﻿OF 
  THE 
  WESTERN 
  ISLES 
  OF 
  SCOTLAND. 
  197 
  

  

  I 
  was 
  primarily 
  responsible. 
  As 
  I 
  stated 
  at 
  the 
  time, 
  Professor 
  J. 
  

   D. 
  Forbes, 
  in 
  1846, 
  brought 
  forward 
  evidence 
  which 
  he 
  believed 
  

   showed 
  that 
  the 
  " 
  hypersthene-rocks 
  " 
  (gabbros) 
  are 
  intrusive 
  in, 
  

   and 
  therefore 
  younger 
  than, 
  the 
  granites 
  * 
  ; 
  and 
  Professor 
  Zirkel, 
  

   in 
  1871, 
  arrived 
  at 
  the 
  same 
  conclusion 
  f 
  . 
  I 
  certainly 
  found 
  evi- 
  

   dence 
  in 
  the 
  central 
  part 
  of 
  Mull 
  and 
  elsewhere 
  which 
  I 
  think 
  is 
  con- 
  

   clusive 
  on 
  the 
  point 
  ; 
  and 
  the 
  responsible 
  officers 
  of 
  the 
  Geological 
  

   Survey 
  of 
  Ireland, 
  two 
  years 
  later, 
  published 
  an 
  account 
  of 
  the 
  rela- 
  

   tions 
  between 
  the 
  acid 
  and 
  basic 
  rocks 
  in 
  County 
  Antrim 
  which 
  

   are 
  in 
  striking 
  agreement 
  with 
  these 
  results 
  t. 
  

  

  The 
  recently 
  published 
  memoir, 
  however, 
  aims 
  at 
  showing 
  that, 
  

   both 
  in 
  the 
  Western 
  Isles 
  and 
  in 
  Antrim, 
  the 
  granitic 
  and 
  other 
  

   acid 
  rocks 
  are 
  younger 
  than 
  the 
  gabbros 
  (T. 
  E. 
  S. 
  £. 
  1888, 
  pp. 
  151- 
  

   171 
  &c). 
  

  

  I 
  must 
  here, 
  however, 
  point 
  out 
  an 
  unfortunate 
  misunderstanding 
  

   into 
  which 
  the 
  author 
  of 
  the 
  recent 
  memoir 
  has 
  fallen 
  with 
  respect 
  

   to 
  my 
  views 
  concerning 
  the 
  relations 
  of 
  the 
  different 
  volcanic 
  rock- 
  

   masses. 
  

  

  Though 
  I 
  have 
  insisted 
  that 
  the 
  acid 
  lavas 
  (felstones) 
  were, 
  as 
  a 
  

   wJiole, 
  ejected 
  before 
  the 
  basic 
  ones 
  (basalts), 
  yet 
  I 
  most 
  clearly 
  and 
  

   emphatically 
  pointed 
  out 
  that 
  the 
  great 
  masses 
  of 
  granite 
  are 
  intru- 
  

   sive 
  in, 
  and 
  therefore 
  younger 
  than, 
  the 
  great 
  mass 
  of 
  the 
  felstone- 
  

   lavas. 
  In 
  proof 
  of 
  this 
  I 
  gave 
  a 
  section, 
  seen 
  at 
  the 
  summit 
  of 
  Beinn 
  

   na 
  Duatharach 
  (Beinn 
  Uaig), 
  in 
  the 
  Island 
  of 
  Mull 
  (Q, 
  J. 
  G. 
  S. 
  1874, 
  

   p. 
  246, 
  fig. 
  1), 
  which, 
  with 
  its 
  legend, 
  as 
  given 
  in 
  1874, 
  is 
  exactly 
  

   reproduced 
  on 
  the 
  next 
  page 
  (p. 
  198). 
  

  

  I 
  cannot 
  help 
  thinking 
  that 
  much 
  of 
  the 
  difference 
  of 
  view 
  ex- 
  

   pressed 
  in 
  the 
  recent 
  memoir, 
  as 
  compared 
  with 
  the 
  paper 
  of 
  1874, 
  

   is 
  due 
  to 
  the 
  employment 
  of 
  the 
  same 
  terms 
  with 
  a 
  different 
  signifi- 
  

   cation. 
  

  

  At 
  the 
  very 
  outset 
  of 
  my 
  paper 
  I 
  stated 
  that 
  I 
  should 
  employ 
  

   petrographical 
  names 
  (with 
  certain 
  specified 
  exceptions) 
  exactly 
  as 
  

   they 
  were 
  defined 
  in 
  Prof. 
  Zirkel's 
  admirable 
  ' 
  Lehrbuch 
  der 
  Petro- 
  

   graphie 
  ' 
  (Q. 
  J. 
  G. 
  S. 
  1874, 
  p. 
  233). 
  I 
  described 
  ray 
  " 
  basic, 
  

   basaltic, 
  or 
  pyroxenic 
  " 
  rocks 
  as 
  in 
  all 
  cases 
  containing 
  olivine 
  

   (Q. 
  J. 
  G. 
  S. 
  1874, 
  pp. 
  233-236 
  &c), 
  and 
  I 
  used 
  the 
  old 
  English 
  

   name 
  of 
  " 
  felstones 
  " 
  as 
  a 
  convenient 
  one 
  for 
  all 
  lavas 
  in 
  which 
  

   olivine 
  was 
  not 
  an 
  essential 
  constituent. 
  In 
  employing 
  the 
  term 
  

   " 
  felstone 
  " 
  I 
  desired 
  to 
  make 
  it 
  serve 
  as 
  a 
  field- 
  geologist's 
  name 
  for 
  

   very 
  varied 
  types 
  of 
  rock, 
  more 
  acid 
  than 
  the 
  oli 
  vine-basalts, 
  which 
  

   I 
  saw 
  would 
  require 
  much 
  time 
  and 
  labour 
  to 
  be 
  expended 
  upon 
  

   them 
  before 
  exact 
  diagnosis 
  was 
  possible. 
  In 
  the 
  present 
  chaotic 
  

   condition 
  of 
  petrographic 
  nomenclature, 
  opinions 
  will 
  no 
  doubt 
  

   differ 
  as 
  to 
  the 
  desirability 
  of 
  limiting 
  the 
  definition 
  of 
  basalt 
  in 
  the 
  

   manner 
  indicated. 
  Two 
  years 
  later 
  I 
  pointed 
  out 
  the 
  desirability 
  of 
  

  

  * 
  Edinb. 
  New 
  Phil. 
  Journ. 
  new 
  ser. 
  vol. 
  xl. 
  (1845-6) 
  p. 
  86. 
  

  

  t 
  Zeitschr. 
  d. 
  d. 
  geol. 
  Gesellsch. 
  Jahrg. 
  1871, 
  p. 
  90. 
  

  

  + 
  Geol. 
  Surv. 
  Ireland. 
  Memoir 
  on 
  Sheets 
  21, 
  28, 
  & 
  29 
  (1876), 
  p. 
  17. 
  Doubt 
  

   has 
  now 
  been 
  thrown 
  on 
  the 
  accuracy 
  of 
  these 
  maps 
  and 
  memoirs 
  by 
  the 
  author 
  

   of 
  the 
  recent 
  memoir 
  (T. 
  R. 
  S. 
  E. 
  p. 
  171, 
  footnote). 
  

  

  