﻿OF 
  THE 
  WESTERN 
  ISLES 
  OF 
  SCOTLAND. 
  203 
  

  

  the 
  great 
  volcanic 
  centres 
  of 
  the 
  Western 
  Isles 
  of 
  Scotland 
  (he. 
  cit. 
  

   pp. 
  94, 
  142. 
  Compare 
  Q. 
  J. 
  G. 
  S. 
  1874, 
  pp. 
  256-7). 
  

  

  So 
  far, 
  indeed, 
  as 
  I 
  have 
  been 
  able 
  to 
  discover, 
  there 
  are 
  three, 
  

   and 
  only 
  three, 
  reasons 
  adduced 
  by 
  the 
  author 
  of 
  the 
  recent 
  memoir 
  

   for 
  rejecting 
  the 
  explanation 
  which 
  I 
  offered 
  of 
  the 
  phenomena 
  in 
  

   1874; 
  and 
  these 
  we 
  may 
  now 
  proceed 
  to 
  consider. 
  

  

  First. 
  It 
  is 
  said 
  that 
  there 
  is 
  no 
  thickening 
  of 
  the 
  sheets 
  of 
  

   basaltic 
  lava 
  as 
  we 
  approach 
  the 
  supposed 
  central 
  vents 
  (T. 
  B. 
  S. 
  E. 
  

   1888, 
  pp. 
  99-100). 
  

  

  I 
  may 
  point 
  out, 
  in 
  answer 
  to 
  this, 
  that 
  I 
  have 
  nowhere 
  suggested 
  

   that 
  the 
  basaltic 
  lava- 
  sheets 
  were 
  thicker 
  nearer 
  their 
  points 
  of 
  

   origin 
  than 
  further 
  away. 
  On 
  the 
  contrary, 
  I 
  have 
  again 
  and 
  again 
  

   insisted 
  upon 
  the 
  striking 
  proofs 
  which 
  exist 
  of 
  the 
  extreme 
  liquidity 
  

   of 
  the 
  basaltic 
  lavas 
  ; 
  and 
  such 
  liquidity 
  would 
  militate 
  against 
  the 
  

   thickening 
  of 
  the 
  lavas 
  as 
  we 
  approach 
  their 
  points 
  of 
  eruption. 
  

   Very 
  liquid 
  lavas 
  of 
  this 
  type 
  exhibit 
  abnormal 
  thicknesses 
  where 
  

   they 
  have 
  flowed 
  into 
  hollows 
  (those 
  of 
  Skaptar 
  Jokull 
  are 
  said 
  to 
  

   have 
  attained 
  a 
  depth 
  of 
  over 
  600 
  feet 
  in 
  certain 
  ravines) 
  ; 
  but 
  I 
  

   know 
  of 
  no 
  observations 
  which 
  would 
  warrant 
  the 
  conclusion 
  that 
  

   a 
  progressive 
  diminution 
  in 
  thickness 
  can 
  be 
  detected 
  in 
  such 
  lava- 
  

   currents 
  as 
  they 
  are 
  traced 
  away 
  from 
  their 
  point 
  of 
  ejection. 
  

   Further, 
  I 
  have 
  insisted 
  that 
  many 
  of 
  the 
  currents 
  of 
  basaltic 
  lava 
  

   must 
  have 
  flowed, 
  not 
  from 
  a 
  central 
  crater, 
  but 
  from 
  parasitical 
  

   cones 
  on 
  the 
  flanks 
  of 
  the 
  great 
  volcanoes, 
  and 
  of 
  such 
  parasitical 
  

   cones 
  I 
  described 
  a 
  conspicuous 
  example 
  in 
  S'Airde 
  Beinn 
  (Sarsta 
  

   Beinn). 
  (See 
  Q. 
  J 
  . 
  G. 
  S. 
  1874, 
  pp. 
  2^-26Q, 
  and 
  compare 
  T. 
  B. 
  S. 
  E. 
  

   1888, 
  pp. 
  103-104.) 
  

  

  On 
  the 
  other 
  hand, 
  I 
  have 
  always 
  asserted 
  that, 
  as 
  we 
  approach 
  

   the 
  great 
  centres 
  of 
  eruption, 
  we 
  find 
  the 
  short 
  and 
  bulky 
  andesitic 
  

   and 
  more 
  acid 
  lava-streams 
  increasing 
  in 
  number 
  till 
  they 
  form 
  a 
  

   considerable 
  portion 
  of 
  the 
  whole 
  mass 
  (Q. 
  J. 
  G. 
  S. 
  1874, 
  p. 
  248). 
  

   My 
  critic 
  admits 
  the 
  importance 
  of 
  these 
  felstone-lavas 
  in 
  the 
  case 
  

   of 
  Beinn 
  More 
  in 
  Mull, 
  but 
  with 
  respect 
  to 
  other 
  localities 
  he 
  

   .appears 
  to 
  have 
  overlooked 
  their 
  great 
  number 
  and 
  significance. 
  

   This 
  seems 
  to 
  arise 
  from 
  the 
  fact 
  that 
  he 
  regards 
  all 
  the 
  masses 
  

   of 
  acid 
  rocks 
  which 
  lie 
  below 
  the 
  basalts 
  in 
  Mull, 
  Bum, 
  Skye, 
  

   and 
  Baasay 
  as 
  being 
  intrusive. 
  He 
  makes 
  the 
  important 
  admission, 
  

   however, 
  that 
  " 
  as 
  we 
  retire 
  from 
  the 
  mountain-tract 
  [that 
  is 
  from 
  

   the 
  great 
  centre 
  of 
  eruption 
  in 
  Mull] 
  into 
  the 
  undisturbed 
  basalts 
  

   of 
  the 
  plateau, 
  these 
  acid 
  intercalations 
  gradually 
  disappear." 
  (T. 
  B. 
  

   S. 
  E. 
  1888, 
  p. 
  172.) 
  

  

  In 
  the 
  account 
  which 
  I 
  propose 
  to 
  give 
  of 
  the 
  remarkably 
  varied 
  

   series 
  of 
  intermediate 
  and 
  acid 
  lavas 
  at 
  the 
  five 
  great 
  centres 
  of 
  

   eruption, 
  I 
  shall 
  show 
  how 
  they 
  graduate 
  insensibly 
  into 
  dioritic 
  and 
  

   granitic 
  types 
  on 
  the 
  one 
  hand 
  ; 
  while 
  on 
  the 
  other 
  they 
  exhibit, 
  

   where 
  they 
  approach 
  the 
  great 
  central 
  vents, 
  those 
  curious 
  and 
  

   interesting 
  modifications 
  of 
  their 
  constituent 
  minerals, 
  from 
  the 
  

   action 
  upon 
  them 
  of 
  acid 
  vapours, 
  which 
  are 
  so 
  characteristic 
  of 
  the 
  

   rocks 
  which 
  have 
  been 
  called 
  " 
  propylites." 
  

  

  