﻿J. 
  W. 
  JT/DD 
  ON 
  THE 
  SECONDARY 
  ROCKS 
  OE 
  SCOTLAND. 
  251 
  

  

  and 
  scoriae, 
  enclosing 
  fragments 
  of 
  the 
  Lower 
  Silurian 
  and 
  Jurassic 
  

   rocks 
  which 
  constitute 
  the 
  foundation 
  on 
  which 
  the 
  volcanic 
  rocks 
  

   of 
  Ardnamurchan 
  are 
  piled. 
  

  

  The 
  whole 
  of 
  the 
  western 
  extremity 
  of 
  the 
  peninsula 
  of 
  Ardna- 
  

   murchan 
  is 
  constituted 
  by 
  masses 
  composed 
  of 
  many 
  varieties 
  of 
  

   gabbro, 
  which 
  form 
  a 
  number 
  of 
  wild 
  and 
  barren 
  mountains. 
  At 
  

   the 
  western 
  extremity 
  of 
  the 
  peninsula 
  the 
  pyroxenic 
  ingredient 
  of 
  

   the 
  gabbro 
  is 
  either 
  diallage 
  or 
  hypersthene 
  ; 
  but 
  as 
  we 
  go 
  east- 
  

   ward 
  these 
  are 
  found 
  to 
  be 
  replaced 
  in 
  varying 
  degrees 
  by 
  other 
  

   members 
  of 
  the 
  pyroxene 
  family, 
  and 
  the 
  gabbro 
  graduates 
  into 
  

   dolerite. 
  

  

  Of 
  the 
  intrusive 
  character 
  of 
  this 
  great 
  mass 
  of 
  highly 
  crystalline 
  

   basic 
  rocks, 
  and 
  its 
  posteriority 
  alike 
  to 
  the 
  Secondary 
  strata 
  and 
  

   the 
  felstone 
  lavas 
  and 
  felsites, 
  we 
  have 
  the 
  most 
  ample 
  proof. 
  

   Along 
  the 
  southern 
  side 
  of 
  the 
  peninsula, 
  on 
  either 
  side 
  of 
  the 
  cape 
  

   called 
  Stron 
  Beg, 
  the 
  strata 
  of 
  Lias 
  and 
  Inferior 
  Oolite 
  are 
  seen 
  to 
  

   be 
  upheaved 
  at 
  very 
  high 
  angles 
  on 
  the 
  flanks 
  of 
  the 
  masses 
  of 
  

   gabbro 
  ; 
  and 
  as 
  we 
  approach 
  the 
  latter 
  the 
  sedimentary 
  deposits 
  

   are 
  found 
  to 
  undergo 
  the 
  most 
  wonderful 
  metamorphism 
  ; 
  sand- 
  

   stones 
  are 
  seen 
  passing 
  into 
  quartzites, 
  limestones 
  into 
  marble, 
  and 
  

   clay 
  into 
  hard 
  shale, 
  slaty 
  rock 
  and 
  " 
  Lydian 
  stone." 
  Not 
  less 
  striking 
  

   are 
  the 
  effects 
  produced 
  by 
  the 
  gabbros 
  on 
  the 
  earlier 
  volcanic 
  rocks, 
  

   the 
  felstone 
  lavas 
  which 
  overlie 
  the 
  Secondary 
  strata. 
  In 
  proxi- 
  

   mity 
  to 
  the 
  gabbros, 
  these 
  felstone 
  lavas 
  are 
  seen 
  to 
  have 
  been 
  up- 
  

   heaved 
  at 
  high 
  angles 
  and 
  to 
  have 
  acquired 
  a 
  peculiar 
  platy 
  structure 
  

   and 
  splintery 
  fracture, 
  combined 
  in 
  many 
  cases 
  with 
  the 
  development 
  

   of 
  a 
  probably 
  preexisting 
  banded 
  coloration. 
  Even 
  the 
  crystalline 
  

   felsites 
  have 
  not 
  escaped 
  the 
  general 
  metamorphism, 
  but 
  exhibit 
  

   peculiarities 
  of 
  texture 
  and 
  fracture 
  near 
  their 
  junction 
  with 
  the 
  

   gabbros. 
  

  

  Secondary 
  strata, 
  felstone 
  lavas, 
  and 
  felsite 
  masses 
  are 
  alike 
  

   penetrated 
  in 
  all 
  directions 
  by 
  innumerable 
  veins, 
  dykes, 
  and 
  sheets 
  

   of 
  gabbro, 
  dolerite, 
  and 
  basalt 
  ; 
  while, 
  to 
  the 
  eastward, 
  the 
  vestiges 
  

   of 
  old 
  basaltic 
  lava-flows 
  are 
  seen 
  resting 
  indifferently 
  upon 
  the 
  

   Lower 
  Silurian 
  gneiss, 
  the 
  Secondary 
  strata, 
  and 
  the 
  felspathic 
  

   lavas 
  of 
  the 
  earlier 
  period 
  of 
  eruption. 
  

  

  Here 
  then, 
  in 
  spite 
  of 
  the 
  more 
  fragmentary 
  character 
  which 
  the 
  

   relics 
  have 
  assumed 
  in 
  consequence 
  of 
  the 
  enormous 
  amount 
  of 
  

   denudation 
  which 
  they 
  have 
  suffered, 
  their 
  analogy 
  with 
  the 
  rocks 
  

   of 
  Mull 
  is 
  unmistakable 
  and 
  points 
  to 
  precisely 
  to 
  the 
  same 
  suc- 
  

   cession 
  of 
  events. 
  It 
  is 
  evident 
  that 
  in 
  Ardnamurchan 
  we 
  have 
  

   preserved 
  the 
  " 
  sector 
  " 
  of 
  a 
  great 
  circular 
  volcanic 
  pile, 
  which, 
  like 
  

   that 
  of 
  Mull, 
  consisted 
  of 
  an 
  older 
  interior 
  mass 
  composed 
  of 
  

   igneous 
  rocks 
  of 
  the 
  acid 
  class, 
  intrusive, 
  laval, 
  and 
  fragmentary, 
  

   which 
  mass 
  was 
  pierced 
  and 
  enveloped 
  by 
  the 
  basic 
  products 
  of 
  a 
  

   second 
  period 
  of 
  eruption. 
  

  

  The 
  manner 
  in 
  which, 
  by 
  means 
  of 
  denudation, 
  the 
  internal 
  

   structure 
  of 
  the 
  volcanic 
  masses 
  of 
  Ardnamurchan 
  has 
  been 
  laid 
  

   bare 
  to 
  our 
  view 
  in 
  sea-cliffs 
  and 
  deep 
  ravines, 
  renders 
  this 
  frag- 
  

   ment 
  of 
  a 
  volcano 
  of 
  special 
  interest 
  to 
  the 
  geologist. 
  Here 
  he 
  ia 
  

  

  