﻿300 
  3. 
  W. 
  JTJDD 
  ON 
  THE 
  SECONDARY 
  ROCKS 
  OE 
  SCOTLAND. 
  

  

  Circumscribed 
  though 
  the 
  observations 
  of 
  the 
  geologist 
  necessarily 
  

   are, 
  within 
  the 
  infinitesimal 
  periods 
  of 
  human 
  chronology, 
  he 
  may 
  

   nevertheless 
  learn 
  to 
  triumph 
  over 
  the 
  painful 
  limitations 
  of 
  time 
  

   by 
  availing 
  himself 
  of 
  the 
  opportunity 
  which 
  he 
  has 
  of 
  studying 
  

   the 
  same 
  series 
  of 
  operations 
  at 
  various 
  stages 
  of 
  its 
  progress, 
  as 
  pre- 
  

   sented 
  in 
  different 
  examples. 
  As 
  was 
  so 
  well 
  expressed 
  by 
  Playfair, 
  

   "It 
  is 
  true 
  we 
  do 
  not 
  see 
  the 
  successive 
  steps 
  of 
  this 
  progress 
  

   exemplified 
  in 
  the 
  states 
  of 
  the 
  same 
  individual 
  rock, 
  but 
  we 
  see 
  

   them 
  clearly 
  in 
  different 
  individuals; 
  and 
  the 
  conviction 
  thus 
  

   produced, 
  when 
  the 
  phenomena 
  are 
  sufficiently 
  multiplied 
  and 
  

   varied, 
  is 
  as 
  irresistible 
  as 
  if 
  we 
  saw 
  the 
  changes 
  actually 
  effected 
  

   in 
  the 
  moment 
  of 
  observation"*. 
  

  

  To 
  this 
  mode 
  of 
  reasoning 
  the 
  geologist 
  is 
  continually 
  compelled 
  

   to 
  resort. 
  If, 
  as 
  I 
  believe, 
  it 
  is 
  demonstrated 
  that 
  in 
  wide-spread 
  

   intrusive 
  masses 
  of 
  granite 
  and 
  gabbro 
  or 
  serpentinous 
  rocks 
  — 
  in 
  

   the 
  more 
  or 
  less 
  isolated 
  mountain-groups 
  of 
  granitic 
  and 
  doleritic 
  

   rocks 
  — 
  in 
  the 
  great 
  volcanic 
  mountains 
  in 
  varying 
  stages 
  of 
  decay 
  

   and 
  destruction 
  by 
  denudation 
  — 
  and 
  in 
  the 
  long 
  lines 
  of 
  parasitical 
  

   cones 
  which 
  surround 
  them, 
  we 
  witness 
  but 
  different 
  portions 
  of 
  

   the 
  same 
  grand 
  series 
  of 
  phenomena, 
  then 
  the 
  geologist 
  is 
  enabled 
  

   to 
  picture 
  to 
  himself 
  the 
  characteristics 
  and 
  results 
  of 
  volcanic 
  

   activity 
  in 
  all 
  its 
  developments 
  and 
  under 
  all 
  its 
  phases. 
  

  

  Thus 
  he 
  is 
  led 
  to 
  the 
  conclusion 
  that 
  the 
  distinction 
  between 
  

   Plutonic, 
  Trappean, 
  and 
  Yolcanic 
  rocks, 
  convenient 
  though 
  it 
  may 
  

   be 
  in 
  practice, 
  is 
  a 
  purely 
  artificial 
  one, 
  and 
  that 
  the 
  whole 
  of 
  

   these 
  must 
  be 
  regarded 
  as 
  the 
  products 
  of 
  the 
  same 
  igneous 
  forces 
  

   when 
  operating 
  under 
  different 
  conditions. 
  "While, 
  on 
  the 
  one 
  hand, 
  

   we 
  are 
  led 
  to 
  conclude 
  that 
  great 
  tracts 
  of 
  granite, 
  like 
  that 
  of 
  Lein- 
  

   ster, 
  may 
  once 
  have 
  been 
  surmounted 
  by 
  vast 
  volcanic 
  piles, 
  we 
  

   cannot, 
  on 
  the 
  other 
  hand, 
  doubt 
  that 
  the 
  subaerial 
  volcanic 
  phe- 
  

   nomena 
  of 
  Iceland, 
  Sicily, 
  and 
  the 
  Andes 
  are 
  accompanied 
  by 
  

   innumerable 
  igneous 
  injections 
  in 
  subjacent 
  strata 
  and 
  the 
  forma- 
  

   tion 
  of 
  masses 
  of 
  granite, 
  syenite, 
  diorite, 
  and 
  gabbro 
  at 
  great 
  

   depths 
  beneath 
  them. 
  

  

  EXPLANATION 
  OF 
  PLATES 
  XXII. 
  & 
  XXIII. 
  

  

  In 
  these 
  Plates 
  an 
  attempt 
  has 
  been 
  made 
  to 
  illustrate 
  the 
  relations 
  of 
  the 
  

   great 
  masses 
  of 
  volcanic 
  rocks 
  in 
  the 
  Hebrides 
  to 
  one 
  another, 
  and 
  to 
  the 
  sedi- 
  

   mentary 
  deposits 
  which 
  they 
  have 
  been 
  the 
  means 
  of 
  preserving. 
  

  

  The 
  sketch 
  map 
  on 
  Plate 
  XXII. 
  is 
  an 
  attempt 
  to 
  exhibit 
  the 
  relations 
  of 
  the 
  

   outcrops 
  of 
  the 
  great 
  intrusive 
  bosses, 
  sheets, 
  and 
  dykes 
  of 
  igneous 
  rock 
  to 
  the 
  

   lavas, 
  agglomerates, 
  and 
  older 
  strata 
  in 
  the 
  island 
  of 
  Mull 
  ; 
  but 
  this, 
  owing 
  to 
  

   the 
  small 
  scale 
  employed, 
  could 
  only 
  be 
  accomplished 
  in 
  a 
  somewhat 
  diagram- 
  

   matic 
  manner. 
  To 
  display 
  the 
  complicated 
  interlacings 
  of 
  the 
  almost 
  innume- 
  

   rable 
  igneous 
  intrusions 
  of 
  the 
  district 
  would 
  require 
  maps 
  of 
  great 
  accuracy 
  on 
  

   the 
  largest 
  scale, 
  and 
  even 
  then 
  could 
  only 
  be 
  attempted 
  after 
  a 
  very 
  detailed 
  

   and 
  minute 
  survey 
  of 
  the 
  area. 
  Unfortunately 
  the 
  Inner 
  Hebrides 
  have 
  not 
  

   yet 
  been 
  surveyed 
  by 
  the 
  Ordnance 
  Department 
  ; 
  and, 
  although 
  the 
  coast-lines 
  

   are 
  well 
  laid 
  down 
  on 
  the 
  Admiralty 
  Charts, 
  the 
  geologist 
  meets 
  with 
  almost 
  

  

  * 
  Illustrations 
  of 
  the 
  Huttonian 
  Theory, 
  p. 
  101. 
  

  

  