﻿232 
  J. 
  W. 
  JXJDD 
  ON 
  THE 
  SECONDARY 
  ROCKS 
  OE 
  SCOTLAND. 
  

  

  fied 
  reservoirs 
  and 
  rivers, 
  through 
  the 
  removal 
  of 
  all 
  traces 
  of 
  the 
  

   ducts 
  which 
  once 
  connected 
  them 
  with 
  one 
  another, 
  and 
  which 
  must 
  

   have 
  necessarily 
  existed 
  at 
  higher 
  elevations 
  than 
  either 
  of 
  them. 
  

  

  In 
  the 
  following 
  pages 
  I 
  hope 
  to 
  be 
  able 
  to 
  supply 
  a 
  complete 
  

   and 
  satisfactory 
  solution 
  of 
  the 
  interesting 
  question 
  of 
  the 
  position 
  

   of 
  the 
  vents 
  from 
  which 
  the 
  great 
  lava 
  streams 
  so 
  conspicuous 
  in 
  

   the 
  north-western 
  part 
  of 
  the 
  British 
  archipelago 
  originally 
  flowed 
  ; 
  

   and 
  even 
  of 
  the 
  dimensions 
  and 
  features 
  of 
  these 
  old 
  volcanoes 
  I 
  

   hope 
  to 
  be 
  able 
  to 
  furnish 
  some 
  data 
  for 
  forming 
  a 
  judgment. 
  At 
  

   the 
  same 
  time 
  I 
  shall 
  exhibit, 
  in 
  its 
  outlines 
  at 
  least, 
  the 
  history 
  of 
  

   the 
  long 
  series 
  of 
  volcanic 
  eruptions 
  to 
  which 
  the 
  rocks 
  of 
  this 
  

   district 
  owe 
  their 
  accumulation. 
  Further, 
  I 
  believe 
  that 
  it 
  will 
  be 
  

   possible, 
  from 
  these 
  interesting 
  examples, 
  to 
  deduce 
  some 
  important 
  

   conclusions 
  as 
  to 
  the 
  modes 
  of 
  operation 
  of 
  volcanic 
  action, 
  especi- 
  

   ally 
  of 
  that 
  by 
  no 
  means 
  insignificant 
  part 
  of 
  it 
  which 
  takes 
  place 
  

   far 
  below 
  the 
  surface, 
  and 
  which, 
  in 
  the 
  nature 
  of 
  things, 
  can 
  never 
  

   be 
  observed 
  by 
  us 
  in 
  volcanoes 
  which 
  are 
  still 
  active 
  or 
  but 
  recently 
  

   extinct 
  ; 
  for 
  in 
  these 
  the 
  mountainous 
  piles 
  of 
  ejected 
  materials 
  

   altogether 
  conceal 
  the 
  underlying 
  rocks. 
  Even 
  in 
  the 
  cases 
  of 
  the 
  

   deepest 
  ravines 
  on 
  the 
  flanks 
  of 
  existing 
  volcanoes 
  (and 
  I 
  do 
  not 
  here 
  

   except 
  even 
  the 
  wonderful 
  natural 
  sections 
  of 
  Somma 
  and 
  the 
  Val 
  del 
  

   Bove), 
  interesting 
  as 
  these 
  undoubtedly 
  are, 
  the 
  insight 
  afforded 
  to 
  

   us 
  of 
  the 
  internal 
  structure 
  of 
  the 
  mountains 
  must 
  be 
  at 
  the 
  best 
  

   very 
  partial 
  indeed 
  ; 
  but 
  in 
  the 
  Hebrides 
  I 
  shall 
  show 
  that 
  we 
  have 
  

   supplied 
  to 
  us 
  that 
  great 
  geological 
  desideratum 
  — 
  a 
  number 
  of 
  

   volcanoes 
  so 
  dissected 
  by 
  the 
  scalpel 
  of 
  denudation 
  as 
  to 
  constitute, 
  

   as 
  it 
  were, 
  a 
  series 
  of 
  anatomical 
  preparations, 
  from 
  which 
  we 
  may 
  

   learn 
  directly 
  the 
  internal 
  structure 
  of 
  the 
  piles, 
  and 
  obtain 
  bases 
  

   for 
  reasoning 
  on 
  the 
  causes 
  to 
  which 
  that 
  structure 
  owes 
  its 
  origin. 
  

   Thus 
  the 
  study 
  of 
  these 
  ancient 
  volcanoes 
  is, 
  as 
  I 
  believe, 
  calculated 
  

   to 
  throw 
  new 
  light 
  upon 
  some 
  of 
  the 
  most 
  difficult 
  problems 
  of 
  

   physical 
  geology. 
  

  

  As 
  I 
  have 
  already 
  intimated, 
  we 
  have, 
  in 
  the 
  west 
  of 
  Scotland, 
  

   the 
  relics 
  of 
  two 
  great 
  and 
  widely 
  separated 
  periods 
  of 
  volcanic 
  

   action 
  : 
  one 
  of 
  these 
  was 
  evidently 
  of 
  earlier 
  date 
  than 
  the 
  depo- 
  

   sition 
  of 
  the 
  Mesozoic 
  strata 
  which 
  unconformably 
  overlie 
  its 
  

   products 
  ; 
  the 
  other 
  was 
  certainly 
  of 
  later 
  date 
  than 
  the 
  formation 
  

   of 
  the 
  Secondary 
  rocks, 
  upon 
  which 
  its 
  products 
  are 
  everywhere 
  

   superposed, 
  and 
  through 
  which 
  they 
  are 
  intruded. 
  As 
  might 
  be 
  

   anticipated, 
  the 
  relations 
  between 
  the 
  various 
  products 
  of 
  the 
  two 
  

   volcanic 
  series 
  and 
  the 
  intermediate 
  Secondary 
  strata 
  are 
  often 
  of 
  the 
  

   most 
  intricate 
  character. 
  

  

  For 
  the 
  purposes 
  of 
  the 
  present 
  argument 
  it 
  will 
  be 
  convenient 
  to 
  

   discuss, 
  in 
  the 
  first 
  instance, 
  the 
  characters 
  presented 
  by 
  the 
  younger 
  

   series 
  of 
  volcanic 
  rocks, 
  as 
  these, 
  owing 
  to 
  their 
  comparatively 
  

   better 
  state 
  of 
  preservation, 
  often 
  throw 
  much 
  light 
  on 
  the 
  internal 
  

   structure 
  and 
  the 
  true 
  relations 
  of 
  volcanic 
  masses. 
  Guided 
  by 
  the 
  

   analogies 
  of 
  this 
  younger 
  series 
  of 
  rocks, 
  we 
  shall 
  be 
  the 
  better 
  

   prepared 
  to 
  decipher 
  the 
  more 
  obscure, 
  because 
  less 
  perfectly 
  pre- 
  

   served, 
  relics 
  of 
  the 
  older 
  series 
  of 
  similar 
  products. 
  

  

  