﻿142 
  MR. 
  A. 
  STRAHAN 
  ON 
  GLACIAL 
  PHENOMENA 
  OF 
  [May 
  1 
  89 
  7, 
  

  

  the 
  lines 
  due 
  to 
  which, 
  however, 
  on 
  the 
  striated 
  platform 
  bear 
  no- 
  

   resemblance 
  to 
  glacial 
  groovings. 
  The 
  striae 
  have, 
  beyond 
  ques- 
  

   tion, 
  been 
  cut 
  into 
  that 
  surface 
  independently 
  of 
  auy 
  structure 
  

   possessed 
  by 
  the 
  rock, 
  and 
  are 
  in 
  all 
  respects 
  characteristic 
  glacial 
  

   markings. 
  

  

  From 
  the 
  character 
  of 
  the 
  markings 
  it 
  is 
  necessary 
  to 
  suppose 
  

   that 
  the 
  sand 
  of 
  which 
  the 
  quartzite 
  consists 
  must 
  have 
  become 
  

   consolidated 
  before 
  tbe 
  ice 
  advanced 
  over 
  it, 
  although 
  it 
  must 
  

   apparently 
  have 
  been 
  newly 
  formed 
  at 
  that 
  time. 
  I 
  suggest, 
  how- 
  

   ever, 
  that 
  it 
  may 
  not 
  have 
  been 
  literally 
  the 
  latest 
  deposit, 
  but 
  that 
  

   it 
  may 
  have 
  been 
  buried, 
  more 
  or 
  less 
  consolidated, 
  and 
  only 
  brought 
  

   into 
  contact 
  with 
  the 
  ice 
  by 
  the 
  removal 
  of 
  all 
  overlying 
  sediment 
  

   that 
  had 
  not 
  become 
  compacted. 
  

  

  The 
  evidence 
  detailed 
  above 
  seems 
  to 
  leave 
  no 
  room 
  for 
  doubt 
  

   that 
  we 
  have 
  here 
  an 
  intercalation 
  of 
  a 
  true 
  glacial 
  till 
  l 
  in 
  the 
  

   Gaisa 
  formation, 
  for 
  it 
  provides 
  an 
  answer 
  to 
  every 
  alternative 
  theory. 
  

   Three 
  possible 
  explanations 
  of 
  the 
  section 
  had 
  suggested 
  themselves 
  

   to 
  me 
  before 
  my 
  visit 
  to 
  the 
  spot 
  : 
  — 
  

  

  (1) 
  That 
  the 
  striae 
  had 
  been 
  made 
  during 
  the 
  Glacial 
  Epoch 
  or 
  

  

  subsequently. 
  

  

  (2) 
  That 
  the 
  till 
  had 
  been 
  forced 
  in 
  between 
  the 
  ledges 
  of 
  

  

  sandstone 
  during 
  that 
  epoch. 
  

  

  (3) 
  That 
  the 
  till 
  was 
  not 
  a 
  glacially-formed 
  deposit, 
  but 
  a 
  

  

  crush- 
  conglomerate 
  or 
  a 
  fault-breccia, 
  and 
  that 
  the 
  striated 
  

   surface 
  beneath 
  it 
  was 
  the 
  floor 
  of 
  a 
  thrust-plane. 
  

  

  The 
  first 
  of 
  these 
  is 
  disposed 
  of 
  by 
  the 
  fact 
  that 
  the 
  striae 
  

   unquestionably 
  pass 
  beneath 
  the 
  till. 
  

  

  The 
  second 
  is 
  negatived 
  by 
  the 
  fact 
  that 
  the 
  till 
  existed 
  before 
  

   the 
  sandstones 
  above 
  it 
  were 
  deposited, 
  for 
  the 
  obstruction 
  offered 
  

   by 
  it 
  to 
  the 
  even 
  distribution 
  of 
  the 
  sand, 
  until 
  it 
  was 
  finally 
  buried, 
  

   is 
  perfectly 
  clear. 
  

  

  The 
  third, 
  which 
  had 
  seemed 
  the 
  most 
  probable, 
  is 
  equally 
  

   untenable. 
  A 
  crush-conglomerate 
  or 
  a 
  fault-breccia 
  is 
  made 
  up 
  

   of 
  the 
  fragments 
  of 
  the 
  strata 
  which 
  it 
  traverses 
  ; 
  the 
  till, 
  on 
  the 
  

   contrary, 
  consists 
  largely 
  of 
  boulders 
  and 
  debris 
  of 
  granitic 
  rocks, 
  

   which 
  can 
  nowhere 
  form 
  part 
  of 
  the 
  Gaisa 
  formation. 
  Nor 
  does 
  

   the 
  striated 
  floor 
  present 
  any 
  of 
  the 
  features 
  of 
  a 
  fault 
  or 
  thrust- 
  

   plane, 
  for 
  the 
  markings 
  are 
  confined 
  to 
  the 
  one 
  surface, 
  and 
  do 
  not 
  

   occur 
  along 
  more 
  than 
  one 
  plane, 
  as 
  happens 
  with 
  slickenside, 
  nor 
  is 
  

   there 
  any 
  indication 
  of 
  shattering 
  in 
  the 
  rocks 
  either 
  in 
  a 
  direction 
  

   parallel 
  to 
  the 
  grooved 
  surface 
  or 
  in 
  any 
  other. 
  That 
  such 
  a 
  fault 
  

   or 
  thrust-plane 
  should 
  exist 
  here 
  is 
  highly 
  improbable, 
  for 
  there 
  is 
  

   no 
  sign 
  in 
  the 
  neighbourhood 
  of 
  such 
  disturbance 
  as 
  the 
  theory 
  

   demands. 
  Lastly, 
  the 
  relation 
  of 
  the 
  till 
  to 
  the 
  sandstones 
  above 
  

   it 
  is 
  alone 
  sufficient 
  to 
  disprove 
  the 
  hypothesis. 
  

  

  1 
  The 
  name 
  ' 
  till 
  ' 
  is 
  used 
  here, 
  not 
  as 
  indicating 
  the 
  presence 
  of 
  clay, 
  but 
  as 
  

   descriptive 
  of 
  a 
  material 
  formed 
  by 
  the 
  mixing 
  together 
  by 
  glacial 
  action 
  of 
  

   the 
  debris 
  of 
  rocks 
  of 
  any 
  composition. 
  

  

  