﻿202 
  PEOF. 
  J. 
  TV. 
  JT7DD 
  OX 
  THE 
  TEETIAEr 
  VOLCANOES 
  

  

  It 
  is 
  admitted 
  that 
  very 
  numerous 
  intrusions 
  of 
  both 
  acid 
  and 
  

   basic 
  rocks 
  took 
  place 
  at 
  just 
  those 
  five 
  centres 
  which 
  were 
  indicated 
  

   by 
  me 
  in 
  1874 
  — 
  namely 
  Mull, 
  Ardnamurchan, 
  Rum, 
  Skye, 
  and 
  St. 
  

   Kilda 
  (T. 
  R. 
  S. 
  E. 
  1888, 
  pp. 
  84, 
  122-170). 
  It 
  is 
  agreed 
  that 
  it 
  is 
  

   quite 
  impossible 
  to 
  draw 
  any 
  sharp 
  lines 
  of 
  distinction 
  between 
  the 
  

   materials 
  — 
  whether 
  acid 
  or 
  basic 
  in 
  composition 
  — 
  which 
  constitute 
  

   these 
  intrusions 
  and 
  the 
  lavas 
  which 
  during 
  the 
  same 
  period 
  were 
  

   poured 
  out 
  at 
  the 
  surface 
  (Joe. 
  cit. 
  pp. 
  76-81, 
  122, 
  125, 
  178, 
  &c). 
  

   The 
  existence 
  of 
  enormous 
  masses 
  of 
  volcanic 
  agglomerates 
  and 
  

   breccias, 
  the 
  result 
  of 
  explosive 
  action 
  near 
  these 
  centres, 
  is 
  granted, 
  

   although 
  an 
  attempt 
  is 
  made 
  to 
  minimize 
  their 
  importance 
  (loc. 
  cit. 
  

   pp. 
  83, 
  87, 
  108, 
  109, 
  134). 
  It 
  is 
  even 
  allowed 
  that 
  there 
  is 
  proof 
  

   of 
  a 
  crowding 
  of 
  the 
  vents 
  of 
  eruption 
  in 
  the 
  neighbourhood 
  of 
  these 
  

   five 
  great 
  centres 
  which 
  have 
  been 
  indicated 
  (Joe. 
  cit. 
  p. 
  121). 
  

  

  Still 
  more 
  significant 
  are 
  the 
  admissions 
  that 
  there 
  is 
  no 
  evidence 
  

   of 
  the 
  outflow 
  of 
  basaltic 
  lavas 
  having 
  ceased 
  before 
  the 
  extravasa- 
  

   tion 
  of 
  the 
  gabbro-bosses, 
  and 
  that 
  it 
  is 
  difficult 
  to 
  suppose 
  that 
  

   none 
  of 
  the 
  dykes 
  " 
  communicated 
  with 
  the 
  surface 
  and 
  gave 
  rise 
  to 
  

   outpouring 
  of 
  basalt 
  and 
  the 
  ejection 
  of 
  dust 
  and 
  stones" 
  (loc. 
  cit. 
  

   p. 
  121). 
  It 
  is 
  granted 
  that 
  " 
  the 
  points 
  of 
  extravasation 
  of 
  the 
  

   materials,'" 
  " 
  which 
  ultimately 
  solidified 
  as 
  dolerites, 
  gabbros, 
  tro- 
  

   ctolites, 
  picrites," 
  &c, 
  " 
  were 
  mainly 
  determined 
  by 
  the 
  positions 
  of 
  

   the 
  larger 
  or 
  more 
  closely 
  clustered 
  vents 
  of 
  the 
  plateau-period, 
  

   where 
  lines 
  of 
  weakness 
  consequently 
  existed 
  in 
  the 
  terrestrial 
  

   crust. 
  Rising 
  as 
  huge 
  bosses 
  through 
  such 
  weak 
  places, 
  the 
  gabbros 
  

   and 
  associated 
  rocks 
  raised 
  up 
  the 
  overlying 
  bedded 
  basalts, 
  and 
  

   forced 
  themselves 
  between 
  them, 
  forming 
  thus 
  a 
  fringe 
  of 
  finer- 
  

   grained 
  intrusive 
  sills 
  and 
  veins 
  around 
  the 
  central 
  amorphous 
  cores 
  

   of 
  more 
  crystalline 
  material. 
  Whether, 
  in 
  any 
  of 
  these 
  vast 
  domes 
  

   of 
  upheaval, 
  the 
  summit 
  was 
  disrupted, 
  so 
  as 
  to 
  allow 
  the 
  basic 
  

   intrusion 
  to 
  flow 
  out 
  as 
  lava 
  at 
  the 
  surface, 
  cannot 
  now 
  be 
  told, 
  

   owing 
  to 
  the 
  enormous 
  subsequent 
  denudation" 
  (loc. 
  cit. 
  p. 
  183). 
  

  

  It 
  is 
  only 
  necessary 
  to 
  grant 
  the 
  very 
  high 
  probability 
  of 
  this 
  last 
  

   suggestion 
  being 
  the 
  true 
  one, 
  to 
  bring 
  the 
  views 
  of 
  the 
  author 
  of 
  

   the 
  recent 
  memoir 
  into 
  complete 
  accord 
  with 
  my 
  own. 
  I 
  have 
  

   argued 
  that 
  the 
  very 
  complete 
  gradation 
  which 
  is 
  now 
  admitted 
  to 
  

   exist 
  between 
  the 
  gabbros 
  of 
  the 
  bosses 
  and 
  the 
  ophitic 
  basalts 
  of 
  

   the 
  plateaux 
  points 
  to 
  the 
  conclusion 
  that 
  — 
  all 
  through 
  the 
  vast 
  

   periods 
  during 
  which 
  the 
  basalts 
  were 
  being 
  poured 
  out 
  at 
  the 
  

   surface, 
  from 
  a 
  central 
  crater 
  or 
  from 
  parasitical 
  vents 
  — 
  masses 
  of 
  

   the 
  same 
  material 
  were 
  finding 
  their 
  way 
  into 
  lower 
  portions 
  of 
  the 
  

   great 
  cone 
  and 
  among 
  the 
  subjacent 
  rock-masses, 
  and 
  consolidating 
  

   in 
  a 
  more 
  highly 
  crystalline 
  form. 
  My 
  critic, 
  on 
  the 
  other 
  hand 
  — 
  

   I 
  fail 
  to 
  see 
  upon 
  what 
  grounds, 
  — 
  asserts 
  that 
  the 
  intrusion 
  of 
  the 
  

   gabbros 
  took 
  place 
  after 
  all, 
  or 
  nearly 
  all, 
  the 
  basalts 
  had 
  been 
  

   erupted. 
  

  

  Lastly, 
  it 
  is 
  admitted 
  that 
  the 
  central 
  subsidence 
  — 
  to 
  which 
  I 
  

   particularly 
  called 
  attention 
  as 
  being 
  similar 
  to 
  that 
  which 
  Darwin 
  

   and 
  other 
  authors 
  had 
  shown 
  to 
  have 
  taken 
  place 
  in 
  the 
  case 
  of 
  

   many 
  volcanic 
  centres 
  of 
  eruption 
  — 
  really 
  occurred 
  in 
  the 
  case 
  of 
  

  

  