﻿TUE 
  EVIDENCE 
  OF 
  FORMER 
  GLACIAL 
  PERIODS. 
  221 
  

  

  which 
  never 
  were 
  land-surfaces. 
  Many 
  of 
  them 
  no 
  doubt 
  have 
  been 
  

   repeatedly 
  above 
  the 
  sea-level, 
  and 
  may 
  once 
  have 
  possessed 
  land- 
  

   surfaces 
  ; 
  but 
  these, 
  with 
  the 
  exception 
  of 
  the 
  under-clays 
  of 
  the 
  

   various 
  coal-measures, 
  the 
  dirt-beds 
  of 
  Portland, 
  and 
  one 
  or 
  two 
  more 
  

   patches, 
  have 
  all 
  been 
  denuded 
  away. 
  The 
  important 
  bearing 
  which 
  

   this 
  consideration 
  has 
  on 
  the 
  nature 
  of 
  the 
  evidence 
  which 
  we 
  cau 
  

   now 
  expect 
  to 
  find 
  of 
  the 
  existence 
  of 
  former 
  glacial 
  epochs 
  has 
  

   certainly 
  been 
  very 
  much 
  overlooked. 
  

  

  If 
  we 
  examine 
  the 
  matter 
  fully, 
  we 
  shall 
  be 
  led 
  to 
  conclude 
  

   that 
  the 
  transformation 
  of 
  a 
  land-surface 
  into 
  a 
  sea-bottom 
  will 
  

   probably 
  completely 
  obliterate 
  every 
  trace 
  of 
  glaciation 
  which 
  that 
  

   land-surface 
  may 
  once 
  have 
  presented. 
  "We 
  cannot, 
  for 
  example, 
  

   expect 
  to 
  meet 
  with 
  polished 
  and 
  striated 
  stones 
  belonging 
  to 
  a 
  

   former 
  land-glaciation 
  ; 
  for 
  such 
  stones 
  are 
  not 
  carried 
  down 
  bodily 
  

   and 
  unchanged 
  by 
  our 
  rivers 
  and 
  deposited 
  in 
  the 
  sea. 
  They 
  become 
  

   broken 
  up 
  by 
  subaerial 
  agencies 
  into 
  gravel, 
  sand, 
  and 
  clay, 
  and 
  in 
  

   this 
  condition 
  are 
  transported 
  seawards. 
  Even 
  if 
  we 
  supposed 
  it 
  

   possible 
  that 
  the 
  stones 
  and 
  boulders 
  derived 
  from 
  a 
  mass 
  of 
  till 
  

   could 
  be 
  carried 
  down 
  to 
  sea 
  hy 
  river-action, 
  still 
  these 
  stones 
  would 
  

   certainly 
  be 
  deprived 
  of 
  all 
  their 
  ice-markings, 
  and 
  become 
  water- 
  

   worn 
  and 
  rounded 
  on 
  the 
  way. 
  Prof. 
  James 
  Geikie 
  states 
  that 
  the 
  

   great 
  accumulations 
  of 
  gravel 
  which 
  occur 
  so 
  abundantly 
  in 
  the 
  low 
  

   grounds 
  of 
  Switzerland, 
  and 
  which 
  are, 
  undoubtedly, 
  merely 
  the 
  

   re-arranged 
  materials 
  originally 
  brought 
  down 
  from 
  the 
  Alps 
  as 
  till 
  

   and 
  as 
  moraines 
  by 
  the 
  glaciers 
  during 
  the 
  glacial 
  epoch, 
  rarely 
  or 
  

   never 
  yield 
  a 
  single 
  scratched 
  or 
  glaciated 
  stone. 
  The 
  action 
  of 
  the 
  

   rivers 
  escaping 
  from 
  the 
  melting 
  ice 
  has 
  succeeded 
  in 
  obliterating 
  all 
  

   trace 
  of 
  striae. 
  It 
  is 
  the 
  same, 
  he 
  says, 
  with 
  the 
  heaps 
  of 
  gravel 
  and 
  

   sand 
  in 
  the 
  lower 
  grounds 
  of 
  Sweden 
  and 
  Norway, 
  Scotland 
  and 
  

   Ireland. 
  These 
  deposits 
  are 
  evidently 
  in 
  the 
  first 
  place 
  merely 
  the 
  

   materials 
  carried 
  down 
  by 
  the 
  swollen 
  rivers 
  that 
  issued 
  from 
  the 
  

   gradually 
  melting 
  ice-fields 
  and 
  glaciers. 
  The 
  stones 
  of 
  the 
  gravel 
  

   derived 
  from 
  the 
  demolition 
  of 
  moraines 
  and 
  till 
  have 
  lost 
  all 
  their 
  

   striae 
  and 
  become 
  in 
  most 
  cases 
  well 
  rounded 
  and 
  water-worn. 
  

   Further, 
  we 
  cannot 
  expect 
  to 
  find 
  Baulder-clay 
  among 
  the 
  stratified 
  

   rocks 
  ; 
  for 
  Boulder-clay 
  is 
  not 
  carried 
  down 
  as 
  such 
  and 
  deposited 
  in 
  

   the 
  sea, 
  but 
  under 
  the 
  influence 
  of 
  the 
  denuding 
  agents 
  becomes 
  

   broken 
  up 
  into 
  s»ft 
  mud, 
  clay, 
  sand, 
  and 
  gravel, 
  as 
  it 
  is 
  gradually 
  

   peeled 
  off 
  the 
  land 
  and 
  swept 
  seawards. 
  Patches 
  of 
  Boulder-clay 
  

   may 
  have 
  been 
  now 
  and 
  again 
  forced 
  into 
  the 
  sea 
  by 
  ice 
  and 
  

   eventually 
  become 
  covered 
  up 
  ; 
  but 
  such 
  cases 
  are 
  wholly 
  exceptional, 
  

   and 
  their 
  absence 
  in 
  any 
  formation 
  cannot 
  fairly 
  be 
  adduced 
  as 
  a 
  

   proof 
  that 
  that 
  formation 
  does 
  not 
  belong 
  to 
  a 
  glacial 
  period. 
  

  

  It 
  may, 
  however, 
  be 
  replied 
  that 
  there 
  is 
  one 
  kind 
  of 
  evidence 
  of 
  

   former 
  glacial 
  periods 
  which 
  we 
  ought 
  to 
  expect 
  in 
  the 
  stratified 
  

   rocks, 
  viz., 
  the 
  presence 
  of 
  large 
  erratic 
  blocks 
  imbedded 
  in 
  strata 
  

   which, 
  from 
  their 
  constitution, 
  have 
  evidently 
  been 
  formed 
  in 
  still 
  

   water. 
  But 
  even 
  allowing 
  this 
  to 
  be 
  the 
  case, 
  we 
  cannot 
  regard 
  the 
  

   absence 
  of 
  such 
  blocks 
  as 
  proof 
  that 
  no 
  glacial 
  period 
  occurred 
  

   during 
  the 
  time 
  of 
  the 
  formation 
  of 
  the 
  strata 
  : 
  for 
  their 
  mere 
  

  

  