﻿OF 
  THE 
  WESTERN 
  ISLES 
  OF 
  SCOTLAND. 
  205 
  

  

  position, 
  coincide 
  perfectly 
  with 
  the 
  rocks 
  composing 
  the 
  lavas 
  ; 
  but 
  

   in 
  the 
  varieties 
  of 
  their 
  texture 
  and 
  mineralogical 
  constitution 
  they 
  

   exhibit 
  a 
  much 
  wider 
  range. 
  Thus 
  while 
  we 
  find 
  veins 
  of 
  basalt 
  in 
  

   which 
  the 
  rock-structure 
  is 
  identical 
  with 
  that 
  of 
  many 
  of 
  the 
  lavas, 
  

   we 
  find 
  also 
  others 
  in 
  which 
  the 
  rock 
  passes 
  into 
  a 
  glass 
  — 
  tachylyte 
  ; 
  

   while 
  others, 
  again, 
  are 
  composed 
  of 
  the 
  highly 
  crystalline, 
  or 
  granite- 
  

   gabbro- 
  rocks. 
  Similarly, 
  felstone- 
  veins 
  are 
  related 
  to 
  those 
  of 
  

   pitchstone 
  and 
  obsidian 
  on 
  the 
  one 
  hand, 
  and 
  to 
  masses 
  of 
  felsite, 
  

   syenite-granite, 
  and 
  granite 
  on 
  the 
  other." 
  (Q. 
  J. 
  Gr. 
  S. 
  1874, 
  

   p. 
  238.) 
  

  

  Not 
  only 
  did 
  I 
  refrain 
  from 
  asserting 
  that 
  an 
  intrusive 
  mass 
  of 
  

   gabbro 
  and 
  dolerite 
  could 
  be 
  traced 
  passing 
  into 
  the 
  basaltic 
  lava- 
  

   current, 
  of 
  which 
  it 
  formed 
  the 
  fllled-up 
  duct 
  leading 
  to 
  the 
  surface, 
  

   but 
  in 
  my 
  diagrammatic 
  sections 
  drawn 
  to 
  illustrate 
  what 
  I 
  supposed 
  

   to 
  be 
  the 
  real 
  interpretation 
  of 
  the 
  phenomena, 
  I 
  represented 
  the 
  

   supposed 
  connexion 
  between 
  the 
  great, 
  nearly 
  horizontal 
  sheets 
  of 
  

   intrusive 
  gabbro 
  and 
  dolerite 
  and 
  the 
  superficial 
  lavas 
  by 
  dotted 
  

   lines 
  (see 
  Q. 
  J. 
  G. 
  S. 
  1874, 
  pi. 
  xxiii. 
  fig. 
  3). 
  

  

  The 
  fact 
  is, 
  I 
  perceived 
  then, 
  as 
  I 
  do 
  now, 
  that 
  only 
  by 
  the 
  most 
  

   extraordinary 
  series 
  of 
  accidents 
  could 
  we 
  expect 
  a 
  mass 
  which 
  has 
  

   consolidated 
  at 
  such 
  a 
  depth 
  as 
  to 
  form 
  a 
  gabbro, 
  to 
  have 
  its 
  con- 
  

   nexion 
  with 
  the 
  lava-current 
  that 
  reached 
  the 
  surface 
  preserved 
  in 
  

   such 
  a 
  way 
  as 
  to 
  permit 
  of 
  the 
  change 
  being 
  followed 
  step 
  by 
  step. 
  

  

  How 
  fully 
  my 
  critic 
  has 
  adopted, 
  and 
  how 
  clearly 
  he 
  has 
  expressed 
  

   the 
  views 
  which 
  I 
  put 
  forth 
  concerning 
  the 
  actual 
  transitions 
  from 
  

   gabbro 
  into 
  dolerite, 
  from 
  dolerite 
  into 
  basalt, 
  and 
  from 
  basalt 
  into 
  

   tachylyte, 
  I 
  have 
  already 
  pointed 
  out. 
  He 
  states 
  quite 
  truly 
  that 
  

   in 
  the 
  gabbro-bosses 
  " 
  there 
  are 
  evidences 
  of 
  successive 
  discharges 
  

   or 
  extravasations 
  of 
  crystalline 
  materials, 
  during 
  a 
  probably 
  pro- 
  

   tracted 
  period 
  of 
  time, 
  and 
  in 
  the 
  intricate 
  network 
  of 
  veins 
  crossing 
  

   each 
  other, 
  and 
  the 
  general 
  body 
  of 
  the 
  rock 
  in 
  every 
  direction," 
  

  

  (the 
  geologist) 
  ; 
  ' 
  will 
  recognize 
  the 
  repeated 
  renewals 
  of 
  

  

  subterranean 
  energy." 
  (T. 
  E. 
  S. 
  E., 
  1888, 
  p. 
  130.) 
  

  

  It 
  will 
  thus 
  be 
  seen 
  that 
  we 
  both 
  believe 
  that 
  the 
  basalts 
  of 
  the 
  

   plateaux 
  and 
  the 
  gabbro-masses 
  around 
  the 
  five 
  great 
  centres 
  were 
  

   alike 
  formed 
  by 
  a 
  number 
  of 
  successive 
  manifestations 
  of 
  the 
  volcanic 
  

   forces 
  extending 
  over 
  enormous 
  periods 
  of 
  time. 
  We 
  are 
  at 
  one 
  

   in 
  the 
  opinion 
  that 
  in 
  chemical 
  composition, 
  and 
  in 
  mineralogical 
  

   constitution, 
  the 
  gabbros 
  and 
  tbe 
  basalts 
  show 
  the 
  most 
  striking 
  

   analogies 
  ; 
  while, 
  in 
  texture, 
  the 
  holocrystalline 
  rocks 
  exhibit 
  re- 
  

   markable 
  transitions 
  into 
  the 
  hypocrystalline 
  ones. 
  But 
  while 
  I 
  

   have 
  argued 
  that 
  the 
  same 
  series 
  of 
  volcanic 
  throes 
  which 
  brought 
  

   the 
  basalts 
  to 
  the 
  surface, 
  may 
  have 
  produced 
  at 
  the 
  same 
  time 
  

   the 
  vertical 
  and 
  horizontal 
  fissures, 
  and 
  injected 
  them 
  with 
  the 
  

   materials 
  of 
  the 
  gabbro, 
  my 
  critic 
  asserts 
  that 
  the 
  gabbro-injec- 
  

   tions 
  belong 
  to 
  a 
  distinct 
  and 
  later 
  period 
  than 
  the 
  outflow 
  of 
  the 
  

   basalts. 
  

  

  It 
  is 
  scarcely 
  necessary 
  to 
  remark 
  that 
  the 
  explanation 
  I 
  have 
  

   given 
  is 
  by 
  no 
  means 
  inconsistent 
  with 
  the 
  view 
  that 
  many 
  of 
  the 
  

  

  