﻿232 
  T. 
  M. 
  KEADE 
  ON 
  THE 
  CHALK-MASSES 
  

  

  Origin 
  and 
  Mode 
  of 
  Transport. 
  

  

  An 
  attentive 
  consideration 
  of 
  the 
  forms 
  and 
  mode 
  of 
  occurrence 
  

   of 
  these 
  masses 
  will, 
  I 
  think, 
  throw 
  considerable 
  light 
  upon 
  their 
  

   origin. 
  They 
  have 
  by 
  various 
  observers 
  been 
  attributed 
  (1st) 
  to 
  

   diluvial 
  currents, 
  (2nd) 
  to 
  icebergs, 
  (3rd) 
  to 
  land-ice 
  either 
  as 
  a 
  glacier 
  

   or 
  ice-sheet 
  ; 
  but 
  the 
  subject 
  is 
  so 
  difficult 
  that 
  most 
  observers 
  have 
  

   contented 
  themselves 
  with 
  these 
  general 
  statements, 
  no 
  attempt 
  

   that 
  I 
  know 
  of 
  having 
  been 
  made 
  to 
  investigate 
  the 
  matter 
  in 
  detail. 
  

   There 
  is, 
  I 
  think, 
  a 
  general 
  agreement 
  among 
  the 
  older 
  geologists 
  

   that 
  they 
  have 
  been 
  floated 
  into 
  their 
  present 
  positions 
  " 
  somehow 
  " 
  

   by 
  ice. 
  

  

  It 
  is 
  a 
  striking 
  fact, 
  and 
  one 
  to 
  be 
  duly 
  weighed, 
  that 
  only 
  rocks 
  

   of 
  a 
  certain 
  nature 
  are 
  found 
  occurring 
  of 
  these 
  huge 
  dimensions 
  in 
  

   the 
  Drift. 
  The 
  harder 
  and 
  primary 
  rocks, 
  the 
  sandstones, 
  gneisses, 
  

   granites, 
  syenites 
  and 
  basalts 
  never 
  occur 
  in 
  this 
  form 
  ; 
  they 
  may 
  be 
  

   found 
  as 
  rounded 
  or 
  cubical 
  boulders 
  of 
  many 
  tons' 
  weight, 
  but 
  

   none 
  of 
  a 
  size 
  like 
  these. 
  In 
  none 
  of 
  the 
  sections 
  in 
  Lancashire 
  

   are 
  such 
  masses 
  found, 
  nor 
  are 
  the 
  beds 
  of 
  Drift 
  contorted, 
  though 
  

   they 
  are 
  much 
  fuller 
  of 
  far-travelled 
  planed 
  and 
  grooved 
  erratics 
  

   and 
  boulders 
  and 
  other 
  indications 
  of 
  glacial 
  action. 
  Masses 
  of 
  

   Marlstone 
  Kock-bed, 
  of 
  oolite, 
  and 
  of 
  chalk 
  and 
  gault 
  of 
  enormous 
  

   proportions 
  have 
  been 
  described 
  by 
  Morris, 
  Judd, 
  Skertchley, 
  Bonney, 
  

   and 
  others 
  as 
  found 
  imbedded 
  in 
  the 
  Boulder-clay 
  in 
  Lincolnshire, 
  

   Rutland, 
  &c. 
  ; 
  but 
  we 
  find 
  they 
  possess 
  this 
  common 
  characteristic 
  : 
  

   they 
  are 
  long, 
  shallow, 
  and 
  narrow. 
  They, 
  in 
  fact, 
  are 
  of 
  just 
  

   the 
  form 
  that 
  would 
  be 
  taken 
  by 
  masses 
  breaking 
  off 
  from 
  a 
  cliff 
  or 
  

   escarpment. 
  

  

  The 
  geological 
  formations 
  from 
  which 
  they 
  are 
  derived 
  are 
  dis- 
  

   tinguished 
  by 
  possessing 
  numerous 
  springs 
  ; 
  the 
  beds 
  are 
  either 
  

   entirely 
  porous 
  or 
  they 
  lie 
  upon 
  sands 
  or 
  clays*. 
  

  

  I 
  take 
  it 
  for 
  certain 
  that 
  the 
  origin 
  of 
  such 
  travelled 
  masses 
  of 
  

   rock 
  is 
  to 
  be 
  sought 
  in 
  cliffs 
  and 
  escarpments. 
  Then 
  how 
  could 
  

   such 
  enormous 
  masses 
  be 
  conveyed 
  to 
  a 
  distance 
  from 
  the 
  parent 
  

   rock. 
  It 
  stands 
  to 
  reason 
  that 
  the 
  transporting 
  agent, 
  whatever 
  it 
  

   may 
  have 
  been, 
  could 
  convey 
  them 
  many 
  miles 
  as 
  easily 
  as 
  one 
  mile. 
  

   Professor 
  Judd 
  considers 
  that 
  some 
  of 
  the 
  boulders 
  he 
  describes 
  from 
  

   the 
  Marlstone 
  Rock-bed 
  have 
  travelled 
  not 
  less 
  than 
  30 
  miles. 
  Land- 
  

   ice, 
  I 
  think, 
  is 
  out 
  of 
  the 
  question 
  ; 
  the 
  effect 
  of 
  a 
  glacier 
  is 
  to 
  grind 
  

   its 
  bed 
  into 
  an 
  impalpable 
  mud 
  ; 
  and 
  I 
  cannot 
  conceive 
  any 
  means 
  

   by 
  which 
  an 
  ice-sheet 
  could 
  tear 
  up 
  masses 
  from 
  the 
  bed 
  of 
  the 
  rock 
  

   it 
  passes 
  over 
  and 
  detach 
  them 
  in 
  this 
  way. 
  An 
  iceberg 
  is 
  only 
  the 
  

   detached 
  termination 
  of 
  a 
  glacier 
  or 
  sheet 
  of 
  ice 
  ; 
  and 
  the 
  same 
  diffi- 
  

   culty 
  applies 
  to 
  this 
  mode 
  of 
  origin, 
  with 
  the 
  additional 
  difficulty 
  of 
  

   requiring 
  a 
  great 
  depth 
  of 
  water 
  for 
  flotation 
  transport. 
  

  

  The 
  position 
  of 
  the 
  shell-beds 
  under 
  and 
  over 
  the 
  chalk 
  boulder 
  

   (fig. 
  9, 
  a, 
  b) 
  appears 
  to 
  me 
  to 
  show 
  conclusively 
  that 
  it 
  has 
  been 
  

  

  * 
  Professor 
  Judd 
  says 
  (Geology 
  of 
  Eutland, 
  p. 
  265) 
  : 
  — 
  "The 
  frequent 
  alter- 
  

   nation 
  within 
  the 
  district 
  under 
  description 
  of 
  pervious 
  beds 
  of 
  limestone 
  and 
  

   sand 
  with 
  impervious 
  clays 
  gives 
  rise 
  to 
  numerous 
  springs," 
  &c. 
  

  

  