﻿256 
  J. 
  W. 
  JTJDD 
  ON 
  THE 
  SECONDARY 
  ROCKS 
  OF 
  SCOTLAND. 
  

  

  10. 
  Comparison 
  of 
  the 
  great 
  Tertiary 
  Volcanoes. 
  — 
  It 
  has 
  been 
  

   shown 
  in 
  the 
  preceding 
  pages 
  that 
  we 
  have 
  evidence 
  of 
  the 
  exist- 
  

   ence 
  during 
  the 
  Tertiary 
  period 
  of 
  five 
  great 
  centres 
  of 
  volcanic 
  

   action 
  in 
  the 
  northern 
  portion 
  of 
  the 
  Hebrides 
  — 
  and 
  that 
  at 
  each 
  

   of 
  these 
  points 
  a 
  great 
  volcanic 
  mountain 
  once 
  existed, 
  the 
  frag- 
  

   mentary, 
  but 
  unmistakable, 
  relics 
  of 
  which 
  still 
  remain 
  for 
  our 
  

   study. 
  A 
  problem 
  which, 
  however, 
  will 
  naturally 
  suggest 
  itself 
  

   to 
  every 
  mind 
  is 
  that 
  of 
  the 
  causes 
  which 
  have 
  led 
  to 
  the 
  very 
  dif- 
  

   ferent 
  states 
  of 
  preservation 
  of 
  volcanoes 
  which 
  appear 
  to 
  have 
  been 
  

   formed 
  contemporaneously. 
  The 
  portion 
  of 
  this 
  inquiry 
  which 
  is 
  of 
  

   more 
  especial 
  interest 
  to 
  the 
  geologist 
  is 
  that 
  concerning 
  the 
  reasons 
  

   of 
  that 
  comparatively 
  perfect 
  state 
  of 
  preservation 
  exhibited 
  by 
  the 
  

   volcano 
  of 
  Mull 
  which 
  renders 
  it 
  so 
  admirable 
  a 
  key 
  to 
  the 
  interpre- 
  

   tation 
  of 
  the 
  whole 
  series 
  of 
  phenomena 
  presented 
  by 
  the 
  Western 
  

   Isles 
  of 
  Scotland. 
  

  

  Fortunately 
  the 
  causes 
  of 
  the 
  different 
  extent 
  of 
  denudation 
  in 
  

   the 
  several 
  cases, 
  and 
  of 
  the 
  exceptional 
  state 
  of 
  preservation 
  of 
  

   the 
  volcano 
  of 
  Mull, 
  are 
  sufficiently 
  obvious 
  ; 
  and 
  as 
  the 
  considera- 
  

   tion 
  of 
  these 
  leads 
  us 
  to 
  some 
  highly 
  interesting 
  and 
  important 
  con- 
  

   clusions, 
  I 
  shall 
  now 
  proceed 
  to 
  detail 
  them. 
  

  

  If 
  we 
  compare 
  the 
  volcanoes 
  of 
  Mull 
  and 
  Skye, 
  the 
  differences 
  

   alluded 
  to, 
  in 
  spite 
  of 
  the 
  identity 
  of 
  the 
  characters 
  and 
  relations 
  of 
  

   the 
  various 
  rocks 
  which 
  compose 
  them, 
  are 
  very 
  striking. 
  Thus 
  

   while 
  in 
  the 
  former 
  case 
  we 
  observe 
  the 
  most 
  wonderful 
  interlacing 
  

   of 
  the 
  two 
  kinds 
  of 
  eruptive 
  rock, 
  and 
  the 
  outliers 
  of 
  the 
  great 
  super- 
  

   posed 
  piles 
  of 
  fragmentary 
  materials 
  are 
  still 
  clearly 
  to 
  be 
  traced, 
  in 
  

   the 
  latter 
  case 
  the 
  two 
  great 
  eruptive 
  masses 
  rise 
  side 
  by 
  side, 
  with 
  

   but 
  comparatively 
  few 
  examples 
  of 
  offshoots 
  from 
  the 
  younger 
  into 
  

   the 
  older 
  series, 
  and 
  the 
  lavas, 
  agglomerates, 
  and 
  breccias 
  which 
  

   formed 
  the 
  mass 
  of 
  the 
  volcano 
  have 
  almost 
  wholly 
  disappeared. 
  

  

  A 
  careful 
  study 
  of 
  the 
  rocks 
  of 
  Mull 
  shows 
  the 
  cause 
  of 
  this 
  dif- 
  

   ference 
  to 
  be 
  that 
  the 
  volcanic 
  pile 
  in 
  this 
  island 
  has 
  suffered 
  far 
  less 
  

   from 
  denudation 
  than 
  in 
  Skye, 
  and 
  that 
  in 
  fact 
  the 
  existing 
  surface 
  

   in 
  the 
  former 
  island 
  presents 
  a 
  cross 
  section 
  of 
  a 
  volcano 
  taken 
  at 
  a 
  

   higher 
  level 
  than 
  in 
  the 
  latter. 
  Further, 
  it 
  can 
  be 
  shown 
  that 
  this 
  

   difference 
  is 
  due 
  to 
  a 
  remarkable 
  central 
  subsidence 
  which 
  has 
  taken 
  

   place 
  in 
  the 
  case 
  of 
  the 
  Mull 
  volcano. 
  

  

  When 
  we 
  examine 
  the 
  fine 
  sections 
  of 
  the 
  lavas 
  of 
  the 
  basaltic 
  

   plateaux 
  which 
  are 
  exposed 
  along 
  the 
  shores 
  of 
  the 
  deep 
  fiords 
  of 
  

   Loch 
  Scridain 
  and 
  Loch-na-Kael, 
  we 
  find 
  that 
  the 
  lava 
  sheets, 
  in- 
  

   stead 
  of 
  sloping 
  away 
  from 
  the 
  great 
  central 
  masses 
  of 
  eruptive 
  rock, 
  

   actually 
  for 
  many 
  miles 
  around 
  dip 
  towards 
  them 
  at 
  angles 
  varying 
  

   from 
  2° 
  or 
  3° 
  up 
  to 
  5°, 
  the 
  inclination 
  increasing 
  as 
  we 
  approach 
  the 
  

   volcano. 
  Now, 
  as 
  we 
  are 
  certain 
  that 
  this 
  was 
  not 
  the 
  original 
  po- 
  

   sition 
  of 
  the 
  lava 
  streams, 
  we 
  are 
  led 
  to 
  the 
  conclusion 
  that 
  a 
  subsi- 
  

   dence 
  has 
  taken 
  place 
  in 
  the 
  great 
  central 
  mass 
  of 
  the 
  island, 
  a 
  view 
  

   which 
  is 
  confirmed 
  by 
  an 
  examination 
  of 
  the 
  inclination 
  of 
  the 
  whole 
  

   of 
  the 
  lavas 
  surrounding 
  the 
  volcano. 
  Still 
  further 
  support 
  to 
  the 
  

   inference 
  is 
  afforded 
  by 
  the 
  fact 
  that 
  at 
  certain 
  points, 
  as 
  in 
  the 
  valley 
  

   between 
  Kilfinichan 
  and 
  Gribun, 
  where 
  the 
  Cretaceous 
  rocks 
  under- 
  

  

  