﻿OF 
  THE 
  WESTERN 
  ISLES 
  OE 
  SCOTLAND. 
  193 
  

  

  composition 
  — 
  successive 
  currents 
  being 
  piled 
  upon 
  one 
  another 
  to 
  a 
  

   great 
  thickness. 
  

  

  (2) 
  Vast 
  deposits 
  of 
  volcanic 
  agglomerate 
  (scoria 
  and 
  dust) 
  are 
  

   found, 
  sometimes 
  containing 
  many 
  blocks 
  of 
  non-volcanic 
  materials, 
  

   and 
  thus 
  passing 
  into 
  breccias, 
  which 
  are 
  composed 
  of 
  materials 
  

   derived 
  from 
  rocks 
  underlying 
  the 
  centres 
  of 
  eruption. 
  

  

  (3) 
  Numerous 
  intrusive 
  masses, 
  frequently 
  nearly 
  horizontal 
  in 
  

   their 
  disposition 
  (sheets) 
  and 
  sometimes 
  more 
  or 
  less 
  vertical 
  (dykes), 
  

   are 
  crowded 
  together 
  at 
  these 
  five 
  centres. 
  The 
  largest 
  of 
  these 
  

   intrusive 
  masses, 
  especially 
  the 
  great 
  nearly 
  horizontal 
  sheets, 
  con- 
  

   sist 
  of 
  gabbro 
  and 
  granite, 
  while 
  the 
  smaller 
  ones, 
  the 
  dykes 
  and 
  

   the 
  peripheral 
  portion 
  of 
  the 
  great 
  bosses 
  and 
  sheets, 
  pass 
  into 
  

   dolerites, 
  basalts, 
  and 
  " 
  felstones," 
  exactly 
  similar 
  to 
  the 
  materials 
  

   of 
  the 
  lava-currents. 
  

  

  (4) 
  The 
  basic 
  intrusions 
  tend 
  to 
  form 
  wide-spread 
  sheets, 
  while 
  

   the 
  acid 
  ones 
  assume 
  those 
  more 
  bulky 
  and 
  lenticular 
  forms 
  for 
  

   which 
  the 
  name 
  of 
  " 
  laccolites 
  " 
  has 
  since 
  been 
  proposed 
  by 
  Gilbert. 
  

  

  (5) 
  Both 
  the 
  sedimentary 
  and 
  the 
  igneous 
  rocks, 
  among 
  which 
  

   these 
  masses 
  are 
  intruded, 
  afford 
  very 
  striking 
  illustrations 
  of 
  con- 
  

   tact-metamorphism 
  around 
  the 
  intrusions. 
  

  

  (6) 
  Many 
  minor 
  centres 
  of 
  eruption 
  exist 
  in 
  the 
  area, 
  of 
  which 
  

   the 
  examples 
  of 
  S'Airde 
  Beinn* 
  (Sarsta 
  Beinn) 
  and 
  Beinn 
  Hiant 
  

   (Beinn 
  Shiant) 
  are 
  described 
  as 
  being 
  among 
  the 
  most 
  striking 
  and 
  

   typical. 
  

  

  It 
  is 
  interesting 
  to 
  find 
  every 
  one 
  of 
  these 
  conclusions 
  adopted 
  and 
  

   confirmed 
  in 
  the 
  memoir 
  lately 
  published. 
  

  

  (1) 
  With 
  respect 
  to 
  the 
  centres 
  of 
  eruptive 
  action, 
  it 
  is 
  only 
  neces- 
  

   sary 
  to 
  turn 
  to 
  the 
  table 
  of 
  contents 
  of 
  the 
  memoir 
  recently 
  pub- 
  

   lished 
  to 
  perceive 
  that 
  the 
  display 
  of 
  volcanic 
  activity 
  and 
  the 
  

   development 
  of 
  intrusive 
  masses 
  are 
  alike 
  recognized 
  as 
  occurring 
  

   at 
  just 
  the 
  five 
  points 
  indicated 
  in 
  1874. 
  (See 
  the 
  description 
  of 
  

   these 
  phenomena, 
  T. 
  B. 
  S. 
  E. 
  1888, 
  pp. 
  84-100, 
  122-17-5, 
  and 
  com- 
  

   pare 
  it 
  with 
  Q. 
  J. 
  G. 
  S. 
  1874, 
  pp. 
  242-270.) 
  Near 
  one 
  of 
  these 
  great 
  

   centres, 
  that 
  of 
  Mull, 
  the 
  piled-up 
  lava-streams 
  are 
  asserted, 
  in 
  

   spite 
  of 
  the 
  great 
  denudation 
  they 
  have 
  suffered, 
  to 
  attain 
  a 
  thick- 
  

   ness 
  of 
  over 
  3000 
  feet. 
  (See 
  T. 
  E. 
  S. 
  E. 
  1888, 
  p. 
  91.) 
  

  

  (2) 
  With 
  respect 
  to 
  the 
  vast 
  masses 
  of 
  volcanic 
  agglomerate 
  at 
  

   the 
  several 
  centres 
  of 
  eruption, 
  an 
  attempt 
  is 
  made 
  in 
  the 
  recent 
  

   memoir 
  to 
  minimize 
  their 
  importance. 
  It 
  will 
  be 
  sufficient 
  for 
  my 
  

   purpose 
  to 
  point 
  out 
  the 
  descriptions 
  given 
  of 
  the 
  tuffs 
  of 
  Antrim 
  

   (T. 
  B. 
  S. 
  E. 
  1888, 
  p. 
  87 
  et 
  seq.J, 
  of 
  Mull 
  (ibid. 
  p. 
  83), 
  of 
  Bum 
  

  

  * 
  In 
  1874 
  the 
  only 
  maps 
  available 
  for 
  geological 
  purposes 
  were 
  the 
  Admi- 
  

   ralty 
  Charts 
  (in 
  which 
  only 
  the 
  parts 
  adjacent 
  to 
  the 
  coast-lines 
  are 
  shown 
  

   in 
  detail) 
  and 
  some 
  very 
  defective 
  maps 
  issued 
  by 
  private 
  individuals. 
  In 
  all 
  

   of 
  these 
  a 
  more 
  or 
  less 
  phonetic 
  spelling 
  of 
  the 
  Gaelic 
  names 
  was 
  adopted. 
  In 
  

   the 
  admirable 
  maps 
  of 
  tbe 
  Ordnance 
  Survey, 
  which 
  have 
  since 
  been 
  issued, 
  the 
  

   correct 
  spelling 
  of 
  the 
  Gaelic 
  is 
  given 
  ; 
  but 
  many 
  of 
  the 
  names, 
  as 
  now 
  written, 
  

   have 
  a 
  very 
  unfamiliar 
  appearance 
  to 
  English 
  readers. 
  To 
  avoid 
  confusion, 
  I 
  

   have 
  placed 
  the 
  old 
  spelling 
  in 
  brackets 
  after 
  the 
  new 
  ones 
  employed 
  on 
  the 
  

   maps. 
  

  

  q2 
  

  

  