﻿OF 
  THE 
  WESTERN 
  ISLES 
  OF 
  SCOTLAND. 
  191 
  

  

  previous 
  memoir, 
  and 
  point 
  out 
  how 
  far 
  these 
  have 
  been 
  substan- 
  

   tiated, 
  modified, 
  or 
  refuted 
  by 
  the 
  work 
  of 
  the 
  Director-General 
  of 
  

   the 
  Geological 
  Survey 
  and 
  of 
  different 
  members 
  of 
  his 
  staff. 
  

  

  By 
  far 
  the 
  most 
  important 
  of 
  the 
  results 
  of 
  my 
  study 
  of 
  the 
  

   igneous 
  rocks 
  in 
  the 
  Western 
  Isles 
  of 
  Scotland 
  was 
  the 
  establishment 
  

   of 
  the 
  true 
  relation 
  to 
  one 
  another 
  of 
  the 
  so-called 
  Plutonic 
  and 
  

   Volcanic 
  rocks 
  of 
  that 
  district. 
  Darwin 
  had 
  long 
  before 
  shown 
  

   similar 
  relations 
  to 
  exist 
  between 
  the 
  granites 
  and 
  lavas 
  of 
  the 
  

   Andes 
  ; 
  while 
  Jukes 
  and 
  other 
  writers 
  had 
  insisted 
  upon 
  the 
  grada- 
  

   tion 
  of 
  highly 
  crystalline 
  into 
  glassy 
  rocks 
  as 
  being 
  on 
  a 
  priori 
  

   grounds 
  probable. 
  Bat 
  the 
  demonstration 
  that 
  in 
  a 
  particular 
  area 
  

   there 
  exist 
  the 
  data 
  for 
  illustrating 
  the 
  complete 
  transition 
  of 
  

   granitic 
  into 
  glassy 
  rocks, 
  both 
  in 
  the 
  acid 
  and 
  basic 
  series, 
  has 
  

   been 
  regarded 
  not 
  only 
  in 
  this 
  country, 
  but 
  by 
  many 
  foreign 
  geolo- 
  

   gists, 
  such 
  as 
  Suess, 
  lleyer, 
  and 
  Dana, 
  as 
  marking 
  a 
  distinct 
  and 
  

   important 
  addition 
  to 
  geological 
  knowledge. 
  

  

  A. 
  The 
  Relations 
  between 
  the 
  Plutonic 
  and 
  the 
  Volcanic 
  Bocks 
  

   of 
  the 
  Western 
  Isles 
  of 
  Scotland. 
  

  

  My 
  conclusions 
  on 
  this 
  subject 
  may 
  be 
  briefly 
  summarized 
  as 
  

   follows 
  : 
  — 
  

  

  There 
  exist 
  in 
  the 
  Western 
  Isles 
  of 
  Scotland 
  true 
  Plutonic 
  rocks 
  

   {granites 
  and 
  gabbros) 
  exhibiting 
  a 
  perfectly 
  noncrystalline 
  structure, 
  

   and 
  it 
  is 
  possible 
  to 
  trace 
  every 
  gradation 
  from 
  these 
  through 
  

   different 
  microcrystalline 
  and 
  cryptocrystalline 
  rocks 
  into 
  truly 
  

   vitreous 
  ones 
  (pitchstones 
  and 
  tachylytes). 
  The 
  distinction 
  between 
  

   Plutonic 
  and 
  Volcanic 
  rocks 
  — 
  -however 
  convenient 
  and 
  necessary 
  it 
  

   may 
  be 
  in 
  practice 
  — 
  is 
  a 
  purely 
  arbitrary 
  one, 
  some 
  lavas 
  being 
  

   more 
  highly 
  crystalline 
  than 
  certain 
  portions 
  of 
  intrusive 
  masses 
  ; 
  for 
  

   the 
  degree 
  of 
  crystallization 
  in 
  each 
  case 
  is 
  determined 
  by 
  the 
  con- 
  

   ditions 
  under 
  which 
  the 
  originally 
  molten 
  masses 
  have 
  been 
  placed. 
  

   (Quart, 
  Journ. 
  Geol. 
  Soc. 
  1874, 
  pp. 
  233-248). 
  

  

  I 
  need 
  not 
  remind 
  the 
  members 
  of 
  this 
  Society 
  that 
  these 
  views 
  — 
  

   though 
  they 
  were 
  very 
  warmly 
  accepted 
  bj 
  many 
  fellow-workers 
  in 
  

   this 
  country 
  — 
  were 
  received 
  in 
  many 
  quarters 
  abroad 
  with 
  much 
  

   opposition 
  and 
  even 
  with 
  ridicule. 
  Equally 
  unnecessary 
  is 
  it 
  to 
  

   refer 
  to 
  subsequent 
  papers 
  in 
  which 
  I 
  endeavoured 
  to 
  support 
  and 
  

   defend 
  my 
  conclusions 
  by 
  a 
  reference 
  to 
  other 
  areas, 
  or 
  by 
  giving 
  

   more 
  detailed 
  descriptions 
  of 
  some 
  of 
  the 
  Scottish 
  rocks 
  *. 
  

  

  Within 
  the 
  last 
  few 
  years, 
  however, 
  a 
  number 
  of 
  foreign 
  geologists 
  

   have 
  described 
  relations 
  very 
  similar 
  to 
  those 
  which 
  I 
  pointed 
  out 
  

   in 
  Scotland 
  as 
  existing 
  between 
  the 
  Plutonic 
  and 
  Volcanic 
  rocks 
  of 
  

   other 
  areas 
  ; 
  and 
  the 
  conclusions 
  which 
  they 
  have 
  arrived 
  at 
  are 
  

   almost 
  identical 
  with 
  my 
  own. 
  This 
  is 
  especially 
  the 
  case 
  with 
  

  

  * 
  See 
  especially 
  the 
  Memoir 
  " 
  On 
  the 
  Ancient 
  Volcano 
  of 
  Schemnitz, 
  Hun- 
  

   gary," 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xxxii. 
  (1876), 
  p. 
  292, 
  and 
  ' 
  Volcanoes, 
  what 
  

   they 
  are 
  and 
  what 
  they 
  teach' 
  (1881), 
  pp. 
  53-56; 
  also 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  

   vol. 
  xli. 
  (1885), 
  pp. 
  354-418, 
  and 
  vol. 
  xlii. 
  (1886), 
  pp. 
  49-97. 
  

   Q. 
  J. 
  G. 
  S. 
  No. 
  178. 
  g 
  

  

  