﻿198 
  PROF. 
  J. 
  W. 
  J 
  TDD 
  0> 
  T 
  THE 
  TEETIAET 
  VOLCANOES 
  

  

  removing 
  from 
  the 
  acid 
  rocks 
  the 
  less 
  highly 
  silicated 
  types 
  which 
  

   had 
  all 
  been 
  included 
  under 
  that 
  head 
  by 
  Bunsen, 
  Durocher, 
  Cotta, 
  

   and 
  others, 
  and 
  erecting 
  them 
  into 
  a 
  group 
  by 
  themselves 
  * 
  — 
  the 
  

   " 
  intermediate 
  " 
  rocks. 
  But 
  I 
  think 
  that 
  I 
  cannot 
  fairly 
  be 
  charged 
  

   with 
  leaving 
  in 
  doubt 
  the 
  sense 
  in 
  which 
  I 
  employed 
  the 
  term 
  

   basalt 
  in 
  the 
  year 
  187-1; 
  and 
  I 
  have 
  since 
  described 
  these 
  " 
  ophitic 
  

  

  Rocics 
  forming 
  the 
  Summit 
  of 
  Beinn 
  Uaig, 
  Isle 
  of 
  Mull. 
  

  

  a. 
  Felstone-lavas, 
  with 
  agglomerates 
  +. 
  

  

  b. 
  Syenite-granite, 
  graduating 
  into 
  felsite. 
  

  

  olivine-basalts 
  " 
  of 
  the 
  district 
  in 
  considerable 
  detail 
  J. 
  I 
  was 
  very 
  

   careful 
  to 
  point 
  out 
  that 
  many 
  of 
  my 
  " 
  felstones 
  " 
  are 
  as 
  dark- 
  

   coloured 
  as 
  the 
  basalts, 
  but 
  that 
  they 
  might 
  be 
  distinguished 
  by 
  their 
  

   macroscopic 
  as 
  well 
  as 
  by 
  their 
  microscopic 
  characters 
  (Q. 
  J. 
  G. 
  S. 
  

   1874, 
  p. 
  236). 
  

  

  Many 
  of 
  the 
  cases 
  cited 
  in 
  the 
  recently 
  published 
  memoir 
  as 
  intru- 
  

   sions 
  of 
  granite 
  and 
  felsite 
  into 
  basalt 
  — 
  and 
  on 
  which 
  so 
  much 
  stress 
  

   is 
  laid 
  — 
  will, 
  I 
  think, 
  be 
  found 
  to 
  be, 
  as 
  at 
  Beinn 
  Uaig, 
  intrusions 
  

   into 
  the 
  dark-coloured 
  " 
  felstones" 
  of 
  my 
  first 
  period 
  of 
  eruption 
  §. 
  

  

  I 
  undoubtedly 
  stated 
  that, 
  as 
  a 
  whole, 
  the 
  great 
  ejections 
  of 
  acid 
  

   material, 
  both 
  as 
  intrusive 
  masses 
  and 
  as 
  lavas 
  or 
  tuffs, 
  took 
  place 
  

   before 
  that 
  of 
  the 
  gabbros 
  and 
  basalts. 
  But 
  considering 
  how 
  vast 
  a 
  

  

  * 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xxxiii. 
  (1876), 
  p. 
  295. 
  

  

  t 
  These 
  " 
  felstone-lavas 
  " 
  are 
  of 
  very 
  dark 
  grey, 
  almost 
  black 
  colour, 
  and 
  

   might 
  easily 
  be 
  mistaken 
  for 
  basalts. 
  Microscopically 
  studied, 
  however, 
  they 
  

   are 
  found 
  to 
  present 
  the 
  very 
  closest 
  analogies 
  with 
  the 
  rock 
  described 
  by 
  

   M. 
  Breon 
  from 
  near 
  Stikkisholmur, 
  Iceland 
  (see 
  his 
  Geologie 
  de 
  ITslande 
  &c. 
  

   p. 
  23, 
  pi. 
  iii. 
  fig. 
  1). 
  This 
  rock, 
  which, 
  like 
  that 
  of 
  Beinn 
  Uaig, 
  contains 
  no 
  

   olivine, 
  but 
  has 
  a 
  microstructure 
  very 
  similar 
  to 
  that 
  of 
  many 
  andesites, 
  is 
  

   classed 
  by 
  Breon 
  as 
  an 
  " 
  augite-andesite."' 
  In 
  the 
  second 
  edition 
  of 
  his 
  

   ' 
  Massigen 
  Gesteine' 
  (1887), 
  p. 
  687, 
  Professor 
  Rosenbusch 
  adopts 
  this 
  reference 
  

   of 
  the 
  Stikkisholmur-rock 
  to 
  the 
  augite-andesites, 
  and 
  compares 
  it 
  with 
  rocks 
  of 
  

   a 
  similar 
  type 
  described 
  by 
  Forstner 
  in 
  Pantellaria. 
  The 
  " 
  syenite-granite" 
  is 
  

   the 
  usual 
  drusy 
  (miarolitic) 
  granite 
  of 
  the 
  district, 
  which 
  graduates 
  through 
  

   various 
  granophvric 
  modifications 
  into 
  a 
  distinctly 
  rhvolitic 
  rock. 
  

  

  + 
  Ibid, 
  vol. 
  xliii. 
  (1886) 
  p. 
  49. 
  

  

  [§ 
  A 
  reference 
  to 
  the 
  discussion 
  which 
  followed 
  the 
  reading 
  of 
  this 
  paper 
  will 
  

   show 
  that 
  the 
  author 
  of 
  the 
  memoir 
  of 
  1888 
  groups 
  together 
  the 
  andesites 
  and 
  

   basalts 
  as 
  distinct 
  from 
  the 
  acid 
  rocks. 
  See 
  p. 
  219.] 
  

  

  