﻿J. 
  W. 
  JTJDD 
  ON 
  THE 
  SECONDARY 
  ROCKS 
  OF 
  SCOTLAND. 
  257 
  

  

  lying 
  the 
  lavas 
  are 
  suddenly 
  cut 
  off, 
  we 
  have 
  clear 
  evidence 
  of 
  the 
  

   existence 
  of 
  faults 
  the 
  downthrow 
  of 
  which 
  is 
  in 
  all 
  cases 
  towards 
  

   the 
  great 
  central 
  mass. 
  

  

  By 
  a 
  study 
  of 
  the 
  section 
  of 
  Craig 
  Craggen 
  in 
  connexion 
  with 
  

   those 
  exposed 
  at 
  some 
  other 
  points, 
  we 
  are 
  led 
  to 
  infer 
  that 
  a 
  similar 
  

   subsidence 
  took 
  place 
  after 
  the 
  period 
  of 
  the 
  eruption 
  of 
  the 
  acid 
  

   lavas, 
  and 
  before 
  the 
  outpouring 
  of 
  the 
  basaltic 
  lavas 
  from 
  the 
  same 
  

   centres 
  commenced. 
  

  

  Now 
  it 
  is 
  a 
  most 
  interesting 
  fact 
  that 
  precisely 
  similar 
  subsidences 
  

   beneath 
  cones 
  of 
  comparatively 
  recent 
  date 
  have 
  been 
  shown 
  to 
  exist 
  

   in 
  the 
  case 
  of 
  a 
  volcano 
  in 
  the 
  Cape-Yerde 
  Islands 
  by 
  Mr. 
  Darwin*, 
  

   and 
  in 
  one 
  in 
  New 
  Zealand 
  by 
  Mr. 
  Heaphyf. 
  The 
  phenomena 
  

   described 
  in 
  these 
  two 
  modern 
  examples 
  are 
  perfectly 
  analogous 
  to 
  

   those 
  presented 
  in 
  the 
  ancient 
  volcano 
  of 
  Mull 
  ; 
  indeed 
  Mr. 
  Scrope 
  

   has, 
  in 
  reasoning 
  on 
  the 
  causes 
  of 
  the 
  phenomenon, 
  speculated 
  on 
  

   the 
  probability 
  of 
  its 
  very 
  general 
  occurrence 
  to 
  a 
  greater 
  or 
  less 
  

   extent 
  in 
  the 
  case 
  of 
  many 
  volcanoes 
  J. 
  

  

  According 
  to 
  the 
  observations 
  of 
  Krug 
  von 
  Nidda, 
  Iceland 
  exhi- 
  

   bits 
  a 
  precisely 
  similar 
  subsidence 
  of 
  the 
  great 
  volcanic 
  centres 
  to 
  

   that 
  which 
  we 
  have 
  been 
  describing 
  in 
  Mull, 
  and 
  on 
  even 
  a 
  grander 
  

   scale 
  §. 
  

  

  Prom 
  the 
  relative 
  positions 
  of 
  the 
  central 
  masses 
  of 
  granite 
  and 
  

   gabbro 
  in 
  the 
  great 
  Tertiary 
  volcanoes 
  of 
  the 
  Hebrides 
  we 
  are 
  led 
  

   to 
  infer 
  that 
  changes 
  must 
  have 
  taken 
  place 
  in 
  the 
  position 
  of 
  the 
  

   axes 
  of 
  eruption 
  of 
  several 
  of 
  them 
  in 
  the 
  same 
  manner 
  as 
  has 
  been 
  

   so 
  admirably 
  illustrated 
  in 
  the 
  case 
  of 
  Etna 
  by 
  Sir 
  Charles 
  Lyell 
  

   (vide 
  Phil. 
  Trans, 
  for 
  1858, 
  pp. 
  738-744, 
  and 
  'Principles 
  of 
  Geology 
  ' 
  

   11th 
  ed. 
  vol. 
  ii. 
  pp. 
  9-14). 
  

  

  With 
  the 
  consideration 
  of 
  the 
  causes 
  of 
  the 
  phenomenon 
  of 
  the 
  

   central 
  subsidence 
  of 
  volcanic 
  piles, 
  however, 
  we 
  are 
  not 
  here 
  con- 
  

   cerned 
  ; 
  but 
  its 
  effect 
  in 
  the 
  island 
  of 
  Mull, 
  where 
  it 
  is 
  exhibited 
  

   in 
  so 
  marked 
  a 
  manner, 
  has 
  been 
  the 
  exceptional 
  preservation 
  of 
  a 
  

   series 
  of 
  rocks 
  which 
  afford 
  us 
  the 
  clearest 
  insight 
  into 
  many 
  important 
  

   matters 
  connected 
  with 
  the 
  history 
  of 
  these 
  ancient 
  volcanoes. 
  

  

  I 
  have 
  already 
  had 
  occasion 
  to 
  allude 
  incidentally 
  to 
  the 
  wild 
  and 
  

   rugged 
  character 
  of 
  the 
  scenery 
  to 
  which 
  the 
  basic 
  crystalline 
  rocks 
  

  

  * 
  Volcanic 
  Islands, 
  p. 
  9. 
  

  

  t 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xvi. 
  (1860) 
  p. 
  244. 
  

  

  % 
  Volcanos, 
  2nd 
  edit. 
  (1872) 
  p. 
  225. 
  

  

  § 
  Karsten's 
  Archiv, 
  vii. 
  (1834) 
  pp. 
  247-284. 
  Recent 
  studies 
  of 
  the 
  positions 
  

   and 
  relations 
  of 
  the 
  older 
  volcanic 
  rocks 
  of 
  the 
  Lipari 
  Islands 
  have 
  convinced 
  

   me 
  that 
  similar 
  central 
  subsidences 
  must 
  have 
  taken 
  place 
  in 
  connexion 
  with 
  

   the 
  great 
  volcanic 
  piles 
  of 
  that 
  area. 
  It 
  seems 
  not 
  unreasonable 
  to 
  suppose 
  

   that 
  the 
  deflection 
  of 
  the 
  liquefied 
  matter 
  below 
  a 
  volcanic 
  centre 
  to 
  new 
  vents 
  

   may 
  very 
  generally 
  give 
  rise 
  to 
  a 
  certain 
  amount 
  of 
  subsidence 
  in 
  the 
  masses 
  

   of 
  ejected 
  materials 
  which, 
  often 
  to 
  a 
  very 
  great 
  height, 
  have 
  been 
  piled 
  about 
  

   it 
  ; 
  the 
  great 
  weight 
  of 
  the 
  latter 
  would 
  assist 
  in 
  producing 
  the 
  same 
  result. 
  

   Of 
  course 
  such 
  gradual 
  and 
  comparatively 
  slight 
  subsidences 
  will 
  not 
  be 
  

   confounded 
  with, 
  or 
  thought 
  to 
  lend 
  any 
  countenance 
  to, 
  the 
  theory 
  of 
  vio- 
  

   lent 
  engulfment 
  of 
  the 
  central 
  parts 
  of 
  volcanic 
  mountains, 
  to 
  which 
  cause 
  

   some 
  geologists 
  have 
  felt 
  disposed 
  to 
  refer 
  the 
  formation 
  of 
  their 
  great 
  cra- 
  

   teral 
  hollows. 
  

  

  