﻿1. 
  C. 
  Russell—" 
  Plasticity 
  " 
  of 
  Glacial 
  Ice. 
  345 
  

  

  ments 
  having 
  their 
  optic 
  axes 
  in 
  the 
  plane 
  of 
  pressure 
  would 
  

   be 
  changed 
  in 
  shape, 
  as 
  in 
  the 
  experiments 
  cited 
  above, 
  by 
  

   movements 
  along 
  gliding 
  planes. 
  

  

  The 
  stresses 
  in 
  glaciers 
  are 
  manifestly 
  in 
  various 
  directions, 
  

   and 
  owing 
  to 
  the 
  flow 
  of 
  the 
  ice, 
  inequalities 
  of 
  the 
  rock 
  sur- 
  

   faces 
  beneath, 
  etc., 
  must 
  be 
  exceedingly 
  complex. 
  As 
  the 
  

   •crystal 
  fragments 
  of 
  which 
  glaciers 
  are 
  composed 
  are 
  without 
  

   orderly 
  arrangement, 
  it 
  is 
  evident 
  that 
  some 
  of 
  them 
  will 
  be 
  

   properly 
  oriented 
  to 
  be 
  deformed 
  by 
  a 
  differential 
  movement 
  

   of 
  their 
  parts 
  along 
  gliding 
  planes, 
  by 
  pressure 
  acting 
  in 
  any 
  

   direction. 
  Under 
  diverse 
  stresses 
  the 
  resultant 
  motion 
  would 
  

   be 
  in 
  the 
  direction 
  of 
  least 
  resistance. 
  That 
  is, 
  the 
  granular 
  

   ice 
  would 
  behave 
  like 
  a 
  plastic 
  solid. 
  

  

  Owing 
  to 
  diversity 
  in 
  orientation, 
  it 
  follows 
  that 
  adjacent 
  

   granules 
  might 
  be 
  deformed 
  in 
  different 
  directions 
  ; 
  and 
  that 
  

   diverse 
  movements 
  of 
  adjacent 
  granules 
  might 
  result. 
  

  

  The 
  yielding 
  of 
  glacial 
  ice 
  to 
  pressure, 
  we 
  conceive, 
  is 
  due 
  

   to 
  movements 
  along 
  gliding 
  planes 
  in 
  the 
  granules 
  of 
  which 
  it 
  

   is 
  composed. 
  If 
  this 
  process 
  is 
  continued, 
  the 
  granules 
  will 
  

   evidently 
  be 
  destroyed 
  by 
  being 
  divided 
  along 
  the 
  planes 
  on 
  

   which 
  movement 
  occurs, 
  but 
  the 
  resulting 
  subdivisions 
  — 
  or 
  

   " 
  plates" 
  perhaps 
  we 
  may 
  term 
  them, 
  — 
  are 
  re-united 
  probably 
  

   by 
  the 
  process 
  of 
  regelation. 
  The 
  plates 
  which 
  after 
  uniting 
  

   have 
  the 
  same 
  orientation, 
  would 
  form 
  new 
  granules. 
  The 
  

   longer 
  this 
  process 
  is 
  continued, 
  or 
  in 
  other 
  words, 
  the 
  farther 
  

   a 
  glacier 
  flows, 
  the 
  greater 
  the 
  chances 
  that 
  a 
  large 
  number 
  of 
  

   plates 
  will 
  come 
  together 
  with 
  similar 
  orientation 
  and 
  the 
  

   larger 
  will 
  be 
  the 
  resulting 
  granules. 
  

  

  TJnder 
  the 
  hypothesis 
  here 
  suggested, 
  the 
  granules 
  of 
  glacial 
  

   ice 
  are 
  considered 
  as 
  an 
  inheritance 
  from 
  the 
  granules 
  in 
  the 
  

   ice 
  formed 
  by 
  partial 
  melting 
  and 
  re-freezing 
  of 
  neve 
  snow. 
  

   The 
  granules 
  at 
  first 
  are 
  minute, 
  but 
  under 
  the 
  pressure 
  of 
  ice 
  

   at 
  higher 
  levels 
  they 
  yield 
  along 
  gliding 
  planes, 
  and 
  a 
  resultant 
  

   motion 
  similar 
  to 
  that 
  of 
  plastic 
  solids 
  under 
  like 
  conditions 
  

   is 
  initiated. 
  The 
  granules 
  are 
  destroyed 
  by 
  this 
  process, 
  but 
  

   progressive 
  motion 
  leads 
  to 
  the 
  union 
  of 
  a 
  constantly 
  increas- 
  

   ing 
  number 
  of 
  the 
  plates 
  into 
  which 
  they 
  are 
  divided, 
  and 
  the 
  

   growth 
  of 
  larger 
  granules. 
  

  

  As 
  movement 
  in 
  ice 
  crystals, 
  along 
  gliding 
  planes, 
  takes 
  

   place 
  with 
  increased 
  freedom 
  the 
  nearer 
  the 
  temperature 
  

   approaches 
  the 
  melting 
  point, 
  we 
  should 
  expect, 
  under 
  the 
  

   hypothesis 
  here 
  suggested, 
  that 
  glacial 
  motion 
  would 
  fluctuate 
  

   with 
  changes 
  of 
  temperature. 
  This, 
  as 
  is 
  well 
  known, 
  is 
  what 
  

   has 
  been 
  observed. 
  

  

  It 
  is 
  evident 
  that 
  one 
  granule 
  in 
  a 
  mass 
  of 
  glacial 
  ice 
  cannot 
  

   be 
  deformed 
  by 
  movements 
  of 
  its 
  various 
  portions 
  along 
  glid- 
  

   ing 
  planes, 
  unless 
  room 
  for 
  the 
  change 
  of 
  form 
  is 
  available. 
  

   As 
  the 
  lower 
  extremity 
  of 
  a 
  glacier 
  is 
  normally 
  exposed 
  to 
  a 
  

  

  