﻿OF 
  THE 
  WESTERN 
  ISLES 
  OP 
  SCOTLAND. 
  199 
  

  

  period 
  of 
  time 
  must 
  have 
  been 
  covered 
  by 
  these 
  Tertiary 
  volcanic 
  

   outbursts, 
  and 
  how 
  enormous 
  is 
  the 
  thickness 
  of 
  volcanic 
  materials, 
  

   it 
  would 
  not 
  be 
  at 
  all 
  surprising 
  to 
  find 
  that 
  some 
  intrusive 
  granites 
  

   may 
  have 
  been 
  ejected 
  at 
  a 
  later 
  date 
  than 
  some 
  of 
  the 
  earlier- 
  

   formed 
  basalts 
  and 
  gabbros. 
  I 
  have, 
  moreover, 
  always 
  contended 
  

   that 
  some 
  ejections 
  of 
  acid 
  materials 
  belonged 
  to 
  a 
  third 
  period, 
  

   later 
  than 
  that 
  of 
  the 
  ejection 
  of 
  either 
  the 
  felstones 
  or 
  the 
  basalts. 
  

  

  There 
  are 
  many 
  observations 
  made 
  by 
  the 
  author 
  of 
  the 
  recently 
  

   published 
  memoir 
  which, 
  while 
  they 
  are 
  strikingly 
  confirmatory 
  of 
  

   the 
  view 
  that 
  the 
  acid 
  rocks 
  are, 
  as 
  a 
  general 
  rule, 
  older 
  than 
  the 
  

   basic 
  ones, 
  appear 
  to 
  me 
  to 
  be 
  quite 
  irreconcilable 
  with 
  the 
  opposite 
  

   opinion. 
  

  

  Thus 
  it 
  is 
  stated 
  that 
  " 
  there 
  are 
  two 
  horizons 
  on 
  which 
  protrusions 
  

   of 
  acid 
  materials 
  have 
  been 
  specially 
  abundant. 
  One 
  of 
  these 
  is 
  the 
  

   base 
  of 
  the 
  bedded 
  basalts 
  of 
  the 
  plateau, 
  the 
  other 
  is 
  at 
  the 
  bottom 
  

   of 
  the 
  thick 
  sheets 
  of 
  gabbro 
  " 
  (T. 
  K. 
  S. 
  E. 
  1888, 
  p. 
  176). 
  Abundant 
  

   evidence 
  is 
  given 
  in 
  many 
  parts 
  of 
  the 
  memoir 
  of 
  this 
  significant 
  

   circumstance 
  (loc. 
  cit. 
  pp. 
  116, 
  117, 
  172, 
  173, 
  &c). 
  

  

  It 
  will, 
  of 
  course, 
  be 
  seen 
  that 
  this 
  fact 
  of 
  the 
  infraposition 
  of 
  

   the 
  acid 
  to 
  the 
  basic 
  rocks 
  points 
  to 
  the 
  earlier 
  age 
  of 
  the 
  first- 
  

   named 
  rocks 
  ; 
  and 
  the 
  author 
  of 
  the 
  memoir 
  in 
  one 
  case 
  mentions 
  an 
  

   acid 
  rock 
  which 
  is 
  apparently 
  split 
  up 
  by 
  " 
  a 
  sill 
  of 
  dolerite 
  " 
  (loc. 
  

   cit. 
  p. 
  173). 
  But 
  in 
  this 
  and 
  all 
  similar 
  cases 
  he 
  rejects 
  the 
  facts 
  

   which 
  seem 
  to 
  point 
  to 
  the 
  priority 
  of 
  acid 
  to 
  basic 
  rocks, 
  and 
  falls 
  

   back 
  on 
  the 
  explanation 
  that 
  the 
  acid 
  rocks 
  have 
  a 
  " 
  tendency 
  " 
  to 
  be 
  

   intruded 
  just 
  in 
  the 
  positions 
  they 
  would 
  occupy 
  if 
  they 
  were 
  older 
  

   than 
  the 
  basic 
  ones 
  (loc. 
  cit. 
  p. 
  172). 
  I 
  cannot 
  myself 
  accept 
  the 
  view 
  

   that 
  the 
  acid 
  rocks 
  are 
  in 
  all 
  these 
  cases 
  of 
  an 
  intrusive 
  character. 
  

  

  Again, 
  the 
  author 
  of 
  the 
  recent 
  memoir 
  is 
  compelled 
  to 
  admit 
  

   that 
  abundant 
  fragments 
  of 
  felsitic 
  materials 
  occur 
  in 
  the 
  agglome- 
  

   rates 
  alternating 
  with 
  the 
  streams 
  of 
  basaltic 
  lava 
  (loc. 
  cit. 
  pp. 
  81, 
  

   82, 
  83, 
  108, 
  156, 
  157, 
  &c). 
  

  

  jSTow 
  this 
  fact, 
  which 
  I 
  had 
  myself 
  frequently 
  observed 
  — 
  especially 
  

   taken 
  in 
  conjunction 
  with 
  the 
  circumstance 
  that 
  many 
  of 
  these 
  

   " 
  felsitic" 
  fragments 
  present 
  a 
  markedly 
  scoriaceous 
  character 
  — 
  has 
  

   always 
  seemed 
  to 
  me 
  to 
  be 
  only 
  reconcilable 
  with 
  the 
  preexistence 
  

   of 
  acid 
  lavas 
  from 
  which 
  these 
  fragments 
  were 
  derived. 
  

  

  This 
  difficulty 
  appears 
  to 
  have 
  been 
  recognized 
  ; 
  but 
  it 
  is 
  suggested 
  

   that 
  the 
  presence 
  of 
  these 
  derived 
  fragments 
  of 
  acid 
  rock 
  among 
  the 
  

   basaltic 
  ejections 
  is 
  to 
  be 
  accounted 
  for 
  by 
  supposing 
  that 
  the 
  frag- 
  

   ments 
  in 
  question 
  were 
  derived 
  from 
  unejected 
  masses 
  of 
  igneous 
  

   material, 
  that 
  were 
  not 
  actually 
  brought 
  to 
  the 
  surface 
  bodily 
  till 
  a 
  

   much 
  later 
  date 
  ! 
  (loc. 
  cit. 
  p. 
  144). 
  The 
  explanation 
  will 
  be 
  re- 
  

   garded 
  by 
  most 
  geologists, 
  I 
  fear, 
  as 
  a 
  "desperate" 
  one. 
  

  

  Lastly, 
  the 
  author 
  of 
  the 
  recent 
  memoir 
  is 
  compelled 
  to' 
  admit 
  

  

  