﻿OF 
  THE 
  WESTEKN 
  ISLES 
  OE 
  SCOTLAND. 
  201 
  

  

  nated 
  as 
  acid, 
  and 
  of 
  those 
  for 
  which 
  I, 
  two 
  years 
  later, 
  proposed 
  the 
  

   name 
  of 
  " 
  intermediate 
  " 
  rocks 
  *. 
  

  

  I 
  hope 
  shortly 
  to 
  be 
  able 
  to 
  describe 
  some 
  of 
  the 
  chief 
  types 
  of 
  

   these 
  rocks 
  of 
  intermediate 
  composition 
  — 
  the 
  lavas, 
  including 
  a 
  great 
  

   variety 
  of 
  andesites, 
  their 
  altered 
  forms 
  (the 
  " 
  propylites"), 
  and 
  their 
  

   Plutonic 
  representatives 
  (diorites 
  and 
  quartz-diorites). 
  When 
  this 
  

   is 
  done, 
  it 
  will 
  be 
  seen 
  how 
  numerous 
  and 
  varied 
  are 
  the 
  different 
  

   types 
  of 
  these 
  rocks 
  which 
  occur 
  in 
  the 
  Western 
  Isles 
  of 
  Scotland, 
  

   and 
  what 
  striking 
  resemblances 
  they 
  present 
  with 
  masses 
  of 
  the 
  same 
  

   age 
  in 
  the 
  Faroe 
  Isles 
  and 
  in 
  Iceland. 
  Still 
  more 
  remarkable 
  are 
  

   the 
  curious 
  modifications 
  which 
  these 
  rocks 
  in 
  Scotland 
  can 
  be 
  

   shown 
  to 
  have 
  undergone 
  from 
  the 
  action 
  upon 
  them 
  of 
  acid 
  vapours 
  

   when 
  they 
  were 
  situated 
  in 
  close 
  proximity 
  to 
  the 
  great 
  central 
  

   vents. 
  

  

  B. 
  The 
  Existence 
  in 
  the 
  District 
  of 
  great 
  Volcanoes, 
  which 
  have 
  

   been 
  dissected 
  by 
  Denudation. 
  

  

  While 
  there 
  is 
  such 
  a 
  close 
  and 
  remarkable 
  agreement 
  between 
  

   the 
  conclusions 
  enunciated 
  by 
  the 
  author 
  of 
  the 
  recent 
  memoir 
  and 
  

   those 
  published 
  in 
  1874, 
  with 
  regard 
  to 
  the 
  phenomena 
  that 
  may 
  be 
  

   observed 
  in 
  the 
  Western 
  Isles 
  of 
  Scotland 
  ; 
  yet, 
  with 
  respect 
  to 
  

   the 
  interpretation 
  of 
  those 
  phenomena 
  and 
  the 
  general 
  theoretical 
  

   explanation 
  of 
  the 
  mutually 
  admitted 
  facts, 
  there 
  is 
  the 
  widest 
  

   possible 
  divergence 
  between 
  that 
  author 
  and 
  myself. 
  

  

  The 
  generalization 
  which 
  I 
  arrived 
  at 
  in 
  1874, 
  and 
  s 
  to 
  which 
  

   especial 
  exception 
  is 
  now 
  taken, 
  may 
  be 
  stated 
  as 
  follows 
  : 
  — 
  

  

  The 
  relations 
  of 
  the 
  different 
  rock-masses 
  around 
  the 
  five 
  centres 
  

   of 
  eruption 
  in 
  the 
  Western 
  Isles 
  show 
  that 
  these 
  were 
  the 
  sites 
  of 
  

   five 
  great 
  volcanoes. 
  From 
  their 
  central 
  craters, 
  from 
  parasitical 
  

   cones 
  on 
  their 
  flanks, 
  and 
  from 
  fissures 
  opened 
  radially 
  around 
  these 
  

   volcanoes, 
  numerous 
  eruptions, 
  both 
  of 
  the 
  explosive 
  and 
  the 
  effusive 
  

   type, 
  must 
  have 
  taken 
  place. 
  The 
  data 
  now 
  remaining 
  to 
  us 
  in 
  

   these 
  old 
  Tertiary 
  volcanoes, 
  — 
  which 
  have 
  been 
  admirably 
  dissected 
  

   by 
  denudation, 
  — 
  enable 
  us 
  to 
  make 
  an 
  approximate 
  estimate 
  of 
  their 
  

   dimensions, 
  and 
  to 
  determine 
  the 
  general 
  order 
  of 
  appearance 
  of 
  the 
  

   materials 
  which 
  were 
  ejected 
  at 
  the 
  surface, 
  and 
  were 
  simultaneously 
  

   injected 
  into 
  the 
  solid 
  framework 
  of 
  the 
  volcano 
  itself 
  or 
  into 
  the 
  

   stratified 
  rock-masses 
  subjacent 
  to 
  them. 
  

  

  It 
  is 
  gratifying 
  to 
  find 
  that 
  most 
  of 
  the 
  observations 
  upon 
  which 
  

   I 
  based 
  these 
  generalizations 
  are 
  accepted 
  as 
  correct, 
  and, 
  indeed, 
  

   are 
  adopted 
  almost 
  without 
  reserve 
  by 
  the 
  author 
  of 
  the 
  memoir. 
  

  

  * 
  The 
  statement 
  that 
  though 
  more 
  acid 
  rocks 
  " 
  occur 
  abundantly 
  in 
  frag- 
  

   ments 
  in 
  the 
  volcanic 
  rocks 
  and 
  agglomerate 
  of 
  the 
  plateaux, 
  not 
  a 
  single 
  

   instance 
  has 
  been 
  observed 
  of 
  their 
  intercalation 
  as 
  contemporaneous 
  sheets 
  

   among 
  the 
  basalts" 
  (T. 
  R. 
  S. 
  E. 
  1888. 
  p. 
  106). 
  is 
  certainly 
  founded 
  on 
  too 
  par- 
  

   tial 
  observation. 
  The 
  statement 
  is 
  qualified, 
  later 
  on 
  in 
  the 
  account 
  of 
  an 
  

   andesite 
  lava 
  among 
  the 
  basalts 
  of 
  Eigg, 
  and 
  I 
  shall 
  have 
  to 
  describe 
  many 
  cases 
  

   of 
  the 
  same 
  kind 
  in 
  Mull, 
  Skye, 
  &c. 
  

  

  