﻿346 
  /. 
  C. 
  Russell 
  — 
  " 
  Plasticity 
  " 
  of 
  Glacial 
  Ice. 
  

  

  higher 
  temperature 
  than 
  its 
  upper 
  portions, 
  and 
  as 
  the 
  freedom 
  

   of 
  motion 
  in 
  ice 
  crystals 
  is 
  greatest 
  the 
  nearer 
  the 
  temperature 
  

   of 
  the 
  ice 
  approaches 
  the 
  melting 
  point, 
  we 
  should 
  expect, 
  

   other 
  conditions 
  being 
  the 
  same, 
  that 
  the 
  rate 
  of 
  change 
  in 
  the 
  

   granules 
  composing 
  a 
  glacier 
  would 
  be 
  greatest 
  at 
  its 
  lower 
  

   extremity 
  and 
  decrease 
  progressively 
  toward 
  its 
  neve. 
  Condi- 
  

   tions 
  which 
  would 
  permit 
  granules 
  to 
  change 
  their 
  shape 
  would 
  

   thus 
  be 
  transmitted 
  from 
  the 
  lower 
  extremity 
  of 
  a 
  glacier 
  

   toward 
  its 
  source. 
  The 
  deformation 
  of 
  granules 
  may 
  also 
  be 
  

   facilitated 
  by 
  the 
  formation 
  of 
  crevasses. 
  In 
  fact, 
  under 
  the 
  

   hypothesis 
  here 
  considered, 
  crevasses 
  seem 
  to 
  be 
  an 
  important 
  

   element 
  in 
  glacial 
  now. 
  

  

  Granules 
  so 
  situated 
  with 
  reference 
  to 
  the 
  direction 
  of 
  pres- 
  

   sure 
  that 
  they 
  cannot 
  yield 
  along 
  gliding 
  planes, 
  may 
  be 
  moved 
  

   bodily, 
  owing 
  to 
  the 
  movements 
  of 
  adjacent 
  granules, 
  or 
  they 
  

   may 
  be 
  fractured 
  and 
  the 
  fragments 
  united 
  to 
  neighboring 
  

   granules, 
  or 
  themselves 
  acquire 
  additions 
  and 
  form 
  new 
  

   granules. 
  This 
  explanation 
  seems 
  in 
  harmony 
  with 
  what 
  has 
  

   been 
  observed 
  in 
  reference 
  to 
  the 
  gradual 
  increase 
  in 
  the 
  size 
  

   of 
  the 
  granules 
  of 
  glacial 
  ice 
  with 
  the 
  distance 
  it 
  flows 
  ; 
  and 
  

   also 
  with 
  the 
  fact 
  that 
  this 
  gradual 
  increase 
  in 
  size 
  is 
  accom- 
  

   panied 
  by 
  the 
  pressure 
  at 
  all 
  times 
  of 
  small 
  granules. 
  

  

  An 
  objection 
  that 
  may 
  perhaps 
  be 
  advanced 
  to 
  the 
  views 
  

   here 
  expressed 
  in 
  reference 
  to 
  the 
  growth 
  of 
  granules, 
  is 
  that, 
  

   owing 
  to 
  the 
  flow 
  of 
  glaciers, 
  the 
  granules 
  composing 
  them 
  

   would 
  become 
  similarly 
  oriented, 
  that 
  is, 
  the 
  optic 
  axes 
  of 
  the 
  

   crystal 
  fragments 
  would 
  all 
  become 
  parallel 
  to 
  the 
  direction 
  of 
  

   flow. 
  The 
  varying 
  stresses, 
  due 
  to 
  inequalities 
  of 
  channel, 
  the 
  

   disturbing 
  conditions 
  introduced 
  by 
  englacial 
  debris, 
  inequali- 
  

   ties 
  in 
  temperature, 
  etc., 
  would 
  apparently 
  lead 
  to 
  the 
  growth 
  

   of 
  granules 
  with 
  their 
  optic 
  axes 
  variously 
  oriented. 
  

  

  It 
  has 
  been 
  asked 
  : 
  " 
  Why, 
  if 
  glacial 
  ice 
  responds 
  to 
  pressure 
  

   like 
  a 
  plastic 
  solid, 
  do 
  not 
  bowlders 
  on 
  the 
  surface 
  of 
  a 
  glacier 
  

   depress 
  the 
  ice 
  beneath?" 
  I 
  would 
  suggest 
  in 
  reply 
  that 
  a 
  

   bowlder 
  shields 
  the 
  ice 
  beneath 
  from 
  the 
  heat 
  of 
  the 
  sun 
  and 
  

   thus 
  diminishes 
  the 
  conditions 
  favorable 
  to 
  movement 
  along 
  

   the 
  gliding 
  planes 
  in 
  the 
  granules 
  composing 
  it. 
  The 
  ice 
  in 
  

   the 
  shadow 
  of 
  a 
  bowlder 
  is 
  stiffened 
  possibly 
  to 
  a 
  considerable 
  

   depth, 
  and 
  forms 
  a 
  platform 
  on 
  which 
  the 
  protecting 
  rock-mass 
  

   is 
  carried. 
  The 
  larger 
  the 
  bowlder, 
  the 
  broader 
  and 
  deeper 
  

   the 
  area 
  of 
  rigid 
  ice 
  in 
  its 
  shadow. 
  

  

  If 
  the 
  explanation 
  just 
  suggested 
  is 
  correct, 
  measurements 
  of 
  

   glacial 
  flow 
  where 
  narrow 
  medial 
  moraines 
  occur, 
  should 
  show 
  

   a 
  less 
  rapid 
  movement 
  than 
  the 
  adjacent 
  lanes 
  of 
  clear 
  ice. 
  

   This 
  and 
  other 
  tests 
  — 
  especially 
  a 
  study 
  of 
  the 
  ice 
  granules 
  at 
  

   many 
  points 
  in 
  a 
  glacier, 
  — 
  need 
  to 
  be 
  made 
  before 
  the 
  explana- 
  

   tion 
  of 
  the 
  plasticity 
  of 
  glacial 
  ice 
  proposed 
  in 
  this 
  note 
  can 
  be 
  

   considered 
  as 
  more 
  than 
  a 
  working 
  hypothesis. 
  

  

  

  