
  old 
  

   sea-bottoms, 
  with 
  here 
  and 
  there 
  an 
  occasional 
  freshwater 
  deposit. 
  

   Bearing 
  this 
  in 
  mind, 
  what 
  is 
  the 
  sort 
  of 
  evidence 
  which 
  we 
  can 
  

   now 
  hope 
  to 
  find 
  in 
  these 
  old 
  sea-bottoms 
  of 
  the 
  existence 
  of 
  former 
  

   ice-periods 
  ? 
  

  

  All 
  geologists 
  of 
  course 
  admit 
  that 
  the 
  stratified 
  rocks 
  are 
  not 
  old 
  

   land- 
  surfaces, 
  but 
  a 
  series 
  of 
  old 
  sea-bottoms 
  formed 
  out 
  of 
  the 
  

   accumulated 
  material 
  derived 
  from 
  the 
  degradation 
  of 
  primeval 
  land- 
  

   surfaces. 
  And 
  it 
  is 
  true 
  that 
  all 
  land-surfaces 
  once 
  existed 
  as 
  se 
  

   bottoms 
  : 
  but 
  the 
  stratified 
  rocks 
  consist 
  of 
  r 
  series 
  of 
  old 
  sea-bottoms 
  

  

  