﻿J. 
  W. 
  JUDD 
  ON 
  THE 
  SECOXDA.EY 
  EOCKS 
  OP 
  SCOTLAND. 
  293 
  

  

  composed 
  of 
  hornblendic 
  granite, 
  passing 
  by 
  insensible 
  gradations 
  

   into 
  ordinary 
  granite 
  on 
  the 
  one 
  band, 
  and 
  into 
  syenite 
  granite 
  on 
  

   tbe 
  other. 
  The 
  normal 
  character 
  of 
  this 
  rock 
  is 
  that 
  of 
  an 
  aggregate 
  

   of 
  white 
  orthoclase 
  and 
  oligoclase 
  felspar 
  with 
  colourless 
  quartz 
  and 
  

   hornblende, 
  the 
  latter 
  being 
  usually 
  in 
  part 
  replaced 
  by 
  varying 
  

   proportions 
  of 
  lepidomelan 
  ; 
  and 
  the 
  whole 
  mass 
  is 
  rendered 
  beauti- 
  

   fully 
  porphyritic 
  by 
  the 
  dissemination 
  through 
  it 
  of 
  fine 
  crystals 
  of 
  

   orthoclase 
  felspar 
  of 
  a 
  pink 
  colour. 
  Locally, 
  however, 
  it 
  exhibits 
  

   many 
  variations 
  from 
  the 
  typical 
  character. 
  It 
  is 
  traversed, 
  too, 
  in 
  

   every 
  direction 
  by 
  veins 
  of 
  various 
  size, 
  composed 
  usually 
  of 
  

   granite 
  of 
  finer 
  grain, 
  of 
  euritic 
  felsite, 
  or 
  of 
  crystallized 
  quartz 
  and 
  

   felspar. 
  The 
  granitic 
  rocks 
  constitute 
  those 
  great 
  spurs, 
  with 
  sharp 
  

   summit-ridges 
  and 
  steeply 
  sloping 
  sides, 
  which 
  divide 
  the 
  deep 
  corries 
  

   that 
  form 
  so 
  striking 
  a 
  feature 
  in 
  the 
  Beinn 
  Nevis 
  group. 
  

  

  Let 
  us 
  now 
  ascend 
  the 
  central 
  peak 
  and 
  examine 
  its 
  structure. 
  

   Some 
  distance 
  above 
  the 
  well-known 
  lake, 
  which 
  lies 
  in 
  a 
  hollow 
  

   upon 
  the 
  shoulders 
  of 
  the 
  mountain, 
  a 
  remarkable 
  change 
  is 
  found 
  

   to 
  take 
  place 
  in 
  the 
  character 
  of 
  the 
  granitic 
  mass 
  ; 
  it 
  becomes 
  much 
  

   finer-grained, 
  and 
  as 
  we 
  still 
  ascend 
  we 
  find 
  the 
  hornblende 
  and 
  

   mica 
  gradually 
  disappearing, 
  till 
  in 
  the 
  end 
  the 
  rock 
  becomes 
  a 
  finely 
  

   granular 
  felsite 
  of 
  a 
  pale 
  red 
  colour, 
  and 
  often 
  more 
  or 
  less 
  porphy- 
  

   ritic 
  in 
  structure. 
  

  

  The 
  highest 
  portion 
  of 
  the 
  mountain, 
  however, 
  is 
  "composed 
  of 
  a 
  

   mass 
  of 
  rocks 
  of 
  totally 
  different 
  character. 
  Instead 
  of 
  the 
  pale 
  red 
  

   granites, 
  eurites, 
  and 
  felsites, 
  we 
  find 
  dark-blue, 
  grey, 
  greeenish, 
  

   and 
  purplish 
  felstones 
  ; 
  and 
  associated 
  with 
  these 
  are 
  enormous 
  

   masses 
  of 
  volcanic 
  agglomerate, 
  composed 
  for 
  the 
  most 
  part 
  of 
  

   angular 
  fragments 
  of 
  all 
  sizes, 
  of 
  felspathic 
  materials, 
  heaped 
  

   together 
  in 
  the 
  wildest 
  confusion, 
  and 
  compacted 
  into 
  masses 
  of 
  

   great 
  solidity 
  and 
  hardness. 
  

  

  The 
  confusion 
  produced 
  by 
  the 
  ever 
  descending 
  fragments 
  which 
  

   cover 
  the 
  upper 
  slopes 
  of 
  the 
  mountain 
  and 
  constitute 
  such 
  im- 
  

   pressive 
  evidence 
  of 
  the 
  potency 
  of 
  atmospheric 
  agencies 
  in 
  the 
  

   work 
  of 
  denudation, 
  is 
  inimical 
  to 
  a 
  complete 
  study 
  of 
  the 
  relations 
  

   of 
  the 
  rocks 
  forming 
  the 
  summit 
  of 
  the 
  mountain 
  ; 
  but 
  an 
  atten- 
  

   tive 
  examination 
  both 
  of 
  these 
  slopes 
  and 
  of 
  the 
  more 
  accessible 
  of 
  

   the 
  precipices 
  surrounding 
  the 
  corries 
  shows 
  that 
  the 
  felstones 
  form 
  

   great 
  sheets, 
  sometimes 
  exhibiting 
  a 
  rudely 
  columnar 
  structure, 
  

   and 
  that 
  between 
  them 
  lie 
  the 
  enormous 
  masses 
  of 
  volcanic 
  agglo- 
  

   merate, 
  the 
  whole 
  being 
  traversed 
  by 
  innumerable 
  felstone 
  dykes. 
  

  

  From 
  the 
  same 
  cause, 
  namely 
  the 
  abundance 
  of 
  debris 
  on 
  the 
  

   higher 
  slopes 
  of 
  the 
  mountain, 
  the 
  relations 
  of 
  the 
  underlying 
  

   felsites 
  and 
  granite 
  to 
  the 
  great 
  cap 
  composed 
  of 
  felstones 
  and 
  

   agglomerates 
  is 
  not 
  at 
  first 
  sight 
  very 
  apparent. 
  Careful 
  study, 
  

   however, 
  shows 
  that 
  the 
  former 
  rocks 
  send 
  off 
  veins 
  into 
  the 
  latter 
  ; 
  

   and 
  the 
  abundance 
  of 
  the 
  fragments 
  of 
  the 
  granitic 
  rocks, 
  above 
  the 
  

   level 
  at 
  which 
  the 
  lavas 
  and 
  agglomerates 
  commence, 
  shows 
  that 
  

   these 
  veins 
  are 
  by 
  no 
  means 
  few 
  in 
  number. 
  

  

  With 
  regard 
  both 
  to 
  the 
  felstone 
  lavas 
  and 
  the 
  associated 
  agglo- 
  

   merates, 
  the 
  changes 
  produced 
  by 
  the 
  chemical 
  action 
  that 
  has 
  taken 
  

  

  Q. 
  J. 
  G. 
  S. 
  No. 
  119. 
  t 
  

  

  