﻿252 
  J. 
  W. 
  JUDD 
  ON 
  THE 
  SECONDARY 
  EOCKS 
  OE 
  SCOTLAND. 
  

  

  enabled 
  to 
  study 
  in 
  detail 
  the 
  results 
  of 
  the 
  potent 
  subterranean 
  

   actions 
  which 
  went 
  on 
  side 
  by 
  side 
  with 
  those 
  subaerial 
  phenomena 
  

   with 
  which 
  our 
  ideas 
  of 
  volcanic 
  activity 
  are 
  usually 
  associated, 
  but 
  

   which 
  do 
  not 
  constitute 
  the 
  only, 
  nor 
  perhaps 
  even 
  the 
  principal 
  

   effects 
  of 
  these 
  grand 
  outbursts. 
  To 
  the 
  consideration 
  of 
  these 
  

   subterranean 
  phenomena 
  I 
  shall 
  have 
  to 
  return 
  in 
  the 
  sequel. 
  

  

  7. 
  The 
  Volcano 
  of 
  Rum.— 
  Lying 
  to 
  the 
  north-west 
  of 
  the 
  penin- 
  

   sula 
  of 
  Ardnaruurchan 
  we 
  find 
  the 
  group 
  of 
  the 
  Small 
  Isles, 
  comprising 
  

   Rum, 
  Canna, 
  Eigg, 
  and 
  Muck, 
  with 
  several 
  smaller 
  islets. 
  The 
  

   whole 
  of 
  these, 
  with 
  the 
  exception 
  of 
  the 
  first 
  mentioned, 
  are 
  

   evidently 
  isolated 
  fragments 
  of 
  a 
  plateau 
  composed 
  of 
  basaltic 
  lava- 
  

   sheets, 
  which 
  are 
  in 
  some 
  places 
  seen 
  to 
  rest 
  unconformably 
  upon 
  

   various 
  Secondary 
  strata. 
  In 
  the 
  island 
  of 
  Rum, 
  however, 
  we 
  find 
  a 
  

   great 
  mass 
  of 
  eruptive 
  crystalline 
  rocks 
  which 
  have 
  burst 
  through 
  

   the 
  older 
  strata 
  ; 
  and 
  the 
  various 
  igneous 
  products, 
  in 
  their 
  nature, 
  

   positions, 
  and 
  relations, 
  present 
  such 
  remarkable 
  analogies 
  with 
  

   those 
  of 
  Mull 
  and 
  Ardnamurchan 
  as 
  to 
  leave 
  no 
  room 
  for 
  doubt 
  

   that 
  in 
  Rum 
  also 
  we 
  have 
  the 
  basal 
  relics 
  of 
  a 
  great 
  volcanic 
  

   mountain 
  (Section, 
  PI. 
  XXIII. 
  fig. 
  5). 
  

  

  Nothing 
  can 
  be 
  more 
  instructive 
  than 
  the 
  admirable 
  manner 
  in 
  

   which, 
  through 
  denudation, 
  the 
  mutual 
  relations 
  of 
  the 
  stratified 
  

   and 
  eruptive 
  rocks 
  are 
  displayed 
  in 
  the 
  island 
  of 
  Rum. 
  The 
  former 
  

   consist 
  of 
  Cambrian 
  sandstone 
  with 
  some 
  masses 
  of 
  gneissose, 
  

   schistose, 
  and 
  quartzose 
  rocks, 
  probably 
  of 
  Lower 
  Silurian 
  age 
  ; 
  

   these 
  occupy 
  the 
  north-west, 
  north-east, 
  and 
  south-east 
  sides 
  of 
  the 
  

   island, 
  while 
  its 
  interior 
  is 
  formed 
  by 
  masses 
  of 
  eruptive 
  rocks, 
  

   including 
  both 
  granites 
  and 
  gabbros, 
  which 
  rise 
  into 
  lofty 
  moun- 
  

   tains. 
  At 
  a 
  distance 
  from 
  the 
  igneous 
  rocks, 
  the 
  Palaeozoic 
  strata 
  

   exhibit 
  their 
  usual 
  mineralogical 
  characters 
  and 
  their 
  normal 
  strike 
  

   and 
  dip 
  ; 
  but 
  wherever 
  they 
  approach 
  the 
  great 
  central 
  intrusive 
  

   masses 
  they 
  are 
  found 
  to 
  be 
  violently 
  upheaved 
  and 
  often 
  much 
  

   contorted, 
  and 
  are 
  further 
  seen 
  to 
  have 
  undergone 
  a 
  striking 
  meta- 
  

   morphism, 
  which 
  often 
  extends 
  to 
  a 
  considerable 
  distance 
  from 
  the 
  

   igneous 
  rocks 
  ; 
  the 
  more 
  argillaceous 
  or 
  felspathic 
  portions 
  of 
  the 
  

   Cambrian 
  sandstone 
  pass 
  into 
  a 
  highly 
  micaceous 
  black 
  schist, 
  

   and 
  the 
  more 
  siliceous 
  portions 
  into 
  an 
  impure 
  quartzite. 
  The 
  

   whole 
  of 
  the 
  stratified 
  rocks 
  surrounding 
  the 
  intrusive 
  masses 
  of 
  

   granite 
  and 
  gabbro 
  are 
  intersected 
  in 
  all 
  directions 
  by 
  innumerable 
  

   veins 
  and 
  dykes. 
  

  

  Let 
  us 
  now 
  turn 
  our 
  attention 
  to 
  the 
  igneous 
  masses 
  which 
  form 
  

   the 
  central 
  group 
  of 
  mountains, 
  and 
  upon 
  the 
  flanks 
  of 
  which 
  the 
  

   Primary 
  strata 
  lie 
  on 
  three 
  sides 
  of 
  the 
  island, 
  having 
  evidently 
  

   been 
  removed 
  by 
  denudation 
  on 
  the 
  fourth 
  side. 
  These 
  central 
  

   mountains 
  of 
  Rum, 
  as 
  is 
  evident 
  to 
  the 
  most 
  casual 
  observer, 
  are 
  

   composed 
  of 
  two 
  very 
  distinct 
  kinds 
  of 
  rocks, 
  the 
  contrast 
  between 
  

   which, 
  owing 
  to 
  their 
  different 
  modes 
  of 
  weathering, 
  is 
  extremely 
  

   striking. 
  

  

  To 
  the 
  westward, 
  and 
  forming 
  the 
  smooth 
  dome- 
  shaped 
  mountain 
  

   mass 
  of 
  Oreval 
  (1872 
  feet) 
  and 
  its 
  dependencies, 
  is 
  a 
  great 
  develop- 
  

   ment 
  of 
  much-jointed 
  highly 
  felspathic 
  rocks, 
  consisting 
  of 
  granite 
  

  

  