﻿200 
  PROF. 
  J. 
  W. 
  JTTDD 
  ON 
  THE 
  TERTIAEY 
  VOLCANOES 
  

  

  that 
  basaltic 
  dykes 
  are 
  found, 
  sometimes 
  in 
  great 
  numbers, 
  tra- 
  

   versing 
  both 
  the 
  granites 
  and 
  the 
  felstones. 
  In 
  the 
  central 
  district 
  

   of 
  Mull 
  it 
  is 
  stated 
  that 
  the 
  vast 
  masses 
  of 
  granite 
  and 
  other 
  acid 
  

   rocks 
  are 
  penetrated 
  by 
  numerous 
  basalt-veins 
  and 
  dykes 
  ; 
  and 
  that 
  

   these 
  are, 
  indeed, 
  so 
  abundant 
  that 
  the 
  geologist 
  " 
  ceases 
  to 
  take 
  

   note 
  of 
  their 
  presence 
  " 
  (see 
  T. 
  E. 
  S. 
  E. 
  1883, 
  pp. 
  158, 
  159, 
  and 
  

   fig. 
  50). 
  

  

  While, 
  however, 
  I 
  cannot 
  admit 
  that 
  the 
  evidence 
  points 
  to 
  the 
  

   conclusion 
  that 
  the 
  acid 
  rocks 
  of 
  the 
  Western 
  Isles 
  are 
  of 
  younger 
  

   age 
  than 
  the 
  more 
  basic 
  ones, 
  there 
  is 
  one 
  point 
  on 
  which 
  I 
  gladly 
  

   take 
  the 
  opportunity 
  of 
  confessing 
  and 
  correcting 
  an 
  error 
  which 
  

   mv 
  subsequent 
  study 
  of 
  the 
  country 
  convinces 
  me 
  that 
  I 
  fell 
  into 
  in 
  

   1874. 
  

  

  It 
  was 
  at 
  that 
  time 
  inferred 
  by 
  me 
  that 
  a 
  considerable 
  interval 
  

   of 
  time 
  must 
  probably 
  have 
  separated 
  the 
  period 
  of 
  the 
  eruption 
  of 
  

   the 
  more 
  acid 
  lavas 
  (" 
  felstones 
  ") 
  from 
  that 
  of 
  the 
  basalts 
  which 
  

   flowed 
  from 
  the 
  great 
  central 
  volcanoes. 
  Now, 
  although 
  the 
  evi- 
  

   dence 
  is 
  often 
  very 
  clear 
  that 
  the 
  acid 
  rocks 
  had 
  suffered 
  a 
  consider- 
  

   able 
  amount 
  of 
  erosion 
  before 
  they 
  were 
  buried 
  under 
  the 
  outflows 
  

   of 
  basalt, 
  yet 
  there 
  are 
  so 
  many 
  other 
  indications 
  of 
  great 
  denudation 
  

   having 
  taken 
  place 
  at 
  different 
  intervals 
  during 
  the 
  whole 
  of 
  the 
  vast 
  

   periods 
  covered 
  by 
  these 
  Tertiary 
  eruptions, 
  that 
  I 
  am 
  no 
  longer 
  

   prepared 
  to 
  maintain 
  the 
  special 
  importance 
  of 
  this 
  particular 
  period 
  

   of 
  quiescence. 
  

  

  On 
  the 
  contrary, 
  the 
  longer 
  I 
  have 
  studied 
  the 
  district 
  the 
  

   more 
  convincing 
  appears 
  to 
  me 
  to 
  be 
  the 
  evidence 
  of 
  a 
  gradual 
  

   change 
  from 
  the 
  acid 
  to 
  the 
  basic 
  eruptions. 
  Some 
  of 
  the 
  felstones 
  

   are 
  augite-andesites 
  and 
  labradorite-andesites, 
  but 
  little 
  more 
  acid 
  

   in 
  character 
  than 
  the 
  olivine-basalts 
  of 
  the 
  great 
  plateaux 
  ; 
  and 
  

   there 
  are 
  also 
  abundant 
  examples 
  of 
  lavas 
  of 
  more 
  or 
  less 
  acid 
  type 
  

   having 
  been 
  erupted 
  from 
  the 
  central 
  vents 
  from 
  time 
  to 
  time, 
  while 
  

   the 
  basalts 
  were 
  being 
  ejected. 
  But, 
  as 
  I 
  have 
  formerly 
  shown, 
  

   while 
  the 
  extremely 
  liquid 
  basalts 
  flowed 
  to 
  distances 
  of 
  forty 
  or 
  

   fifty 
  miles 
  from 
  their 
  point 
  of 
  origin, 
  the 
  less 
  fluid, 
  acid 
  lavas 
  seldom 
  

   flowed 
  to 
  distances 
  of 
  more 
  than 
  ten 
  miles, 
  and 
  are, 
  consequently, 
  

   found 
  confined 
  to 
  the 
  flanks 
  of 
  the 
  volcanoes 
  from 
  which 
  they 
  

   issued. 
  

  

  Just 
  as 
  there 
  are 
  examples 
  of 
  " 
  felstone 
  "-lavas 
  among 
  the 
  basalts, 
  

   so 
  are 
  there 
  cases 
  in 
  which 
  basaltic 
  currents 
  may 
  be 
  found 
  among 
  

   the 
  older 
  ones 
  of 
  more 
  acid 
  composition, 
  — 
  though 
  it 
  seems 
  to 
  me 
  to 
  

   be 
  clear 
  that 
  the 
  acid 
  series 
  was, 
  as 
  a 
  ivhole, 
  of 
  earlier 
  date 
  than 
  the 
  

   basic 
  one. 
  The 
  " 
  pale-coloured 
  felstones 
  of 
  Beinn 
  More," 
  which 
  are 
  

   claimed 
  by 
  the 
  author 
  of 
  the 
  recent 
  memoir 
  as 
  being 
  posterior 
  to 
  

   the 
  basalts, 
  are, 
  if 
  my 
  interpretation 
  be 
  correct, 
  a 
  part 
  of 
  the 
  older 
  

   volcanic 
  ejections. 
  

  

  The 
  great 
  mistake 
  which 
  I 
  made 
  in 
  1874 
  was, 
  not 
  in 
  insisting 
  

   upon 
  the 
  existence 
  of 
  a 
  great 
  series 
  of 
  "felstones" 
  underlying 
  the 
  

   basalts, 
  but 
  in 
  not 
  recognizing 
  the 
  fact 
  that 
  these 
  felstones 
  include 
  

   representatives 
  both 
  of 
  the 
  rocks 
  which 
  are 
  more 
  properly 
  desig- 
  

  

  