﻿294 
  J. 
  W. 
  JTTDD 
  ON 
  THE 
  SECONDAKY 
  EOCKS 
  OF 
  SCOTLAND. 
  

  

  place 
  iii 
  the 
  mass 
  has 
  been 
  very 
  great. 
  But 
  in 
  this, 
  as 
  in 
  so 
  many 
  

   similar 
  instances, 
  weathering 
  action 
  is 
  found 
  to 
  a 
  great 
  extent 
  

   undoing 
  the 
  work 
  of 
  metamorphism, 
  and 
  revealing 
  original 
  struc- 
  

   tures 
  that 
  had 
  been 
  wholly 
  obscured 
  by 
  it 
  ; 
  thus 
  many 
  rocks, 
  now 
  

   of 
  highly 
  crystalline 
  or 
  compact 
  character, 
  are 
  shown 
  to 
  have 
  been 
  

   originally 
  composed 
  of 
  highly 
  vesicular 
  or 
  scoriaceous 
  materials. 
  

  

  The 
  remarkable 
  characters 
  and 
  relations 
  of 
  the 
  rocks 
  forming 
  

   Beinn 
  Nevis 
  were 
  not 
  unnaturally 
  the 
  cause 
  of 
  much 
  doubt 
  and 
  

   difficulty 
  during 
  the 
  infancy 
  of 
  geological 
  science. 
  Almost 
  the 
  

   only 
  attempt 
  at 
  their 
  explanation 
  hitherto 
  made, 
  consists 
  in 
  the 
  

   suggestion 
  that 
  masses 
  of 
  " 
  porphyry 
  " 
  had 
  been 
  forced 
  through 
  the 
  

   midst 
  of 
  an 
  earlier-formed 
  mountain 
  of 
  granite. 
  

  

  But 
  to 
  any 
  one 
  who 
  has 
  studied 
  the 
  Tertiary 
  volcanic 
  rocks 
  of 
  

   the 
  Hebrides 
  nothing 
  can 
  be 
  clearer 
  than 
  the 
  significance 
  of 
  the 
  

   features 
  exhibited 
  by 
  Beinn 
  Nevis. 
  Indeed 
  it 
  is 
  evidently 
  but 
  a 
  

   repetition, 
  on 
  a 
  grander 
  scale, 
  of 
  many 
  a 
  Tertiary 
  mountain 
  like 
  

   Beinn 
  Uaig 
  in 
  Mull, 
  which 
  we 
  have 
  already 
  described. 
  In 
  both 
  

   alike 
  we 
  see 
  the 
  evidence 
  that 
  while 
  felspathic 
  lava 
  streams 
  and 
  

   scoria? 
  were 
  being 
  erupted 
  at 
  the 
  surface, 
  masses 
  of 
  molten 
  mate- 
  

   rials 
  of 
  the 
  same 
  composition 
  were 
  intruded 
  below 
  them, 
  and 
  by 
  

   slow 
  consolidation 
  forming 
  those 
  bosses 
  of 
  felsite 
  which 
  pass 
  by 
  

   insensible 
  gradations, 
  in 
  their 
  lower 
  and 
  deeper 
  parts, 
  into 
  horn- 
  

   blendic 
  and 
  ordinary 
  granite. 
  That 
  both 
  alike 
  constituted 
  portions 
  

   of 
  great 
  volcanic 
  piles 
  is, 
  I 
  believe, 
  from 
  the 
  facts 
  adduced 
  in 
  this 
  

   memoir, 
  placed 
  beyond 
  dispute. 
  

  

  Scarcely 
  less 
  interesting, 
  in 
  their 
  bearing 
  upon 
  the 
  present 
  inquiry, 
  

   are 
  the 
  phenomena 
  displayed 
  in 
  Glencoe. 
  At 
  the 
  northern 
  end 
  of 
  the 
  

   celebrated 
  pass 
  we 
  find 
  the 
  Lower 
  Silurian 
  strata 
  undergoing 
  consider- 
  

   able 
  disturbance 
  and 
  great 
  metamorphism 
  ; 
  and 
  they 
  are 
  also 
  seen 
  

   to 
  be 
  traversed 
  by 
  a 
  wonderful 
  plexus 
  of 
  veins, 
  dykes, 
  and 
  intrusive 
  

   masses 
  composed 
  sometimes 
  of 
  hornblendic 
  granite, 
  but 
  more 
  usually 
  

   of 
  numerous 
  varieties 
  of 
  red 
  felsite 
  ("porphyry" 
  of 
  authors). 
  The 
  

   stratified 
  rocks 
  are 
  overlain 
  by 
  great 
  masses 
  of 
  felstone, 
  with 
  some 
  

   beds 
  of 
  the 
  so-called 
  "breceiated 
  porphyry".' 
  (consolidated 
  ash, 
  

   scoriae, 
  &c), 
  which 
  are, 
  however, 
  by 
  no 
  means 
  ' 
  so 
  abundant 
  here 
  as 
  

   on 
  Beinn 
  Nevis. 
  These 
  felstones 
  are 
  traversed, 
  like 
  the 
  stratified 
  

   rocks 
  upon 
  which 
  they 
  rest, 
  by 
  an 
  almost 
  infinite 
  number 
  of 
  veins 
  

   and 
  dykes, 
  usually 
  composed 
  of 
  felsite 
  ; 
  and 
  they 
  have 
  in 
  many 
  

   cases 
  undergone 
  a 
  certain 
  amount 
  of 
  alteration 
  from 
  heat, 
  similar 
  to 
  

   that 
  which 
  we 
  have 
  seen 
  taking 
  place 
  in 
  the 
  Tertiary 
  lavas 
  under 
  

   like 
  conditions, 
  and 
  resulting 
  in 
  the 
  development 
  in 
  them 
  of 
  a 
  

   peculiar 
  banded 
  structure 
  and 
  splintery 
  fracture. 
  

  

  These 
  felstones 
  occupy 
  the 
  whole 
  of 
  the 
  central 
  parts 
  of 
  the 
  pass, 
  

   and 
  weather 
  into 
  those 
  striking 
  forms 
  which 
  characterize 
  the 
  

   scenery 
  of 
  this 
  famous 
  spot. 
  At 
  the 
  southern 
  end 
  of 
  the 
  pass 
  

   there 
  appear 
  from 
  underneath 
  these 
  altered 
  felstones, 
  the 
  felsites 
  

   passing 
  into 
  granites, 
  and 
  enclosing 
  masses 
  of 
  often 
  highly 
  altered 
  

   Lower 
  Silurian 
  strata, 
  which 
  constitute 
  the 
  great 
  intrusive 
  mass 
  of 
  

   the 
  Black 
  Mount. 
  

  

  Here 
  it 
  is 
  evident 
  that 
  there 
  has 
  escaped 
  destruction 
  by 
  denuda- 
  

  

  